tango/server/

def.rs

//! Definition of a device model and of its interface: its commands and its attributes.

use std::time::{Duration, SystemTime};
use std::ops::{Bound, RangeBounds};
use std::fmt::{self, Debug};
use std::future::Future;
use std::borrow::Cow;
use std::sync::Arc;

use futures::future::BoxFuture;

use crate::dev::{AttrConfig, AttrDeltaConfig, AttrMemorized, AttrNumber, AttrQuality, AttrRangeConfig, AttrRead, DevState, DisplayLevel};
use crate::value::{AttrData, AttrDataFormat, AttrDataKind, CommandData, CommandDataKind, FromValue, IntoValue, Value};
use crate::error::Result;

use super::{reserved_name, Model, RESERVED_ATTRS, RESERVED_COMMANDS, STATE_NAME, STATUS_NAME};
use super::attr::{unsupported, unsupported_prop};
use super::dev::Dev;


/// Merge a data kind being defined with the one already defined, if any, and give the
/// kind to keep. Both are compatible when they are equal, which is what allows a kind to
/// be both derived from a handler and given explicitly, in any order. Anything else is a
/// contradiction in the definition and therefore panics.
#[track_caller]
fn merge_kind<K: PartialEq + Debug>(def: &str, what: &str, current: Option<K>, new: K) -> K {
    match current {
        Some(current) if current != new => panic!(
            "{def}: the {what} is already defined as {current:?}, incompatible with {new:?}"),
        _ => new,
    }
}

/// Merge a data format being defined with the one already defined, if any, and give the
/// format to keep. Both are compatible when they have the same layout, their maximum
/// dimensions being merged with [`merge_max`], anything else panics.
#[track_caller]
fn merge_format(def: &str, current: Option<AttrDataFormat>, new: AttrDataFormat) -> AttrDataFormat {
    let Some(current) = current else { return new };
    match (&current, &new) {
        (AttrDataFormat::Scalar, AttrDataFormat::Scalar) => AttrDataFormat::Scalar,
        (AttrDataFormat::Spectrum(cur), AttrDataFormat::Spectrum(new)) => {
            AttrDataFormat::Spectrum(merge_max(def, "maximum size", *cur, *new))
        }
        (AttrDataFormat::Image(cur_x, cur_y), AttrDataFormat::Image(new_x, new_y)) => {
            AttrDataFormat::Image(
                merge_max(def, "maximum width", *cur_x, *new_x),
                merge_max(def, "maximum height", *cur_y, *new_y),
            )
        }
        _ => panic!(
            "{def}: the data format is already defined as {current:?}, incompatible with {new:?}"),
    }
}

/// Merge two maximum dimensions of a data format, zero standing for an undefined maximum
/// and therefore being compatible with any other value. Two different defined maximums
/// are a contradiction in the definition and therefore panic.
#[track_caller]
fn merge_max(def: &str, what: &str, current: usize, new: usize) -> usize {
    match (current, new) {
        (0, new) => new,
        (current, 0) => current,
        (current, new) if current != new => panic!(
            "{def}: the {what} is already defined as {current}, incompatible with {new}"),
        (current, _) => current,
    }
}

/// The two bounds of a range being defined, either of which is absent for a bound that is
/// left open, which is how an attribute says it has no minimum or no maximum rather than
/// having one it never reaches.
#[track_caller]
fn range_config<T, R>(def: &str, what: &str, range: R) -> AttrRangeConfig
where
    T: Into<AttrNumber> + Copy,
    R: RangeBounds<T>,
{
    AttrRangeConfig {
        min: bound_value(def, what, range.start_bound()),
        max: bound_value(def, what, range.end_bound()),
    }
}

/// A single bound of such a range, as the number the attribute holds: a range written with
/// integer bounds gives integer limits, one written with floating bounds gives floating
/// ones, and it is up to the definition to bound an attribute with its own kind of number.
///
/// An exclusive bound has no representation in Tango, the bounds being values a client
/// compares against, so it is refused rather than silently taken for the inclusive one it
/// is not.
#[track_caller]
fn bound_value<T>(def: &str, what: &str, bound: Bound<&T>) -> Option<AttrNumber>
where
    T: Into<AttrNumber> + Copy,
{
    match bound {
        Bound::Included(value) => Some((*value).into()),
        Bound::Unbounded => None,
        Bound::Excluded(_) => panic!(
            "{def}: the {what} cannot have an exclusive bound, Tango only has inclusive ones"),
    }
}

/// A placeholder shown by [`Debug`] in place of a handler, a closure having nothing to
/// show about itself.
struct HandlerDebug;

impl Debug for HandlerDebug {
    #[inline]
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str("..")
    }
}

/// Implement [`Debug`] for the handler enums, showing which access to the model the
/// handler takes and whether it is asynchronous, which is all there is to show about it.
macro_rules! impl_handler_debug {
    ($($handler:ident),* $(,)?) => { $(
        impl<M: Model> Debug for $handler<M> {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                f.write_str(match self {
                    Self::SharedSync(_) => "SharedSync(..)",
                    Self::ExclusiveSync(_) => "ExclusiveSync(..)",
                    Self::SharedAsync(_) => "SharedAsync(..)",
                    Self::ExclusiveAsync(_) => "ExclusiveAsync(..)",
                })
            }
        }
    )* };
}

impl_handler_debug!(StateFn, StatusFn, ExecFn, ReadFn, WriteFn);

/// A builder for defining a device model and interface.
pub struct ClassDef<M: Model> {
    /// The name of the device class.
    pub(super) name: Cow<'static, str>,
    /// The description every device of that class gives of itself, which is what the
    /// description request of a device answers.
    pub(super) doc: Cow<'static, str>,
    /// Where the documentation of that class is, which is what a client browsing the
    /// control system opens when it is asked to show what a device is. Absent for a class
    /// that documents itself nowhere, which is what a client is then told.
    pub(super) doc_url: Option<Cow<'static, str>>,
    /// The function building the model when the device is initialized.
    pub(super) init: InitFn<M>,
    /// The function called when the device is deleted, given the model back.
    pub(super) delete: Option<DeleteFn<M>>,
    /// The function giving the state of the device, absent to always give [`DevState::On`].
    pub(super) state: Option<StateFn<M>>,
    /// The function giving the status of the device, absent to always give "Device is on".
    pub(super) status: Option<StatusFn<M>>,
    /// The commands every device of that class has, which is the usual way of defining
    /// one: a device is given all of them when it is initialized, and only adds to that
    /// what its own properties decide, see [`Dev::add_command`].
    ///
    /// Behind an [`Arc`] because a definition is fixed once the class is built and shared
    /// by every device of it, only the state the server keeps around it being per-device.
    pub(super) commands: Vec<Arc<CommandDef<M>>>,
    /// The attributes every device of that class has, see [`Self::commands`].
    pub(super) attrs: Vec<Arc<AttrDef<M>>>,
    /// The definition of the State attribute every device has, which is read through the
    /// state function but configured like any other, see [`Self::state_config`].
    pub(super) state_attr: Arc<AttrDef<M>>,
    /// The definition of the Status attribute every device has, see [`Self::state_attr`].
    pub(super) status_attr: Arc<AttrDef<M>>,
}

/// The function building the model of a device, either synchronous or asynchronous, see
/// [`ClassDef::new`] and [`ClassDef::new_async`].
pub enum InitFn<M: Model> {
    Sync(Box<dyn for<'ctx> Fn(&'ctx Dev<M>) -> Result<M> + Send + Sync>),
    Async(Box<dyn for<'a> Fn(&'a Dev<M>) -> BoxFuture<'a, Result<M>> + Send + Sync>),
}
/// The function called when the model of a device is deleted, see [`ClassDef::delete`],
/// either synchronous or asynchronous.
pub enum DeleteFn<M: Model> {
    Sync(Box<dyn for<'ctx> Fn(M, &'ctx Dev<M>) + Send + Sync>),
    Async(Box<dyn for<'a> Fn(M, &'a Dev<M>) -> BoxFuture<'a, ()> + Send + Sync>),
}

/// The state function of a device, with a shared or an exclusive access to the model,
/// either synchronous or asynchronous.
///
/// An asynchronous function borrows the model, the device and, for those that take one,
/// the argument for as long as its future runs, hence the single lifetime they share.
pub enum StateFn<M: Model> {
    SharedSync(Box<dyn for<'m, 'ctx> Fn(&'m M, &'ctx Dev<M>) -> Result<DevState> + Send + Sync>),
    ExclusiveSync(Box<dyn for<'m, 'ctx> Fn(&'m mut M, &'ctx Dev<M>) -> Result<DevState> + Send + Sync>),
    SharedAsync(Box<dyn for<'a> Fn(&'a M, &'a Dev<M>) -> BoxFuture<'a, Result<DevState>> + Send + Sync>),
    ExclusiveAsync(Box<dyn for<'a> Fn(&'a mut M, &'a Dev<M>) -> BoxFuture<'a, Result<DevState>> + Send + Sync>),
}

/// The status function of a device, see [`StateFn`].
pub enum StatusFn<M: Model> {
    SharedSync(Box<dyn for<'m, 'ctx> Fn(&'m M, &'ctx Dev<M>) -> Result<Cow<'m, str>> + Send + Sync>),
    ExclusiveSync(Box<dyn for<'m, 'ctx> Fn(&'m mut M, &'ctx Dev<M>) -> Result<Cow<'m, str>> + Send + Sync>),
    SharedAsync(Box<dyn for<'a> Fn(&'a M, &'a Dev<M>) -> BoxFuture<'a, Result<Cow<'a, str>>> + Send + Sync>),
    ExclusiveAsync(Box<dyn for<'a> Fn(&'a mut M, &'a Dev<M>) -> BoxFuture<'a, Result<Cow<'a, str>>> + Send + Sync>),
}

/// An asynchronous function of the model and of the device alone, whose result is given
/// as-is, which is what the state and the status functions are. See [`ClassDef::state_async`].
///
/// The model is taken as `R`, being either `&'a M` or `&'a mut M` depending on the access
/// the function asks for, and the future may borrow from both the model and the device,
/// which is what an `async fn` method of the model does.
pub trait HandlerAsync<'a, R, M: Model, O> {
    /// Call the function, giving the future of its result.
    fn call(&self, model: R, dev: &'a Dev<M>) -> BoxFuture<'a, Result<O>>;
}

impl<'a, R, M: Model, O, F, Fut> HandlerAsync<'a, R, M, O> for F
where
    F: Fn(R, &'a Dev<M>) -> Fut,
    Fut: Future<Output = Result<O>> + Send + 'a,
{
    #[inline]
    fn call(&self, model: R, dev: &'a Dev<M>) -> BoxFuture<'a, Result<O>> {
        Box::pin(self(model, dev))
    }
}

/// An asynchronous function building the model of a device, see [`ClassDef::new_async`].
///
/// The future may borrow the device, which is what an `async fn` taking it does.
pub trait InitAsync<'a, M: Model> {
    /// Call the function, giving the future of the model.
    fn call(&self, dev: &'a Dev<M>) -> BoxFuture<'a, Result<M>>;
}

impl<'a, M: Model, F, Fut> InitAsync<'a, M> for F
where
    F: Fn(&'a Dev<M>) -> Fut,
    Fut: Future<Output = Result<M>> + Send + 'a,
{
    #[inline]
    fn call(&self, dev: &'a Dev<M>) -> BoxFuture<'a, Result<M>> {
        Box::pin(self(dev))
    }
}

/// An asynchronous function deleting the model of a device, see [`ClassDef::delete_async`].
///
/// The future owns the model and may borrow the device, which is what an `async fn` taking
/// them does.
pub trait DeleteAsync<'a, M: Model> {
    /// Call the function, giving the future of the deletion.
    fn call(&self, model: M, dev: &'a Dev<M>) -> BoxFuture<'a, ()>;
}

impl<'a, M: Model, F, Fut> DeleteAsync<'a, M> for F
where
    F: Fn(M, &'a Dev<M>) -> Fut,
    Fut: Future<Output = ()> + Send + 'a,
{
    #[inline]
    fn call(&self, model: M, dev: &'a Dev<M>) -> BoxFuture<'a, ()> {
        Box::pin(self(model, dev))
    }
}

impl<M: Model> ClassDef<M> {

    /// Create a new device definition, the given function building the model of each
    /// device of that class when it is initialized.
    pub fn new<N, I>(name: N, init: I) -> Self
    where
        N: Into<Cow<'static, str>>,
        I: for<'ctx> Fn(&'ctx Dev<M>) -> Result<M> + Send + Sync + 'static,
    {
        Self::with_init(name.into(), InitFn::Sync(Box::new(init)))
    }

    /// Create a new device definition, the given function building the model of each
    /// device of that class asynchronously when it is initialized. This is the raw form of
    /// [`Self::new_async`], for a closure that boxes its future itself.
    pub fn new_async_with<N, I>(name: N, init: I) -> Self
    where
        N: Into<Cow<'static, str>>,
        I: for<'a> Fn(&'a Dev<M>) -> BoxFuture<'a, Result<M>> + Send + Sync + 'static,
    {
        Self::with_init(name.into(), InitFn::Async(Box::new(init)))
    }

    /// Create a new device definition, the given function building the model of each
    /// device of that class asynchronously when it is initialized, typically an `async fn`
    /// taking the device.
    ///
    /// The model of the device is locked exclusively until the future completes, as it is
    /// for a synchronous one, see [`Dev::init`]. The future is polled by the server itself
    /// when nothing else of the initialization may block, so it must never block, see
    /// [`Self::state_async`].
    pub fn new_async<N, I>(name: N, init: I) -> Self
    where
        N: Into<Cow<'static, str>>,
        I: for<'a> InitAsync<'a, M> + Send + Sync + 'static,
    {
        Self::new_async_with(name, move |dev| init.call(dev))
    }

    fn with_init(name: Cow<'static, str>, init: InitFn<M>) -> Self {
        Self {
            name,
            doc: Cow::Borrowed("A Tango device"),
            doc_url: None,
            init,
            delete: None,
            state: None,
            status: None,
            commands: Vec::new(),
            attrs: Vec::new(),
            state_attr: state_attr(AttrConfig {
                doc: Some(Cow::Borrowed("The state of the device")),
                label: Some(Cow::Borrowed(STATE_NAME)),
                ..AttrConfig::default()
            }),
            status_attr: status_attr(AttrConfig {
                doc: Some(Cow::Borrowed("The status of the device")),
                label: Some(Cow::Borrowed(STATUS_NAME)),
                ..AttrConfig::default()
            }),
        }
    }

    /// Create a new device definition whose model is built with its default value.
    pub fn new_default<N>(name: N) -> Self
    where
        M: Default,
        N: Into<Cow<'static, str>>,
    {
        Self::new(name, |_ctx| Ok(M::default()))
    }

    /// Set the description the devices of that class give of themselves, which is what a
    /// client asking a device what it is gets back, and what a class defines once for all
    /// of its devices.
    pub fn doc<D>(mut self, doc: D) -> Self
    where
        D: Into<Cow<'static, str>>,
    {
        self.doc = doc.into();
        self
    }

    /// Set where the documentation of that class is, which is what a client shows as the
    /// documentation of every device of it: a page describing what the class does, not
    /// what one device of it is doing, which is its status.
    pub fn doc_url<U>(mut self, url: U) -> Self
    where
        U: Into<Cow<'static, str>>,
    {
        self.doc_url = Some(url.into());
        self
    }

    /// Define the function called when the model of a device of that class is deleted,
    /// which is given the model back so that it may release its resources: before every
    /// initialization but the first, which is what the Init command of a device is, and
    /// once the device is no longer served.
    pub fn delete<F>(mut self, delete: F) -> Self
    where
        F: for<'ctx> Fn(M, &'ctx Dev<M>) + Send + Sync + 'static,
    {
        self.delete = Some(DeleteFn::Sync(Box::new(delete)));
        self
    }

    /// Define the function called when the model of a device of that class is deleted,
    /// asynchronously. This is the raw form of [`Self::delete_async`], for a closure that
    /// boxes its future itself.
    pub fn delete_async_with<F>(mut self, delete: F) -> Self
    where
        F: for<'a> Fn(M, &'a Dev<M>) -> BoxFuture<'a, ()> + Send + Sync + 'static,
    {
        self.delete = Some(DeleteFn::Async(Box::new(delete)));
        self
    }

    /// Define the function called when the model of a device of that class is deleted,
    /// asynchronously, typically an `async fn` taking the model and the device, see
    /// [`Self::delete`].
    ///
    /// The model of the device is locked exclusively until the future completes, as it is
    /// for a synchronous one. The future is polled by the server itself when nothing else
    /// of the deletion may block, so it must never block, see [`Self::state_async`].
    pub fn delete_async<F>(self, delete: F) -> Self
    where
        F: for<'a> DeleteAsync<'a, M> + Send + Sync + 'static,
    {
        self.delete_async_with(move |model, dev| delete.call(model, dev))
    }

    /// Define the function giving the state of the device, with a shared access to the
    /// model, which allows parallel execution of such functions.
    pub fn state<F>(mut self, state: F) -> Self
    where
        F: for<'m, 'ctx> Fn(&'m M, &'ctx Dev<M>) -> Result<DevState> + Send + Sync + 'static,
    {
        self.state = Some(StateFn::SharedSync(Box::new(state)));
        self
    }

    /// Define the function giving the state of the device, with an exclusive access to
    /// the model, therefore no other command or attribute can access it in parallel.
    pub fn state_exclusive<F>(mut self, state: F) -> Self
    where
        F: for<'m, 'ctx> Fn(&'m mut M, &'ctx Dev<M>) -> Result<DevState> + Send + Sync + 'static,
    {
        self.state = Some(StateFn::ExclusiveSync(Box::new(state)));
        self
    }

    /// Define the function giving the state of the device, asynchronously and with a
    /// shared access to the model. This is the raw form of [`Self::state_async`], for a
    /// closure that boxes its future itself.
    pub fn state_async_with<F>(mut self, state: F) -> Self
    where
        F: for<'a> Fn(&'a M, &'a Dev<M>) -> BoxFuture<'a, Result<DevState>> + Send + Sync + 'static,
    {
        self.state = Some(StateFn::SharedAsync(Box::new(state)));
        self
    }

    /// Define the function giving the state of the device, asynchronously and with a
    /// shared access to the model, typically an `async fn` method of the model.
    ///
    /// The model stays borrowed until the future completes, and the future is polled by
    /// the server itself, so it must never block: a function that has to wait on anything
    /// else than another future offloads that wait to a thread of its own.
    pub fn state_async<F>(self, state: F) -> Self
    where
        F: for<'a> HandlerAsync<'a, &'a M, M, DevState> + Send + Sync + 'static,
    {
        self.state_async_with(move |model, dev| state.call(model, dev))
    }

    /// Define the function giving the state of the device, asynchronously and with an
    /// exclusive access to the model. This is the raw form of [`Self::state_exclusive_async`],
    /// see [`Self::state_async_with`].
    pub fn state_exclusive_async_with<F>(mut self, state: F) -> Self
    where
        F: for<'a> Fn(&'a mut M, &'a Dev<M>) -> BoxFuture<'a, Result<DevState>> + Send + Sync + 'static,
    {
        self.state = Some(StateFn::ExclusiveAsync(Box::new(state)));
        self
    }

    /// Define the function giving the state of the device, asynchronously and with an
    /// exclusive access to the model, which no other command or attribute can then access
    /// until the future completes. See [`Self::state_async`].
    pub fn state_exclusive_async<F>(self, state: F) -> Self
    where
        F: for<'a> HandlerAsync<'a, &'a mut M, M, DevState> + Send + Sync + 'static,
    {
        self.state_exclusive_async_with(move |model, dev| state.call(model, dev))
    }

    /// Define the function giving the status of the device, with a shared access to the
    /// model, the status being allowed to borrow from it.
    pub fn status<F>(mut self, status: F) -> Self
    where
        F: for<'m, 'ctx> Fn(&'m M, &'ctx Dev<M>) -> Result<Cow<'m, str>> + Send + Sync + 'static,
    {
        self.status = Some(StatusFn::SharedSync(Box::new(status)));
        self
    }

    /// Define the function giving the status of the device, with an exclusive access to
    /// the model, the status being allowed to borrow from it.
    pub fn status_exclusive<F>(mut self, status: F) -> Self
    where
        F: for<'m, 'ctx> Fn(&'m mut M, &'ctx Dev<M>) -> Result<Cow<'m, str>> + Send + Sync + 'static,
    {
        self.status = Some(StatusFn::ExclusiveSync(Box::new(status)));
        self
    }

    /// Define the function giving the status of the device, asynchronously and with a
    /// shared access to the model. This is the raw form of [`Self::status_async`], see
    /// [`Self::state_async_with`].
    pub fn status_async_with<F>(mut self, status: F) -> Self
    where
        F: for<'a> Fn(&'a M, &'a Dev<M>) -> BoxFuture<'a, Result<Cow<'a, str>>> + Send + Sync + 'static,
    {
        self.status = Some(StatusFn::SharedAsync(Box::new(status)));
        self
    }

    /// Define the function giving the status of the device, asynchronously and with a
    /// shared access to the model, the status being allowed to borrow from it. See
    /// [`Self::state_async`].
    pub fn status_async<F>(self, status: F) -> Self
    where
        F: for<'a> HandlerAsync<'a, &'a M, M, Cow<'a, str>> + Send + Sync + 'static,
    {
        self.status_async_with(move |model, dev| status.call(model, dev))
    }

    /// Define the function giving the status of the device, asynchronously and with an
    /// exclusive access to the model. This is the raw form of
    /// [`Self::status_exclusive_async`], see [`Self::state_async_with`].
    pub fn status_exclusive_async_with<F>(mut self, status: F) -> Self
    where
        F: for<'a> Fn(&'a mut M, &'a Dev<M>) -> BoxFuture<'a, Result<Cow<'a, str>>> + Send + Sync + 'static,
    {
        self.status = Some(StatusFn::ExclusiveAsync(Box::new(status)));
        self
    }

    /// Define the function giving the status of the device, asynchronously and with an
    /// exclusive access to the model, see [`Self::state_exclusive_async`].
    pub fn status_exclusive_async<F>(self, status: F) -> Self
    where
        F: for<'a> HandlerAsync<'a, &'a mut M, M, Cow<'a, str>> + Send + Sync + 'static,
    {
        self.status_exclusive_async_with(move |model, dev| status.call(model, dev))
    }

    /// Define a command every device of that class has, which is what defining a command
    /// usually means: the devices of a class have the same interface, and a device only
    /// adds to its own the part of it that its properties decide, see [`Dev::add_command`].
    ///
    /// # Panics
    ///
    /// If the class already defines a command of that name, or if the name is one of the
    /// commands the server answers itself, being `State`, `Status` and `Init`.
    #[track_caller]
    pub fn command(mut self, def: CommandDef<M>) -> Self {
        check_own_name(&self.name, "command", &def.name, RESERVED_COMMANDS);
        if self.commands.iter().any(|other| other.name.eq_ignore_ascii_case(&def.name)) {
            panic!("{}: the command {} is already defined", self.name, def.name);
        }
        self.commands.push(Arc::new(def));
        self
    }

    /// Define an attribute every device of that class has, see [`Self::command`].
    ///
    /// # Panics
    ///
    /// If the class already defines an attribute of that name, or if the name is one of
    /// the attributes the server answers itself, being `State` and `Status`.
    #[track_caller]
    pub fn attr(mut self, def: AttrDef<M>) -> Self {
        check_own_name(&self.name, "attribute", &def.name, RESERVED_ATTRS);
        if let Err(err) = def.check() {
            panic!("{}: {err}", self.name);
        }
        if self.attrs.iter().any(|other| other.name.eq_ignore_ascii_case(&def.name)) {
            panic!("{}: the attribute {} is already defined", self.name, def.name);
        }
        self.attrs.push(Arc::new(def));
        self
    }

    /// Set the configuration the State attribute of every device of that class starts
    /// from, which the database and clients then override like the one of any attribute.
    /// The default one only gives it a label and a description.
    ///
    /// # Panics
    ///
    /// If the configuration gives a value to a property a state does not support, such as
    /// a bound or an event criterion, see [`AttrConfig`].
    #[track_caller]
    pub fn state_config(mut self, config: AttrConfig) -> Self {
        self.state_attr = state_attr(config);
        self
    }

    /// Set the configuration the Status attribute of every device of that class starts
    /// from, see [`Self::state_config`].
    ///
    /// # Panics
    ///
    /// If the configuration gives a value to a property a string does not support, see
    /// [`Self::state_config`].
    #[track_caller]
    pub fn status_config(mut self, config: AttrConfig) -> Self {
        self.status_attr = status_attr(config);
        self
    }

    /// Declare that the devices of that class push the events of their State themselves,
    /// change and archive, see [`AttrDef::push_change_event`].
    pub fn push_state_events(mut self, change: Option<bool>, archive: Option<bool>) -> Self {
        builtin_pushed(&mut self.state_attr, change, archive);
        self
    }

    /// Declare that the devices of that class push the events of their Status themselves,
    /// see [`Self::push_state_events`].
    pub fn push_status_events(mut self, change: Option<bool>, archive: Option<bool>) -> Self {
        builtin_pushed(&mut self.status_attr, change, archive);
        self
    }

}

/// The definition of the State attribute, with that configuration.
#[track_caller]
fn state_attr<M: Model>(config: AttrConfig) -> Arc<AttrDef<M>> {
    builtin_attr(STATE_NAME, AttrDataKind::State, config)
}

/// The definition of the Status attribute, with that configuration.
#[track_caller]
fn status_attr<M: Model>(config: AttrConfig) -> Arc<AttrDef<M>> {
    builtin_attr(STATUS_NAME, AttrDataKind::String, config)
}

/// Declare the events pushed for one of the attributes the server answers itself, whose
/// definition only its class holds while it is built.
fn builtin_pushed<M: Model>(def: &mut Arc<AttrDef<M>>, change: Option<bool>, archive: Option<bool>) {
    let def = Arc::get_mut(def).expect("a class being built is the only owner of its definitions");
    def.pushed = PushedEvents { change, archive };
}

/// The definition of one of the attributes the server answers itself, a read-only scalar
/// of that kind with that configuration.
#[track_caller]
fn builtin_attr<M: Model>(name: &'static str, kind: AttrDataKind, config: AttrConfig) -> Arc<AttrDef<M>> {
    let mut def = AttrDef::new(name).kind(kind).scalar();
    def.display_level = DisplayLevel::Operator;
    def.config = config;
    if let Err(err) = def.check() {
        panic!("{err}");
    }
    Arc::new(def)
}

/// Refuse a name that belongs to the server rather than to the class: the state, the
/// status and the initialization of a device are answered by the server itself, whatever
/// its class, so a definition of that name would never be reached and is a mistake worth
/// pointing out where it is written rather than leaving to be found at runtime.
#[track_caller]
fn check_own_name(class: &str, what: &str, name: &str, reserved: &[&str]) {
    if let Some(reserved) = reserved_name(name, reserved) {
        panic!("{class}: the {what} {name} cannot be defined by a class, \
            {reserved} is answered by the server itself");
    }
}

impl<M: Model> Debug for ClassDef<M> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("ClassDef")
            .field("name", &self.name)
            .field("doc", &self.doc)
            .field("doc_url", &self.doc_url)
            .field("init", &HandlerDebug)
            .field("delete", &self.delete.as_ref().map(|_| HandlerDebug))
            .field("state", &self.state)
            .field("status", &self.status)
            .field("commands", &self.commands)
            .field("attrs", &self.attrs)
            .field("state_attr", &self.state_attr)
            .field("status_attr", &self.status_attr)
            .finish()
    }
}

/// A builder for defining a command of a device.
pub struct CommandDef<M: Model> {
    /// The name of the command.
    pub(super) name: Cow<'static, str>,
    /// The function executing the command.
    pub(super) exec: Option<ExecFn<M>>,
    /// The advertised kind of the argument, derived from the execution function, absent
    /// until it is defined and then defaulting to [`CommandDataKind::Void`].
    pub(super) in_kind: Option<CommandDataKind>,
    /// The documentation of the argument.
    pub(super) in_doc: Cow<'static, str>,
    /// The advertised kind of the result, derived from the execution function, absent
    /// until it is defined and then defaulting to [`CommandDataKind::Void`].
    pub(super) out_kind: Option<CommandDataKind>,
    /// The documentation of the result.
    pub(super) out_doc: Cow<'static, str>,
    /// The display level of the command, which is all a command has of a configuration:
    /// unlike an attribute, nothing about it can be changed once the class is defined.
    pub(super) display_level: DisplayLevel,
    /// The polling period this command asks for, absent for one that is not polled unless
    /// the database says otherwise.
    pub(super) poll_period: Option<Duration>,
}

/// The execution function of a command, with a shared or an exclusive access to the model,
/// either synchronous or asynchronous, see [`StateFn`].
pub enum ExecFn<M: Model> {
    SharedSync(Box<dyn Fn(&M, &Dev<M>, Value) -> Result<Value> + Send + Sync>),
    ExclusiveSync(Box<dyn Fn(&mut M, &Dev<M>, Value) -> Result<Value> + Send + Sync>),
    SharedAsync(Box<dyn for<'a> Fn(&'a M, &'a Dev<M>, Value) -> BoxFuture<'a, Result<Value>> + Send + Sync>),
    ExclusiveAsync(Box<dyn for<'a> Fn(&'a mut M, &'a Dev<M>, Value) -> BoxFuture<'a, Result<Value>> + Send + Sync>),
}

/// An execution function of a command with a shared access to the model, whose result may
/// borrow from it until it is converted into a value. See [`CommandDef::exec`].
pub trait ExecShared<'m, M: Model, I> {
    /// The advertised kind of the argument, derived from its type.
    const IN_KIND: CommandDataKind;
    /// The advertised kind of the result, derived from its type.
    const OUT_KIND: CommandDataKind;
    /// Convert the given argument, execute the command and convert its result.
    fn exec(&self, model: &'m M, dev: &Dev<M>, input: Value) -> Result<Value>;
}

impl<'m, M: Model, I, O, F> ExecShared<'m, M, I> for F
where
    I: FromValue + CommandData,
    O: IntoValue + CommandData,
    F: Fn(&'m M, &Dev<M>, I) -> Result<O>,
{

    const IN_KIND: CommandDataKind = I::DATA_KIND;
    const OUT_KIND: CommandDataKind = O::DATA_KIND;

    #[inline]
    fn exec(&self, model: &'m M, dev: &Dev<M>, input: Value) -> Result<Value> {
        Ok(self(model, dev, I::from_value(input)?)?.into_value())
    }
    
}

/// The exclusive counterpart of [`ExecShared`], see [`CommandDef::exec_exclusive`].
pub trait ExecExclusive<'m, M: Model, I> {
    /// The advertised kind of the argument, derived from its type.
    const IN_KIND: CommandDataKind;
    /// The advertised kind of the result, derived from its type.
    const OUT_KIND: CommandDataKind;
    /// Convert the given argument, execute the command and convert its result.
    fn exec(&self, model: &'m mut M, dev: &Dev<M>, input: Value) -> Result<Value>;
}

impl<'m, M: Model, I, O, F> ExecExclusive<'m, M, I> for F
where
    I: FromValue + CommandData,
    O: IntoValue + CommandData,
    F: Fn(&'m mut M, &Dev<M>, I) -> Result<O>,
{

    const IN_KIND: CommandDataKind = I::DATA_KIND;
    const OUT_KIND: CommandDataKind = O::DATA_KIND;

    #[inline]
    fn exec(&self, model: &'m mut M, dev: &Dev<M>, input: Value) -> Result<Value> {
        Ok(self(model, dev, I::from_value(input)?)?.into_value())
    }

}

/// An asynchronous execution function of a command, see [`CommandDef::exec_async`].
///
/// The model is taken as `R`, being either `&'a M` or `&'a mut M`, see [`HandlerAsync`],
/// and the future may borrow from the model and the device, its result being allowed to
/// borrow from both until it is converted into a value.
pub trait ExecAsync<'a, R, M: Model, I> {
    /// The advertised kind of the argument, derived from its type.
    const IN_KIND: CommandDataKind;
    /// The advertised kind of the result, derived from its type.
    const OUT_KIND: CommandDataKind;
    /// Convert the given argument and give the future executing the command and
    /// converting its result.
    fn exec(&self, model: R, dev: &'a Dev<M>, input: Value) -> BoxFuture<'a, Result<Value>>;
}

impl<'a, R, M: Model, I, O, F, Fut> ExecAsync<'a, R, M, I> for F
where
    I: FromValue + CommandData,
    O: IntoValue + CommandData,
    F: Fn(R, &'a Dev<M>, I) -> Fut,
    Fut: Future<Output = Result<O>> + Send + 'a,
{

    const IN_KIND: CommandDataKind = I::DATA_KIND;
    const OUT_KIND: CommandDataKind = O::DATA_KIND;

    #[inline]
    fn exec(&self, model: R, dev: &'a Dev<M>, input: Value) -> BoxFuture<'a, Result<Value>> {
        // The function is called right away rather than on the first poll, so that
        // whatever it starts before its first await is started with the request.
        let fut = I::from_value(input).map(|input| self(model, dev, input));
        Box::pin(async move { Ok(fut?.await?.into_value()) })
    }

}

impl<M: Model> CommandDef<M> {

    /// Create a new command definition, without any execution function.
    pub fn new<N>(name: N) -> Self
    where
        N: Into<Cow<'static, str>>,
    {
        Self {
            name: name.into(),
            exec: None,
            in_kind: None,
            in_doc: Cow::Borrowed(""),
            out_kind: None,
            out_doc: Cow::Borrowed(""),
            display_level: DisplayLevel::default(),
            poll_period: None,
        }
    }

    /// Set the documentation of the command argument (argin).
    ///
    /// A command has no description of its own, Tango only telling a client what each
    /// side of it is, so what the command does has to be said in one of these two.
    pub fn in_doc<D>(mut self, doc: D) -> Self
    where
        D: Into<Cow<'static, str>>,
    {
        self.in_doc = doc.into();
        self
    }

    /// Set the documentation of the command result (argout).
    pub fn out_doc<D>(mut self, doc: D) -> Self
    where
        D: Into<Cow<'static, str>>,
    {
        self.out_doc = doc.into();
        self
    }

    /// Set the display level of the command, [`DisplayLevel::Expert`] keeping it out of
    /// the panels an operator is shown without keeping it from anyone who asks for it.
    pub fn display_level(mut self, level: DisplayLevel) -> Self {
        self.display_level = level;
        self
    }

    /// Ask for the command to be polled at that period, its result being kept in the
    /// polling buffer of the device the way a polled attribute value is, which is what
    /// `State` and `Status` are usually given.
    ///
    /// Only a command that takes no argument can be polled, there being no argument for
    /// the server to invent, and the database has the last word on the period.
    pub fn polled(mut self, period: Duration) -> Self {
        self.poll_period = Some(period);
        self
    }

    /// Set the advertised kind of the command argument (argin). This is only needed for
    /// the execution functions that take a raw [`Value`] argument, the typed ones derive
    /// it from their input type.
    ///
    /// # Panics
    ///
    /// If the argument kind has already been defined, explicitly or by an execution
    /// function, as a different kind.
    #[track_caller]
    pub fn in_kind(mut self, kind: CommandDataKind) -> Self {
        self.in_kind = Some(merge_kind(&self.name, "argument kind", self.in_kind.take(), kind));
        self
    }

    /// Set the advertised kind of the command result (argout). This is only needed for
    /// the execution functions that return a raw [`Value`], the typed ones derive it from
    /// their output type.
    ///
    /// # Panics
    ///
    /// If the result kind has already been defined, explicitly or by an execution
    /// function, as a different kind.
    #[track_caller]
    pub fn out_kind(mut self, kind: CommandDataKind) -> Self {
        self.out_kind = Some(merge_kind(&self.name, "result kind", self.out_kind.take(), kind));
        self
    }

    /// Define the execution function of this command, with a shared access to the model.
    ///
    /// This is the raw form of [`Self::exec`]: the closure is given the argument
    /// [`Value`] as-is and returns one, converting what it borrows from the model itself,
    /// which is what makes a closure usable for such a result. Because no input and output
    /// types are involved, the advertised kinds have to be given with [`Self::in_kind`]
    /// and [`Self::out_kind`].
    pub fn exec_with<F>(mut self, func: F) -> Self
    where
        F: Fn(&M, &Dev<M>, Value) -> Result<Value> + Send + Sync + 'static,
    {
        self.exec = Some(ExecFn::SharedSync(Box::new(func)));
        self
    }

    /// Define the execution function of this command, with a shared access to the model,
    /// which allows parallel execution of such functions.
    ///
    /// The argument and result types are taken from the `ExecShared` implementation of
    /// the function. The result may borrow from the model, being converted into a value
    /// before the model is released, which requires a function item generic over the
    /// model lifetime, typically an associated method of the model.
    ///
    /// # Panics
    ///
    /// If either advertised kind has already been defined as a different kind, see
    /// [`Self::in_kind`] and [`Self::out_kind`].
    #[track_caller]
    pub fn exec<F, I>(self, func: F) -> Self
    where
        F: for<'m> ExecShared<'m, M, I> + Send + Sync + 'static,
    {
        self.in_kind(F::IN_KIND)
            .out_kind(F::OUT_KIND)
            .exec_with(move |model, dev, value| func.exec(model, dev, value))
    }

    /// Define the execution function of this command, with an exclusive access to the
    /// model. This is the raw form of [`Self::exec_exclusive`], see [`Self::exec_with`].
    pub fn exec_exclusive_with<F>(mut self, func: F) -> Self
    where
        F: Fn(&mut M, &Dev<M>, Value) -> Result<Value> + Send + Sync + 'static,
    {
        self.exec = Some(ExecFn::ExclusiveSync(Box::new(func)));
        self
    }

    /// Define the execution function of this command, with an exclusive access to the
    /// model, therefore no other command or attribute can access it in parallel.
    ///
    /// See [`Self::exec`] for the constraints on the argument and result types, this one
    /// is only different in that its function item takes the model by exclusive reference.
    ///
    /// # Panics
    ///
    /// If either advertised kind has already been defined as a different kind, see
    /// [`Self::in_kind`] and [`Self::out_kind`].
    #[track_caller]
    pub fn exec_exclusive<F, I>(self, func: F) -> Self
    where
        F: for<'m> ExecExclusive<'m, M, I> + Send + Sync + 'static,
    {
        self.in_kind(F::IN_KIND)
            .out_kind(F::OUT_KIND)
            .exec_exclusive_with(move |model, dev, value| func.exec(model, dev, value))
    }

    /// Define the execution function of this command, asynchronously and with a shared
    /// access to the model. This is the raw form of [`Self::exec_async`], for a closure
    /// that boxes its future itself, see [`Self::exec_with`].
    pub fn exec_async_with<F>(mut self, func: F) -> Self
    where
        F: for<'a> Fn(&'a M, &'a Dev<M>, Value) -> BoxFuture<'a, Result<Value>> + Send + Sync + 'static,
    {
        self.exec = Some(ExecFn::SharedAsync(Box::new(func)));
        self
    }

    /// Define the execution function of this command, asynchronously and with a shared
    /// access to the model, typically an `async fn` method of the model.
    ///
    /// This is what a command forwarding its execution to other devices is written as:
    /// it holds no thread while it waits on them, and may wait on many of them at once.
    /// The future is polled by the server itself, so it must never block, see
    /// [`ClassDef::state_async`].
    ///
    /// # Panics
    ///
    /// If either advertised kind has already been defined as a different kind, see
    /// [`Self::in_kind`] and [`Self::out_kind`].
    #[track_caller]
    pub fn exec_async<F, I>(self, func: F) -> Self
    where
        F: for<'a> ExecAsync<'a, &'a M, M, I> + Send + Sync + 'static,
    {
        self.in_kind(F::IN_KIND)
            .out_kind(F::OUT_KIND)
            .exec_async_with(move |model, dev, value| func.exec(model, dev, value))
    }

    /// Define the execution function of this command, asynchronously and with an
    /// exclusive access to the model. This is the raw form of [`Self::exec_exclusive_async`],
    /// see [`Self::exec_async_with`].
    pub fn exec_exclusive_async_with<F>(mut self, func: F) -> Self
    where
        F: for<'a> Fn(&'a mut M, &'a Dev<M>, Value) -> BoxFuture<'a, Result<Value>> + Send + Sync + 'static,
    {
        self.exec = Some(ExecFn::ExclusiveAsync(Box::new(func)));
        self
    }

    /// Define the execution function of this command, asynchronously and with an
    /// exclusive access to the model, which no other command or attribute can then access
    /// until the future completes. See [`Self::exec_async`].
    ///
    /// # Panics
    ///
    /// If either advertised kind has already been defined as a different kind, see
    /// [`Self::in_kind`] and [`Self::out_kind`].
    #[track_caller]
    pub fn exec_exclusive_async<F, I>(self, func: F) -> Self
    where
        F: for<'a> ExecAsync<'a, &'a mut M, M, I> + Send + Sync + 'static,
    {
        self.in_kind(F::IN_KIND)
            .out_kind(F::OUT_KIND)
            .exec_exclusive_async_with(move |model, dev, value| func.exec(model, dev, value))
    }

}

impl<M: Model> Debug for CommandDef<M> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("CommandDef")
            .field("name", &self.name)
            .field("exec", &self.exec)
            .field("in_kind", &self.in_kind)
            .field("in_doc", &self.in_doc)
            .field("out_kind", &self.out_kind)
            .field("out_doc", &self.out_doc)
            .field("display_level", &self.display_level)
            .field("poll_period", &self.poll_period)
            .finish()
    }
}

/// A builder for defining an attribute of a device.
pub struct AttrDef<M: Model> {
    /// The name of the attribute.
    pub(super) name: Cow<'static, str>,
    /// The function reading the attribute value, absent for a write-only attribute.
    pub(super) read: Option<ReadFn<M>>,
    /// The function writing the attribute value, absent for a read-only attribute.
    pub(super) write: Option<WriteFn<M>>,
    /// The advertised data kind, derived from the read and write functions.
    pub(super) kind: Option<AttrDataKind>,
    /// The advertised data format, derived from the read and write functions.
    pub(super) format: Option<AttrDataFormat>,
    /// The display level the attribute is shown at, which the class declares and a client
    /// cannot change, unlike the configuration below.
    pub(super) display_level: DisplayLevel,
    /// What the attribute does with the value of a write when the device is initialized
    /// again, absent for one that remembers nothing, which the class declares as well.
    pub(super) memorized: Option<AttrMemorized>,
    /// Everything a client may read about the attribute and, for most of it, change: its
    /// documentation, its units, its bounds and its event criteria. Every device of the
    /// class starts from this one and owns its copy from then on, because the database
    /// overrides it per device and a client changes it per device.
    pub(super) config: AttrConfig,
    /// The polling period this attribute asks for, absent for one that is not polled
    /// unless the database says otherwise.
    pub(super) poll_period: Option<Duration>,
    /// The events the device pushes itself for this attribute, see
    /// [`Self::push_change_event`].
    pub(super) pushed: PushedEvents,
}

/// The events a device declares it pushes itself for an attribute, each with whether the
/// criteria of its configuration are checked against what is pushed, see
/// [`AttrDef::push_change_event`].
#[derive(Debug, Clone, Copy, Default)]
pub(super) struct PushedEvents {
    pub change: Option<bool>,
    pub archive: Option<bool>,
}

/// The read function of an attribute, with a shared or an exclusive access to the model,
/// either synchronous or asynchronous, see [`StateFn`].
pub enum ReadFn<M: Model> {
    SharedSync(Box<dyn Fn(&M, &Dev<M>) -> Result<AttrRead<Value>> + Send + Sync>),
    ExclusiveSync(Box<dyn Fn(&mut M, &Dev<M>) -> Result<AttrRead<Value>> + Send + Sync>),
    SharedAsync(Box<dyn for<'a> Fn(&'a M, &'a Dev<M>) -> BoxFuture<'a, Result<AttrRead<Value>>> + Send + Sync>),
    ExclusiveAsync(Box<dyn for<'a> Fn(&'a mut M, &'a Dev<M>) -> BoxFuture<'a, Result<AttrRead<Value>>> + Send + Sync>),
}

/// The write function of an attribute, with a shared or an exclusive access to the model,
/// either synchronous or asynchronous, see [`StateFn`].
pub enum WriteFn<M: Model> {
    SharedSync(Box<dyn Fn(&M, &Dev<M>, Value) -> Result<()> + Send + Sync>),
    ExclusiveSync(Box<dyn Fn(&mut M, &Dev<M>, Value) -> Result<()> + Send + Sync>),
    SharedAsync(Box<dyn for<'a> Fn(&'a M, &'a Dev<M>, Value) -> BoxFuture<'a, Result<()>> + Send + Sync>),
    ExclusiveAsync(Box<dyn for<'a> Fn(&'a mut M, &'a Dev<M>, Value) -> BoxFuture<'a, Result<()>> + Send + Sync>),
}

/// A trait for abstracting what output types are accepted by read attribute methods: a
/// value alone, its quality and timestamp then being implicit, or a whole [`AttrRead`].
pub trait ReadOutput {
    /// The advertised kind of the value, derived from its type.
    const DATA_KIND: AttrDataKind;
    /// The advertised format of the value, without any maximum dimension.
    const DATA_FORMAT: AttrDataFormat;
    /// Convert this output into a read result, an absent write value asking the server
    /// to fill it with the latest one it has written.
    fn into_read(self) -> AttrRead<Value>;
}

impl<O> ReadOutput for O
where
    O: IntoValue + AttrData,
{
    
    const DATA_KIND: AttrDataKind = O::DATA_KIND;
    const DATA_FORMAT: AttrDataFormat = O::DATA_FORMAT;

    fn into_read(self) -> AttrRead<Value> {
        AttrRead {
            value: Some(self.into_value()),
            // An absent write value asks the server to fill it with the latest one it
            // has written, which is the usual behaviour of a read function.
            // If this is filled by the read function, the server will overwrite its own 
            // write value so that if a next read does not specify the write value, it 
            // will be returning the same one.
            w_value: None,
            quality: AttrQuality::Valid,
            time: SystemTime::now(),
        }
    }
    
}

impl<O> ReadOutput for AttrRead<O>
where
    O: IntoValue + AttrData,
{

    const DATA_KIND: AttrDataKind = O::DATA_KIND;
    const DATA_FORMAT: AttrDataFormat = O::DATA_FORMAT;

    fn into_read(self) -> AttrRead<Value> {
        AttrRead {
            value: self.value.map(IntoValue::into_value),
            w_value: self.w_value.map(IntoValue::into_value),
            quality: self.quality,
            time: self.time,
        }
    }

}

/// A read function with a shared access to the model, whose result may borrow from it
/// until it is converted into a value. See [`AttrDef::read`], the same constraints as
/// [`ExecShared`] apply to the result type.
pub trait ReadShared<'m, M: Model> {
    /// The advertised kind of the value, derived from its type.
    const DATA_KIND: AttrDataKind;
    /// The advertised format of the value, without any maximum dimension.
    const DATA_FORMAT: AttrDataFormat;
    /// Read the value of the attribute and convert it.
    fn read(&self, model: &'m M, dev: &Dev<M>) -> Result<AttrRead<Value>>;
}

impl<'m, M: Model, O, F> ReadShared<'m, M> for F
where
    F: Fn(&'m M, &Dev<M>) -> Result<O>,
    O: ReadOutput,
{
    const DATA_KIND: AttrDataKind = O::DATA_KIND;
    const DATA_FORMAT: AttrDataFormat = O::DATA_FORMAT;

    #[inline]
    fn read(&self, model: &'m M, dev: &Dev<M>) -> Result<AttrRead<Value>> {
        Ok(self(model, dev)?.into_read())
    }
}

/// The exclusive counterpart of [`ReadShared`], see [`AttrDef::read_exclusive`].
pub trait ReadExclusive<'m, M: Model> {
    /// The advertised kind of the value, derived from its type.
    const DATA_KIND: AttrDataKind;
    /// The advertised format of the value, without any maximum dimension.
    const DATA_FORMAT: AttrDataFormat;
    /// Read the value of the attribute and convert it.
    fn read(&self, model: &'m mut M, dev: &Dev<M>) -> Result<AttrRead<Value>>;
}

impl<'m, M: Model, O, F> ReadExclusive<'m, M> for F
where
    F: Fn(&'m mut M, &Dev<M>) -> Result<O>,
    O: ReadOutput,
{
    const DATA_KIND: AttrDataKind = O::DATA_KIND;
    const DATA_FORMAT: AttrDataFormat = O::DATA_FORMAT;

    #[inline]
    fn read(&self, model: &'m mut M, dev: &Dev<M>) -> Result<AttrRead<Value>> {
        Ok(self(model, dev)?.into_read())
    }
}

/// A write function with a shared access to the model. See [`AttrDef::write`].
pub trait WriteShared<M: Model, I> {
    /// The advertised kind of the value, derived from its type.
    const DATA_KIND: AttrDataKind;
    /// The advertised format of the value, without any maximum dimension.
    const DATA_FORMAT: AttrDataFormat;
    /// Convert the given value and write it to the attribute.
    fn write(&self, model: &M, dev: &Dev<M>, value: Value) -> Result<()>;
}

impl<M: Model, I, F> WriteShared<M, I> for F
where
    F: Fn(&M, &Dev<M>, I) -> Result<()>,
    I: FromValue + AttrData,
{
    const DATA_KIND: AttrDataKind = I::DATA_KIND;
    const DATA_FORMAT: AttrDataFormat = I::DATA_FORMAT;

    #[inline]
    fn write(&self, model: &M, dev: &Dev<M>, value: Value) -> Result<()> {
        self(model, dev, I::from_value(value)?)
    }
}

/// The exclusive counterpart of [`WriteShared`], see [`AttrDef::write_exclusive`].
pub trait WriteExclusive<M: Model, I> {
    /// The advertised kind of the value, derived from its type.
    const DATA_KIND: AttrDataKind;
    /// The advertised format of the value, without any maximum dimension.
    const DATA_FORMAT: AttrDataFormat;
    /// Convert the given value and write it to the attribute.
    fn write(&self, model: &mut M, dev: &Dev<M>, value: Value) -> Result<()>;
}

impl<M: Model, I, F> WriteExclusive<M, I> for F
where
    F: Fn(&mut M, &Dev<M>, I) -> Result<()>,
    I: FromValue + AttrData,
{
    const DATA_KIND: AttrDataKind = I::DATA_KIND;
    const DATA_FORMAT: AttrDataFormat = I::DATA_FORMAT;

    #[inline]
    fn write(&self, model: &mut M, dev: &Dev<M>, value: Value) -> Result<()> {
        self(model, dev, I::from_value(value)?)
    }
}

/// An asynchronous read function of an attribute, see [`AttrDef::read_async`].
///
/// The model is taken as `R`, being either `&'a M` or `&'a mut M`, see [`HandlerAsync`],
/// and the value may borrow from the model, with the same result types as [`ReadShared`].
pub trait ReadAsync<'a, R, M: Model> {
    /// The advertised kind of the value, derived from its type.
    const DATA_KIND: AttrDataKind;
    /// The advertised format of the value, without any maximum dimension.
    const DATA_FORMAT: AttrDataFormat;
    /// Give the future reading the value of the attribute and converting it.
    fn read(&self, model: R, dev: &'a Dev<M>) -> BoxFuture<'a, Result<AttrRead<Value>>>;
}

impl<'a, R, M: Model, O, F, Fut> ReadAsync<'a, R, M> for F
where
    F: Fn(R, &'a Dev<M>) -> Fut,
    Fut: Future<Output = Result<O>> + Send + 'a,
    O: ReadOutput,
{
    const DATA_KIND: AttrDataKind = O::DATA_KIND;
    const DATA_FORMAT: AttrDataFormat = O::DATA_FORMAT;

    #[inline]
    fn read(&self, model: R, dev: &'a Dev<M>) -> BoxFuture<'a, Result<AttrRead<Value>>> {
        let fut = self(model, dev);
        Box::pin(async move { Ok(fut.await?.into_read()) })
    }
}

/// An asynchronous write function of an attribute, see [`AttrDef::write_async`].
///
/// The model is taken as `R`, being either `&'a M` or `&'a mut M`, see [`HandlerAsync`].
pub trait WriteAsync<'a, R, M: Model, I> {
    /// The advertised kind of the value, derived from its type.
    const DATA_KIND: AttrDataKind;
    /// The advertised format of the value, without any maximum dimension.
    const DATA_FORMAT: AttrDataFormat;
    /// Convert the given value and give the future writing it to the attribute.
    fn write(&self, model: R, dev: &'a Dev<M>, value: Value) -> BoxFuture<'a, Result<()>>;
}

impl<'a, R, M: Model, I, F, Fut> WriteAsync<'a, R, M, I> for F
where
    F: Fn(R, &'a Dev<M>, I) -> Fut,
    Fut: Future<Output = Result<()>> + Send + 'a,
    I: FromValue + AttrData,
{
    const DATA_KIND: AttrDataKind = I::DATA_KIND;
    const DATA_FORMAT: AttrDataFormat = I::DATA_FORMAT;

    #[inline]
    fn write(&self, model: R, dev: &'a Dev<M>, value: Value) -> BoxFuture<'a, Result<()>> {
        let fut = I::from_value(value).map(|input| self(model, dev, input));
        Box::pin(async move { fut?.await })
    }
}

impl<M: Model> AttrDef<M> {

    /// Create a new attribute definition, without any read or write function.
    pub fn new<N>(name: N) -> Self
    where
        N: Into<Cow<'static, str>>,
    {
        Self {
            name: name.into(),
            read: None,
            write: None,
            kind: None,
            format: None,
            display_level: DisplayLevel::default(),
            memorized: None,
            config: AttrConfig::default(),
            poll_period: None,
            pushed: PushedEvents::default(),
        }
    }

    // --- Documentation --- //

    /// Set the description of the attribute, which is the only part of its configuration
    /// a client cannot change afterwards: it describes what the attribute *is*, and that
    /// is the business of the device, not of whoever reads it.
    pub fn doc<D>(mut self, doc: D) -> Self
    where
        D: Into<Cow<'static, str>>,
    {
        self.config.doc = Some(doc.into());
        self
    }

    /// Set the label of the attribute, the short name a client shows in place of its own,
    /// where "Position" reads better than "position" and "Δθ" than "delta_theta".
    pub fn label<L>(mut self, label: L) -> Self
    where
        L: Into<Cow<'static, str>>,
    {
        self.config.label = Some(label.into());
        self
    }

    /// Set the unit the value is expressed in, as it is shown next to it.
    pub fn unit<U>(mut self, unit: U) -> Self
    where
        U: Into<Cow<'static, str>>,
    {
        self.config.unit = Some(unit.into());
        self
    }

    /// Set the factor going from the value of the attribute to the same quantity in the
    /// international system, so that a client can convert without knowing the unit: 1e-3
    /// for a value in millimetres, the metre being the standard unit of a length.
    pub fn standard_unit(mut self, factor: f64) -> Self {
        self.config.standard_unit = Some(factor);
        self
    }

    /// Set the unit the value is *displayed* in, when it is not the one it is expressed
    /// in: a value in metres a client should show in millimetres.
    pub fn display_unit<U>(mut self, unit: U) -> Self
    where
        U: Into<Cow<'static, str>>,
    {
        self.config.display_unit = Some(unit.into());
        self
    }

    /// Set how the value is formatted for display, as a printf format such as `%8.3f`,
    /// which is what tells a client how many decimals of it are meaningful.
    pub fn display_format<F>(mut self, format: F) -> Self
    where
        F: Into<Cow<'static, str>>,
    {
        self.config.display_format = Some(format.into());
        self
    }

    /// Set the display level of the attribute, [`DisplayLevel::Expert`] hiding it from the
    /// panels an operator is shown without hiding it from anyone who asks for it.
    pub fn display_level(mut self, level: DisplayLevel) -> Self {
        self.display_level = level;
        self
    }

    // --- Bounds --- //

    /// Set the range of the values the attribute accepts being written, outside of which a
    /// write is refused by the server rather than by the device.
    ///
    /// The range is given as a Rust range, either bound of which may be left open for an
    /// attribute that is only bounded on one side:
    ///
    /// ```ignore
    /// AttrDef::new("position").range(-100.0..=100.0)
    /// AttrDef::new("pressure").range(0.0..)
    /// ```
    ///
    /// # Panics
    ///
    /// If a bound is exclusive, which Tango has no way to express.
    #[track_caller]
    pub fn range<T, R>(mut self, range: R) -> Self
    where
        T: Into<AttrNumber> + Copy,
        R: RangeBounds<T>,
    {
        self.config.range = range_config(&self.name, "value range", range);
        self
    }

    /// Set the range outside of which the attribute is in [`AttrQuality::Alarm`], and the
    /// device with it: an alarm is the reading being wrong, not the write being refused.
    ///
    /// # Panics
    ///
    /// If a bound is exclusive, see [`Self::range`].
    #[track_caller]
    pub fn alarm<T, R>(mut self, range: R) -> Self
    where
        T: Into<AttrNumber> + Copy,
        R: RangeBounds<T>,
    {
        self.config.alarm = range_config(&self.name, "alarm range", range);
        self
    }

    /// Set the range outside of which the attribute is in [`AttrQuality::Warning`], which
    /// is the same as an alarm without the device going into [`DevState::Alarm`] for it.
    ///
    /// # Panics
    ///
    /// If a bound is exclusive, see [`Self::range`].
    #[track_caller]
    pub fn warning<T, R>(mut self, range: R) -> Self
    where
        T: Into<AttrNumber> + Copy,
        R: RangeBounds<T>,
    {
        self.config.warning = range_config(&self.name, "warning range", range);
        self
    }

    /// Set the alarm on the read value drifting from the written one: how far apart the
    /// two may be, and for how long before it counts, absent for immediately.
    ///
    /// This is what says that a motor asked to go somewhere is not going there, which no
    /// bound on the value alone can express.
    pub fn delta<V, T>(mut self, value: V, time: T) -> Self
    where
        V: Into<AttrNumber>,
        T: Into<Option<Duration>>,
    {
        self.config.delta = Some(AttrDeltaConfig {
            value: value.into(),
            time: time.into(),
        });
        self
    }

    // --- Events --- //

    /// Set the criteria of the change event: how much the value has to move, absolutely
    /// and relatively, for the server to send one. Either may be left out with [`None`],
    /// and an attribute that declares neither only has the change events its device pushes
    /// by itself.
    ///
    /// ```ignore
    /// AttrDef::new("position").change_event(0.001, None)
    /// ```
    pub fn change_event<A, R>(mut self, abs: A, rel: R) -> Self
    where
        A: Into<Option<f64>>,
        R: Into<Option<f64>>,
    {
        self.config.change_event.abs_change = abs.into();
        self.config.change_event.rel_change = rel.into();
        self
    }

    /// Set the period of the periodic event, which is sent whether the value moved or not
    /// and is how a client tells a device that has nothing to say from one that is gone.
    pub fn periodic_event(mut self, period: Duration) -> Self {
        self.config.periodic_event.period = Some(period);
        self
    }

    /// Set the criteria of the archive event: the same two changes as
    /// [`Self::change_event`], and the longest an archiver may go without an event when
    /// the value does not move.
    pub fn archive_event<A, R, P>(mut self, abs: A, rel: R, period: P) -> Self
    where
        A: Into<Option<f64>>,
        R: Into<Option<f64>>,
        P: Into<Option<Duration>>,
    {
        self.config.archive_event.abs_change = abs.into();
        self.config.archive_event.rel_change = rel.into();
        self.config.archive_event.period = period.into();
        self
    }

    /// Declare that the device pushes the change events of this attribute itself, see
    /// [`Dev::push_change_event`], which is what a client may then subscribe to them for:
    /// with no polling, a subscription to the change events of an attribute that does not
    /// declare them is refused, the way cppTango refuses it.
    ///
    /// With `detect`, the criteria of the change event are to be checked against what is
    /// pushed, see [`Self::change_event`], as cppTango does.
    ///
    /// TODO: Check the criteria, every push being sent for now.
    pub fn push_change_event(mut self, detect: bool) -> Self {
        self.pushed.change = Some(detect);
        self
    }

    /// Declare that the device pushes the archive events of this attribute itself, see
    /// [`Self::push_change_event`].
    pub fn push_archive_event(mut self, detect: bool) -> Self {
        self.pushed.archive = Some(detect);
        self
    }

    // --- Server behaviour --- //

    /// Make the attribute memorized: the server stores the value of every write in the
    /// database and restores it when the device is initialized, the mode saying whether it
    /// is only given back as the write value or written again for real.
    ///
    /// Only a writable scalar can be memorized, there being nothing to remember about a
    /// value the device computes, and a state or an encoded value cannot, which a class
    /// is told when the attribute is given to it, see [`ClassDef::attr`].
    pub fn memorized(mut self, mode: AttrMemorized) -> Self {
        self.memorized = Some(mode);
        self
    }

    /// Check what is only known once the whole definition is built: that this attribute
    /// can be memorized if it asks to be, see [`Self::memorized`], and that its kind
    /// supports every property its configuration gives a value to.
    pub(super) fn check(&self) -> std::result::Result<(), String> {

        if let Some(prop) = unsupported_prop(&self.config, self.kind) {
            return Err(format!("the attribute {} cannot have the property {:?}, {}",
                self.name, prop.name(), unsupported(self.kind)));
        }

        if self.memorized.is_none() {
            return Ok(());
        }

        let reason = if self.write.is_none() {
            "it is not writable"
        } else if !matches!(self.format, None | Some(AttrDataFormat::Scalar)) {
            "it is not a scalar"
        } else if matches!(self.kind, Some(AttrDataKind::State | AttrDataKind::Encoded)) {
            "a value of its kind cannot be memorized"
        } else {
            return Ok(());
        };

        Err(format!("the attribute {} cannot be memorized, {reason}", self.name))
        
    }

    /// Ask for the attribute to be polled at that period, the value each poll reads being
    /// what the server keeps in its polling buffer and answers a cached read with.
    ///
    /// This is the period the class asks for; the database has the last word, so a device
    /// may end up polled at another period, or not polled at all.
    pub fn polled(mut self, period: Duration) -> Self {
        self.poll_period = Some(period);
        self
    }

    /// Set the advertised data kind of the attribute. This is only needed for the read
    /// and write functions that use a raw [`Value`], the typed ones derive it from their
    /// value type.
    ///
    /// # Panics
    ///
    /// If the data kind has already been defined, explicitly or by a read or a write
    /// function, as a different kind.
    #[track_caller]
    pub fn kind(mut self, kind: AttrDataKind) -> Self {
        self.kind = Some(merge_kind(&self.name, "data kind", self.kind.take(), kind));
        self
    }

    /// Set the advertised data format of the attribute, which also gives the maximum
    /// dimensions of a spectrum or an image. This is only needed for the read and write
    /// functions that use a raw [`Value`], the typed ones derive a format without any
    /// maximum dimension from their value type.
    ///
    /// A maximum dimension of zero being undefined, it is compatible with any other one,
    /// so the maximum dimensions of a typed read or write function can be completed by
    /// this method, in any order.
    ///
    /// # Panics
    ///
    /// If the data format has already been defined, explicitly or by a read or a write
    /// function, with a different layout or a different maximum dimension.
    #[track_caller]
    pub fn format(mut self, format: AttrDataFormat) -> Self {
        self.format = Some(merge_format(&self.name, self.format.take(), format));
        self
    }

    /// Declare the attribute to hold a single value, which is [`Self::format`] with the
    /// format written for you.
    ///
    /// # Panics
    ///
    /// If the format has already been defined as another layout, see [`Self::format`].
    #[track_caller]
    pub fn scalar(self) -> Self {
        self.format(AttrDataFormat::Scalar)
    }

    /// Declare the attribute to hold a one dimensional array of at most that many values.
    ///
    /// A typed read or write function already gives the layout, so this is usually only
    /// there for the maximum size, which no value type carries; a maximum of zero leaves
    /// it undeclared, an array of any size then being accepted.
    ///
    /// # Panics
    ///
    /// If the format has already been defined as another layout, or as a spectrum of a
    /// different maximum size, see [`Self::format`].
    #[track_caller]
    pub fn spectrum(self, max_size: usize) -> Self {
        self.format(AttrDataFormat::Spectrum(max_size))
    }

    /// Declare the attribute to hold a two dimensional array of at most that width and
    /// that height, see [`Self::spectrum`].
    ///
    /// # Panics
    ///
    /// If the format has already been defined as another layout, or as an image of
    /// different maximum dimensions, see [`Self::format`].
    #[track_caller]
    pub fn image(self, max_width: usize, max_height: usize) -> Self {
        self.format(AttrDataFormat::Image(max_width, max_height))
    }

    /// Define the read function of this attribute, with a shared access to the model.
    ///
    /// This is the raw form of [`Self::read`]: the closure returns a [`Value`], converting
    /// what it borrows from the model itself, which is what makes a closure usable for
    /// such a result. The advertised kind and format then have to be given with
    /// [`Self::kind`] and [`Self::format`].
    pub fn read_with<F>(mut self, func: F) -> Self
    where
        F: Fn(&M, &Dev<M>) -> Result<AttrRead<Value>> + Send + Sync + 'static,
    {
        self.read = Some(ReadFn::SharedSync(Box::new(func)));
        self
    }

    /// Define the read function of this attribute, with a shared access to the model,
    /// which allows parallel reading of such attributes.
    ///
    /// The value type is taken from the `ReadShared` implementation of the function,
    /// and may borrow from the model, with the same constraints as [`CommandDef::exec`]:
    /// it is then written as a function item generic over the model lifetime, typically
    /// an associated method of the model.
    ///
    /// # Panics
    ///
    /// If the advertised kind or format has already been defined incompatibly, see
    /// [`Self::kind`] and [`Self::format`].
    #[track_caller]
    pub fn read<F>(self, func: F) -> Self
    where
        F: for<'m> ReadShared<'m, M> + Send + Sync + 'static,
    {
        self.kind(F::DATA_KIND)
            .format(F::DATA_FORMAT)
            .read_with(move |model, dev| func.read(model, dev))
    }

    /// Define the read function of this attribute, with an exclusive access to the model.
    /// This is the raw form of [`Self::read_exclusive`], see [`Self::read_with`].
    pub fn read_exclusive_with<F>(mut self, func: F) -> Self
    where
        F: Fn(&mut M, &Dev<M>) -> Result<AttrRead<Value>> + Send + Sync + 'static,
    {
        self.read = Some(ReadFn::ExclusiveSync(Box::new(func)));
        self
    }

    /// Define the read function of this attribute, with an exclusive access to the model,
    /// therefore no other command or attribute can access it in parallel.
    ///
    /// # Panics
    ///
    /// If the advertised kind or format has already been defined incompatibly, see
    /// [`Self::kind`] and [`Self::format`].
    #[track_caller]
    pub fn read_exclusive<F>(self, func: F) -> Self
    where
        F: for<'m> ReadExclusive<'m, M> + Send + Sync + 'static,
    {
        self.kind(F::DATA_KIND)
            .format(F::DATA_FORMAT)
            .read_exclusive_with(move |model, dev| func.read(model, dev))
    }

    /// Define the read function of this attribute, asynchronously and with a shared access
    /// to the model. This is the raw form of [`Self::read_async`], for a closure that
    /// boxes its future itself, see [`Self::read_with`].
    pub fn read_async_with<F>(mut self, func: F) -> Self
    where
        F: for<'a> Fn(&'a M, &'a Dev<M>) -> BoxFuture<'a, Result<AttrRead<Value>>> + Send + Sync + 'static,
    {
        self.read = Some(ReadFn::SharedAsync(Box::new(func)));
        self
    }

    /// Define the read function of this attribute, asynchronously and with a shared access
    /// to the model, typically an `async fn` method of the model, see
    /// [`CommandDef::exec_async`] for what it is for and what it must not do.
    ///
    /// # Panics
    ///
    /// If the advertised kind or format has already been defined incompatibly, see
    /// [`Self::kind`] and [`Self::format`].
    #[track_caller]
    pub fn read_async<F>(self, func: F) -> Self
    where
        F: for<'a> ReadAsync<'a, &'a M, M> + Send + Sync + 'static,
    {
        self.kind(F::DATA_KIND)
            .format(F::DATA_FORMAT)
            .read_async_with(move |model, dev| func.read(model, dev))
    }

    /// Define the read function of this attribute, asynchronously and with an exclusive
    /// access to the model. This is the raw form of [`Self::read_exclusive_async`], see
    /// [`Self::read_async_with`].
    pub fn read_exclusive_async_with<F>(mut self, func: F) -> Self
    where
        F: for<'a> Fn(&'a mut M, &'a Dev<M>) -> BoxFuture<'a, Result<AttrRead<Value>>> + Send + Sync + 'static,
    {
        self.read = Some(ReadFn::ExclusiveAsync(Box::new(func)));
        self
    }

    /// Define the read function of this attribute, asynchronously and with an exclusive
    /// access to the model, which no other command or attribute can then access until the
    /// future completes. See [`Self::read_async`].
    ///
    /// # Panics
    ///
    /// If the advertised kind or format has already been defined incompatibly, see
    /// [`Self::kind`] and [`Self::format`].
    #[track_caller]
    pub fn read_exclusive_async<F>(self, func: F) -> Self
    where
        F: for<'a> ReadAsync<'a, &'a mut M, M> + Send + Sync + 'static,
    {
        self.kind(F::DATA_KIND)
            .format(F::DATA_FORMAT)
            .read_exclusive_async_with(move |model, dev| func.read(model, dev))
    }

    /// Define the write function of this attribute, with a shared access to the model.
    /// This is the raw form of [`Self::write`], the written [`Value`] being given as-is
    /// and the advertised kind and format having to be given with [`Self::kind`] and
    /// [`Self::format`].
    pub fn write_with<F>(mut self, func: F) -> Self
    where
        F: Fn(&M, &Dev<M>, Value) -> Result<()> + Send + Sync + 'static,
    {
        self.write = Some(WriteFn::SharedSync(Box::new(func)));
        self
    }

    /// Define the write function of this attribute, with a shared access to the model.
    ///
    /// The value type is taken from the `WriteShared` implementation of the function,
    /// any [`FromValue`] type, a [`Buf`](crate::buf::Buf) sharing the written value
    /// rather than copying it.
    ///
    /// # Panics
    ///
    /// If the advertised kind or format has already been defined incompatibly, which
    /// notably happens when the read function of the attribute does not use the same
    /// value type, see [`Self::kind`] and [`Self::format`].
    #[track_caller]
    pub fn write<F, I>(self, func: F) -> Self
    where
        F: WriteShared<M, I> + Send + Sync + 'static,
    {
        self.kind(F::DATA_KIND)
            .format(F::DATA_FORMAT)
            .write_with(move |model, dev, value| func.write(model, dev, value))
    }

    /// Define the write function of this attribute, with an exclusive access to the
    /// model. This is the raw form of [`Self::write_exclusive`], see [`Self::write_with`].
    pub fn write_exclusive_with<F>(mut self, func: F) -> Self
    where
        F: Fn(&mut M, &Dev<M>, Value) -> Result<()> + Send + Sync + 'static,
    {
        self.write = Some(WriteFn::ExclusiveSync(Box::new(func)));
        self
    }

    /// Define the write function of this attribute, with an exclusive access to the
    /// model, which is what a write usually needs in order to store the written value.
    ///
    /// # Panics
    ///
    /// If the advertised kind or format has already been defined incompatibly, which
    /// notably happens when the read function of the attribute does not use the same
    /// value type, see [`Self::kind`] and [`Self::format`].
    #[track_caller]
    pub fn write_exclusive<F, I>(self, func: F) -> Self
    where
        F: WriteExclusive<M, I> + Send + Sync + 'static,
    {
        self.kind(F::DATA_KIND)
            .format(F::DATA_FORMAT)
            .write_exclusive_with(move |model, dev, value| func.write(model, dev, value))
    }

    /// Define the write function of this attribute, asynchronously and with a shared
    /// access to the model. This is the raw form of [`Self::write_async`], see
    /// [`Self::read_async_with`].
    pub fn write_async_with<F>(mut self, func: F) -> Self
    where
        F: for<'a> Fn(&'a M, &'a Dev<M>, Value) -> BoxFuture<'a, Result<()>> + Send + Sync + 'static,
    {
        self.write = Some(WriteFn::SharedAsync(Box::new(func)));
        self
    }

    /// Define the write function of this attribute, asynchronously and with a shared
    /// access to the model, see [`Self::read_async`].
    ///
    /// # Panics
    ///
    /// If the advertised kind or format has already been defined incompatibly, see
    /// [`Self::write`].
    #[track_caller]
    pub fn write_async<F, I>(self, func: F) -> Self
    where
        F: for<'a> WriteAsync<'a, &'a M, M, I> + Send + Sync + 'static,
    {
        self.kind(F::DATA_KIND)
            .format(F::DATA_FORMAT)
            .write_async_with(move |model, dev, value| func.write(model, dev, value))
    }

    /// Define the write function of this attribute, asynchronously and with an exclusive
    /// access to the model. This is the raw form of [`Self::write_exclusive_async`], see
    /// [`Self::read_async_with`].
    pub fn write_exclusive_async_with<F>(mut self, func: F) -> Self
    where
        F: for<'a> Fn(&'a mut M, &'a Dev<M>, Value) -> BoxFuture<'a, Result<()>> + Send + Sync + 'static,
    {
        self.write = Some(WriteFn::ExclusiveAsync(Box::new(func)));
        self
    }

    /// Define the write function of this attribute, asynchronously and with an exclusive
    /// access to the model, which no other command or attribute can then access until the
    /// future completes. See [`Self::read_async`].
    ///
    /// # Panics
    ///
    /// If the advertised kind or format has already been defined incompatibly, see
    /// [`Self::write`].
    #[track_caller]
    pub fn write_exclusive_async<F, I>(self, func: F) -> Self
    where
        F: for<'a> WriteAsync<'a, &'a mut M, M, I> + Send + Sync + 'static,
    {
        self.kind(F::DATA_KIND)
            .format(F::DATA_FORMAT)
            .write_exclusive_async_with(move |model, dev, value| func.write(model, dev, value))
    }

}

impl<M: Model> Debug for AttrDef<M> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("AttrDef")
            .field("name", &self.name)
            .field("read", &self.read)
            .field("write", &self.write)
            .field("kind", &self.kind)
            .field("format", &self.format)
            .field("config", &self.config)
            .field("poll_period", &self.poll_period)
            .field("pushed", &self.pushed)
            .finish()
    }
}


#[cfg(test)]
mod tests {

    use super::*;
    use crate::value::Spectrum;

    #[derive(Default)]
    struct Model { count: i32, label: String, values: Vec<f64>, labels: Vec<String> }

    impl Model {

        /// A result borrowed from the model, the lifetime elision binding it to the model
        /// borrow, see [`CommandDef::exec`].
        fn label(&self, _dev: &Dev<Self>, _arg: ()) -> Result<&str> {
            Ok(self.label.as_str())
        }

        fn values(&self, _dev: &Dev<Self>, _arg: ()) -> Result<&[f64]> {
            Ok(&self.values)
        }

        fn set_label(&mut self, _dev: &Dev<Self>, arg: String) -> Result<&str> {
            self.label = arg;
            Ok(self.label.as_str())
        }

        fn add(&self, _dev: &Dev<Self>, arg: i32) -> Result<i32> {
            Ok(self.count + arg)
        }

        fn incr(&mut self, _dev: &Dev<Self>, arg: i32) -> Result<i32> {
            self.count += arg;
            Ok(self.count)
        }

        // --- Attribute handlers, which are only given the device --- //

        fn count(&self, _dev: &Dev<Self>) -> Result<i32> {
            Ok(self.count)
        }

        fn set_count(&mut self, _dev: &Dev<Self>, arg: i32) -> Result<()> {
            self.count = arg;
            Ok(())
        }

        /// A read giving the quality and the timestamp of the value along with it.
        fn count_full(&self, _dev: &Dev<Self>) -> Result<AttrRead<i32>> {
            Ok(AttrRead {
                value: Some(self.count),
                w_value: None,
                quality: AttrQuality::Valid,
                time: SystemTime::now(),
            })
        }

        fn read_label(&self, _dev: &Dev<Self>) -> Result<&str> {
            Ok(self.label.as_str())
        }

        fn read_values(&self, _dev: &Dev<Self>) -> Result<&[f64]> {
            Ok(&self.values)
        }

        /// The handler shared by the commands of a dynamic set, the index of the command
        /// being captured by the closure forwarding to it.
        fn label_n(&self, _dev: &Dev<Self>, index: usize) -> Result<&str> {
            Ok(self.labels[index].as_str())
        }

        /// A read updating the model, therefore needing an exclusive access to it.
        fn tick(&mut self, _dev: &Dev<Self>) -> Result<i32> {
            self.count += 1;
            Ok(self.count)
        }

    }

    /// A result that does not borrow from the model at all.
    fn version(_model: &Model, _dev: &Dev<Model>, _arg: ()) -> Result<&'static str> {
        Ok("1.0.0")
    }

    /// Check that all the intended signatures are accepted, and that both kinds are
    /// always derived from the argument and result types.
    #[test]
    fn exec_signatures() {

        // Scalar in, scalar out, as an associated method of the model.
        let def = CommandDef::<Model>::new("Add").exec(Model::add);
        assert_eq!(def.in_kind, Some(CommandDataKind::Long));
        assert_eq!(def.out_kind, Some(CommandDataKind::Long));

        // Exclusive access to the model.
        let _ = CommandDef::<Model>::new("Incr").exec_exclusive(Model::incr);

        // A closure is still accepted as long as its result does not borrow from the
        // model, but it has to annotate the two reference parameters.
        let _ = CommandDef::<Model>::new("Status")
            .exec(|model: &Model, _dev: &Dev<Model>, (): ()| Ok(model.label.clone()));

        // A closure whose result borrows from the model is only accepted by the raw
        // form, see `closures_borrowing`.
        let def = CommandDef::<Model>::new("Label")
            .out_kind(CommandDataKind::String)
            .exec_with(|model: &Model, dev, value| {
                Ok(model.label(dev, <()>::from_value(value)?)?.into())
            });
        assert_eq!(def.out_kind, Some(CommandDataKind::String));

        // Spectrum in, scalar out.
        let def = CommandDef::<Model>::new("Sum")
            .exec(|_model: &Model, _dev: &Dev<Model>, arg: Vec<f64>| Ok(arg.iter().sum::<f64>()));
        assert_eq!(def.in_kind, Some(CommandDataKind::DoubleArray));
        assert_eq!(def.out_kind, Some(CommandDataKind::Double));

        // Spectrum in and out, owned or borrowed from the argument.
        let _ = CommandDef::<Model>::new("Scale")
            .exec(|_model: &Model, _dev: &Dev<Model>, arg: Vec<f64>| Ok(arg.iter().map(|v| v * 2.0).collect::<Vec<f64>>()));
        let _ = CommandDef::<Model>::new("Reverse")
            .exec(|_model: &Model, _dev: &Dev<Model>, arg: Vec<String>| Ok(arg.into_iter().rev().collect::<Vec<String>>()));
        let _ = CommandDef::<Model>::new("Zeros")
            .exec(|_model: &Model, _dev: &Dev<Model>, (): ()| Ok(Spectrum::<f64>::from(vec![0.0; 4])));

        // A `&'static str` result.
        let def = CommandDef::<Model>::new("Version").exec(version);
        assert_eq!(def.out_kind, Some(CommandDataKind::String));

        // A result borrowed from the model, both kinds still being derived.
        let def = CommandDef::<Model>::new("Label")
            .out_doc("The label of the device")
            .exec(Model::label);
        assert_eq!(def.in_kind, Some(CommandDataKind::Void));
        assert_eq!(def.out_kind, Some(CommandDataKind::String));

        let def = CommandDef::<Model>::new("Values").exec(Model::values);
        assert_eq!(def.out_kind, Some(CommandDataKind::DoubleArray));

        // Exclusive access to the model, with a result borrowed from it.
        let def = CommandDef::<Model>::new("SetLabel").exec_exclusive(Model::set_label);
        assert_eq!(def.in_kind, Some(CommandDataKind::String));
        assert_eq!(def.out_kind, Some(CommandDataKind::String));

        // Raw form, with a result borrowed from the model, the kinds being explicit
        // because they cannot be derived from a raw `Value`.
        let def = CommandDef::<Model>::new("Status")
            .out_kind(CommandDataKind::String)
            .exec_with(|model: &Model, _dev, _value| Ok(model.label.as_str().into()));
        assert_eq!(def.in_kind, None);
        assert_eq!(def.out_kind, Some(CommandDataKind::String));

        // Raw form, borrowing the argument value itself.
        let _ = CommandDef::<Model>::new("Echo")
            .in_kind(CommandDataKind::String)
            .out_kind(CommandDataKind::String)
            .exec_with(|model: &Model, _dev, value| {
                let arg = <&str>::try_from(&value)?;
                Ok(format!("{}{arg}", model.label).into())
            });

        // Raw form with an exclusive access to the model, returning a borrow of it.
        let _ = CommandDef::<Model>::new("SetLabel")
            .in_kind(CommandDataKind::String)
            .out_kind(CommandDataKind::String)
            .exec_exclusive_with(|model: &mut Model, _dev, value| {
                model.label = String::try_from(value)?;
                Ok(model.label.as_str().into())
            });

    }

    /// Check that all the intended read and write signatures are accepted, and that the
    /// kind and the format are always derived from the value type.
    #[test]
    fn attr_signatures() {

        // Scalar read and write, as associated methods of the model.
        let def = AttrDef::<Model>::new("Count")
            .read(Model::count)
            .write_exclusive(Model::set_count);
        assert_eq!(def.kind, Some(AttrDataKind::Long));
        assert_eq!(def.format, Some(AttrDataFormat::Scalar));

        // A value borrowed from the model, scalar or spectrum.
        let def = AttrDef::<Model>::new("Label").read(Model::read_label);
        assert_eq!(def.kind, Some(AttrDataKind::String));
        assert_eq!(def.format, Some(AttrDataFormat::Scalar));

        let def = AttrDef::<Model>::new("Values").read(Model::read_values);
        assert_eq!(def.kind, Some(AttrDataKind::Double));
        assert_eq!(def.format, Some(AttrDataFormat::Spectrum(0)));

        // A full read result, giving the quality and the timestamp.
        let def = AttrDef::<Model>::new("CountFull").read(Model::count_full);
        assert_eq!(def.kind, Some(AttrDataKind::Long));

        // Exclusive access to the model, for a read updating it.
        let _ = AttrDef::<Model>::new("Ticks").read_exclusive(Model::tick);

        // A closure is accepted as long as its result does not borrow from the model.
        let _ = AttrDef::<Model>::new("Double")
            .read(|model: &Model, _dev: &Dev<Model>| Ok(model.count * 2));

        // Raw form, the kind and the format having to be explicit.
        let _ = AttrDef::<Model>::new("Raw")
            .kind(AttrDataKind::Double)
            .format(AttrDataFormat::Spectrum(4096))
            .read_with(|model: &Model, _dev| Ok(AttrRead {
                value: Some(model.values.as_slice().into()),
                w_value: None,
                quality: AttrQuality::Valid,
                time: SystemTime::now(),
            }));

    }

    /// Check that every configuration builder writes the field of [`AttrConfig`] it is
    /// named after, which is what a device of the class then starts from.
    #[test]
    fn attr_config() {

        let def = AttrDef::<Model>::new("Position")
            .read(Model::count)
            .doc("The position of the axis")
            .label("Position")
            .unit("mm")
            .standard_unit(1e-3)
            .display_unit("mm")
            .display_format("%8.3f")
            .display_level(DisplayLevel::Expert)
            .delta(0.5, Duration::from_millis(200))
            .change_event(0.001, None)
            .periodic_event(Duration::from_secs(1))
            .archive_event(0.1, None, Duration::from_secs(10))
            .memorized(AttrMemorized::RestoreAndWrite)
            .polled(Duration::from_millis(100));

        assert_eq!(def.config.doc.as_deref(), Some("The position of the axis"));
        assert_eq!(def.config.label.as_deref(), Some("Position"));
        assert_eq!(def.config.unit.as_deref(), Some("mm"));
        assert_eq!(def.config.standard_unit, Some(1e-3));
        assert_eq!(def.config.display_unit.as_deref(), Some("mm"));
        assert_eq!(def.config.display_format.as_deref(), Some("%8.3f"));
        assert_eq!(def.display_level, DisplayLevel::Expert);

        // The delta is a limit like the bounds are, so it is the number of the attribute
        // and not a double the value would have to be converted to.
        assert_eq!(def.config.delta, Some(AttrDeltaConfig {
            value: AttrNumber::Double(0.5),
            time: Some(Duration::from_millis(200)),
        }));

        // The criteria of the events are not limits: an absolute change is a magnitude and
        // a relative one a percentage, so both stay doubles, and either may be left out.
        assert_eq!(def.config.change_event.abs_change, Some(0.001));
        assert_eq!(def.config.change_event.rel_change, None);
        assert_eq!(def.config.periodic_event.period, Some(Duration::from_secs(1)));
        assert_eq!(def.config.archive_event.abs_change, Some(0.1));
        assert_eq!(def.config.archive_event.period, Some(Duration::from_secs(10)));

        assert_eq!(def.memorized, Some(AttrMemorized::RestoreAndWrite));
        assert_eq!(def.poll_period, Some(Duration::from_millis(100)));

        // What is not defined stays absent rather than being given a value nobody asked
        // for, an attribute with no bound being the usual case.
        let def = AttrDef::<Model>::new("Count").read(Model::count);
        assert_eq!(def.config, AttrConfig::default());
        assert_eq!(def.poll_period, None);

    }

    /// Check the three ways a range is bounded, that a bound left open stays open, and
    /// that a range is the kind of number it was written with.
    #[test]
    fn attr_ranges() {

        let def = AttrDef::<Model>::new("Position")
            .read(Model::count)
            .range(-100.0..=100.0)
            .warning(-90.5..)
            .alarm(..=95.0);

        assert_eq!(def.config.range, AttrRangeConfig {
            min: Some(AttrNumber::Double(-100.0)),
            max: Some(AttrNumber::Double(100.0)),
        });

        // Only bounded below: an attribute that cannot go under that and has no ceiling.
        assert_eq!(def.config.warning, AttrRangeConfig {
            min: Some(AttrNumber::Double(-90.5)),
            max: None,
        });

        // And only bounded above.
        assert_eq!(def.config.alarm, AttrRangeConfig {
            min: None,
            max: Some(AttrNumber::Double(95.0)),
        });

        // An integer attribute is bounded with integers, which are held as such rather
        // than being rounded into a double: this is the whole point of the three kinds.
        let def = AttrDef::<Model>::new("Count")
            .read(Model::count)
            .range(-8..=8i32)
            .alarm(..=u64::MAX);

        assert_eq!(def.config.range, AttrRangeConfig {
            min: Some(AttrNumber::Long(-8)),
            max: Some(AttrNumber::Long(8)),
        });
        assert_eq!(def.config.alarm.max, Some(AttrNumber::ULong(u64::MAX)));

        // Two limits a double cannot tell apart, which is the whole reason they are not
        // doubles: kept as they are they differ, converted they are the same number.
        assert_ne!(AttrNumber::ULong(u64::MAX), AttrNumber::ULong(u64::MAX - 1));
        assert_eq!(AttrNumber::ULong(u64::MAX).as_f64(), AttrNumber::ULong(u64::MAX - 1).as_f64());

        // Two limits of different kinds are equal when they stand for the same number.
        assert_eq!(AttrNumber::Long(2), AttrNumber::Double(2.0));
        assert!(AttrNumber::Long(-1) < AttrNumber::ULong(0));
        assert!(AttrNumber::ULong(u64::MAX) > AttrNumber::ULong(u64::MAX - 1));
        assert_ne!(AttrNumber::Double(f64::NAN), AttrNumber::Double(f64::NAN));

    }

    #[test]
    #[should_panic(expected = "the value range cannot have an exclusive bound")]
    fn exclusive_range_bound() {
        let _ = AttrDef::<Model>::new("Position").range(-100.0..100.0);
    }

    /// Check that the three layout methods are the formats they stand for, and that they
    /// merge with what a handler derived the way [`AttrDef::format`] does.
    #[test]
    fn attr_layouts() {

        let def = AttrDef::<Model>::new("Count").read(Model::count).scalar();
        assert_eq!(def.format, Some(AttrDataFormat::Scalar));

        // The layout comes from the handler and only the maximum size from here, in
        // either order.
        let def = AttrDef::<Model>::new("Values").read(Model::read_values).spectrum(4096);
        assert_eq!(def.format, Some(AttrDataFormat::Spectrum(4096)));

        let def = AttrDef::<Model>::new("Values").spectrum(4096).read(Model::read_values);
        assert_eq!(def.kind, Some(AttrDataKind::Double));
        assert_eq!(def.format, Some(AttrDataFormat::Spectrum(4096)));

        // Nothing is derived from a raw handler, so both are stated outright.
        let def = AttrDef::<Model>::new("Frame")
            .kind(AttrDataKind::Double)
            .image(640, 480);
        assert_eq!(def.format, Some(AttrDataFormat::Image(640, 480)));

        // A maximum of zero is the undefined one, which any size then matches.
        let def = AttrDef::<Model>::new("Values").read(Model::read_values).spectrum(0);
        assert_eq!(def.format, Some(AttrDataFormat::Spectrum(0)));

    }

    #[test]
    #[should_panic(expected = "the data format is already defined as Scalar")]
    fn incompatible_layout() {
        let _ = AttrDef::<Model>::new("Count").read(Model::count).spectrum(8);
    }

    /// Check what a command has of a configuration, which is a good deal less than an
    /// attribute: Tango tells a client the display level and nothing else about it.
    #[test]
    fn command_config() {

        let def = CommandDef::<Model>::new("Add")
            .exec(Model::add)
            .display_level(DisplayLevel::Expert)
            .polled(Duration::from_secs(1));

        assert_eq!(def.display_level, DisplayLevel::Expert);
        assert_eq!(def.poll_period, Some(Duration::from_secs(1)));

        // The level everything is at unless it says otherwise.
        let def = CommandDef::<Model>::new("Add").exec(Model::add);
        assert_eq!(def.display_level, DisplayLevel::Operator);
        assert_eq!(def.poll_period, None);

    }

    /// The description of a class is the one every device of it gives of itself.
    #[test]
    fn class_doc() {
        let def = ClassDef::<Model>::new_default("Motor").doc("A single axis");
        assert_eq!(def.doc, "A single axis");
        let def = ClassDef::<Model>::new_default("Motor");
        assert_eq!(def.doc, "A Tango device");
    }

    /// Check that an explicit kind or format completes the one derived from a handler,
    /// in any order, as long as they do not contradict each other.
    #[test]
    fn compatible_kinds() {

        // The same kind given twice, before or after the execution function.
        let def = CommandDef::<Model>::new("Add")
            .in_kind(CommandDataKind::Long)
            .exec(Model::add)
            .out_kind(CommandDataKind::Long);
        assert_eq!(def.in_kind, Some(CommandDataKind::Long));
        assert_eq!(def.out_kind, Some(CommandDataKind::Long));

        // An undefined maximum size being completed by an explicit format, the derived
        // kind being kept in both orders.
        let def = AttrDef::<Model>::new("Values")
            .read(Model::read_values)
            .format(AttrDataFormat::Spectrum(4096));
        assert_eq!(def.kind, Some(AttrDataKind::Double));
        assert_eq!(def.format, Some(AttrDataFormat::Spectrum(4096)));

        let def = AttrDef::<Model>::new("Values")
            .format(AttrDataFormat::Spectrum(4096))
            .read(Model::read_values);
        assert_eq!(def.format, Some(AttrDataFormat::Spectrum(4096)));

    }

    #[test]
    #[should_panic(expected = "the argument kind is already defined as String")]
    fn incompatible_in_kind() {
        let _ = CommandDef::<Model>::new("Add")
            .in_kind(CommandDataKind::String)
            .exec(Model::add);
    }

    #[test]
    #[should_panic(expected = "the result kind is already defined as Long")]
    fn incompatible_out_kind() {
        let _ = CommandDef::<Model>::new("Add")
            .exec(Model::add)
            .out_kind(CommandDataKind::Long64);
    }

    #[test]
    #[should_panic(expected = "the data kind is already defined as String")]
    fn incompatible_attr_kind() {
        let _ = AttrDef::<Model>::new("Label")
            .read(Model::read_label)
            .write_exclusive(Model::set_count);
    }

    #[test]
    #[should_panic(expected = "the data format is already defined as Scalar")]
    fn incompatible_attr_format() {
        let _ = AttrDef::<Model>::new("Count")
            .read(Model::count)
            .format(AttrDataFormat::Spectrum(8));
    }

    #[test]
    #[should_panic(expected = "the maximum size is already defined as 8")]
    fn incompatible_attr_max() {
        let _ = AttrDef::<Model>::new("Values")
            .format(AttrDataFormat::Spectrum(8))
            .format(AttrDataFormat::Spectrum(4096));
    }


    #[test]
    #[should_panic(expected = "Test: the attribute Label cannot have the property \"max_alarm\", not supported by an attribute of type String")]
    fn unsupported_attr_prop() {
        let _ = ClassDef::<Model>::new_default("Test")
            .attr(AttrDef::new("Label")
                .read(Model::read_label)
                .alarm(..=5));
    }

    #[test]
    #[should_panic(expected = "the attribute State cannot have the property \"min_value\", not supported by an attribute of type State")]
    fn unsupported_state_prop() {
        let _ = ClassDef::<Model>::new_default("Test")
            .state_config(AttrConfig { range: AttrRangeConfig { min: Some(AttrNumber::Long(0)), max: None }, ..AttrConfig::default() });
    }

    #[test]
    #[should_panic(expected = "the attribute Count cannot be memorized, it is not writable")]
    fn memorized_read_only() {
        let _ = ClassDef::<Model>::new_default("Test")
            .attr(AttrDef::new("Count")
                .read(Model::count)
                .memorized(AttrMemorized::RestoreSetPoint));
    }

    #[test]
    #[should_panic(expected = "the attribute Values cannot be memorized, it is not a scalar")]
    fn memorized_spectrum() {
        let _ = ClassDef::<Model>::new_default("Test")
            .attr(AttrDef::new("Values")
                .memorized(AttrMemorized::RestoreAndWrite)
                .kind(AttrDataKind::Double)
                .format(AttrDataFormat::Spectrum(8))
                .write_with(|_model: &Model, _dev, _value| Ok(())));
    }

    /// Check the way a closure borrowing from the model is defined: only the raw form
    /// accepts it, its result being converted into a [`Value`] before the borrow ends,
    /// and the advertised kinds then being explicit.
    #[test]
    fn closures_borrowing() {

        // The dynamic command case: several commands sharing a single handler, each
        // closure capturing the index of its own command.
        for index in 0..3 {
            let def = CommandDef::<Model>::new(format!("Label{index}"))
                .out_kind(CommandDataKind::String)
                .exec_with(move |model: &Model, dev, _value| {
                    Ok(model.label_n(dev, index)?.into())
                });
            assert_eq!(def.out_kind, Some(CommandDataKind::String));
        }

        // With an exclusive access to the model, the result borrowing what was written.
        let _ = CommandDef::<Model>::new("Push")
            .in_kind(CommandDataKind::String)
            .out_kind(CommandDataKind::String)
            .exec_exclusive_with(|model: &mut Model, _dev, value| {
                model.labels.push(String::try_from(value)?);
                Ok(model.labels.last().unwrap().as_str().into())
            });

        // The same for the read function of a dynamic attribute.
        for index in 0..3 {
            let def = AttrDef::<Model>::new(format!("Label{index}"))
                .kind(AttrDataKind::String)
                .format(AttrDataFormat::Scalar)
                .read_with(move |model: &Model, dev| {
                    Ok(AttrRead {
                        value: Some(model.label_n(dev, index)?.into()),
                        w_value: None,
                        quality: AttrQuality::Valid,
                        time: SystemTime::now(),
                    })
                });
            assert_eq!(def.kind, Some(AttrDataKind::String));
        }

    }

    /// A model owning what it borrowed from outside of the device, which is what the
    /// `'static` bound asks of it: a class lives as long as the server serving it, so a
    /// model borrowing from a shorter lifetime has nothing to be borrowed from by then.
    struct Owning { values: Vec<f64> }

    impl Owning {
        fn len(&self, _dev: &Dev<Self>, _arg: ()) -> Result<i32> { Ok(self.values.len() as i32) }
        fn first(&self, _dev: &Dev<Self>) -> Result<f64> { Ok(self.values[0]) }
    }

    /// The definitions of such a model are `'static` themselves, so they can be given to a
    /// class without being tied to the scope they were written in.
    fn owning_defs(prefix: &str) -> (CommandDef<Owning>, AttrDef<Owning>) {
        let command = CommandDef::<Owning>::new("Len")
            .exec(Owning::len)
            .out_doc(prefix.to_owned());
        let attr = AttrDef::<Owning>::new("First")
            .read(Owning::first)
            // A handler capturing what it needs by value, the definition outliving the
            // scope it was built in.
            .write_with({
                let prefix = prefix.to_owned();
                move |_model: &Owning, _dev, _value| {
                    let _ = &prefix;
                    Ok(())
                }
            });
        (command, attr)
    }

    #[test]
    fn model_owned() {
        let values = vec![1.0, 2.0, 3.0];
        let prefix = format!("{} values", values.len());
        let (command, attr) = owning_defs(&prefix);
        drop(prefix);
        assert_eq!(command.out_kind, Some(CommandDataKind::Long));
        assert_eq!(attr.kind, Some(AttrDataKind::Double));
    }

    /// The asynchronous counterparts of the handlers of [`Model`], written as `async fn`
    /// methods, which is what the typed async definitions are meant to take.
    struct AsyncModel { count: i32, label: String }

    impl AsyncModel {

        async fn add(&self, _dev: &Dev<Self>, arg: i32) -> Result<i32> {
            Ok(self.count + arg)
        }

        async fn label(&self, _dev: &Dev<Self>, _arg: ()) -> Result<&str> {
            Ok(self.label.as_str())
        }

        async fn incr(&mut self, _dev: &Dev<Self>, arg: i32) -> Result<i32> {
            self.count += arg;
            Ok(self.count)
        }

        async fn count(&self, _dev: &Dev<Self>) -> Result<i32> {
            Ok(self.count)
        }

        async fn set_count(&mut self, _dev: &Dev<Self>, arg: i32) -> Result<()> {
            self.count = arg;
            Ok(())
        }

        async fn read_label(&self, _dev: &Dev<Self>) -> Result<&str> {
            Ok(self.label.as_str())
        }

        async fn check_label(&self, _dev: &Dev<Self>, arg: String) -> Result<()> {
            let _ = arg.len();
            Ok(())
        }

        async fn tick(&mut self, _dev: &Dev<Self>) -> Result<i32> {
            self.count += 1;
            Ok(self.count)
        }

        async fn state(&self, _dev: &Dev<Self>) -> Result<DevState> {
            Ok(DevState::On)
        }

        async fn status(&mut self, _dev: &Dev<Self>) -> Result<Cow<'_, str>> {
            Ok(Cow::Borrowed(self.label.as_str()))
        }

    }

    /// Check that the async execution functions are accepted as `async fn` methods and as
    /// boxing closures, that both kinds are derived as for the sync ones, and that each
    /// definition is stored with the access it asked for.
    #[test]
    fn exec_async_signatures() {

        let def = CommandDef::<AsyncModel>::new("Add").exec_async(AsyncModel::add);
        assert_eq!(def.in_kind, Some(CommandDataKind::Long));
        assert_eq!(def.out_kind, Some(CommandDataKind::Long));
        assert!(matches!(def.exec, Some(ExecFn::SharedAsync(_))));

        // A result borrowed from the model across the future.
        let def = CommandDef::<AsyncModel>::new("Label").exec_async(AsyncModel::label);
        assert_eq!(def.in_kind, Some(CommandDataKind::Void));
        assert_eq!(def.out_kind, Some(CommandDataKind::String));

        let def = CommandDef::<AsyncModel>::new("Incr").exec_exclusive_async(AsyncModel::incr);
        assert_eq!(def.in_kind, Some(CommandDataKind::Long));
        assert!(matches!(def.exec, Some(ExecFn::ExclusiveAsync(_))));

        // Raw forms, whose closures box their future themselves and give no kind.
        let def = CommandDef::<AsyncModel>::new("Echo")
            .in_kind(CommandDataKind::String)
            .out_kind(CommandDataKind::String)
            .exec_async_with(|model: &AsyncModel, _dev, value| Box::pin(async move {
                let arg = <&str>::try_from(&value)?;
                Ok(format!("{}{arg}", model.label).into())
            }));
        assert_eq!(def.in_kind, Some(CommandDataKind::String));
        assert!(matches!(def.exec, Some(ExecFn::SharedAsync(_))));

        let def = CommandDef::<AsyncModel>::new("SetLabel")
            .exec_exclusive_async_with(|model: &mut AsyncModel, _dev, value| Box::pin(async move {
                model.label = String::try_from(value)?;
                Ok(model.label.as_str().into())
            }));
        assert_eq!(def.in_kind, None);
        assert_eq!(def.out_kind, None);
        assert!(matches!(def.exec, Some(ExecFn::ExclusiveAsync(_))));

    }

    /// Check the same for the async read and write functions of an attribute.
    #[test]
    fn attr_async_signatures() {

        let def = AttrDef::<AsyncModel>::new("Count")
            .read_async(AsyncModel::count)
            .write_exclusive_async(AsyncModel::set_count);
        assert_eq!(def.kind, Some(AttrDataKind::Long));
        assert_eq!(def.format, Some(AttrDataFormat::Scalar));
        assert!(matches!(def.read, Some(ReadFn::SharedAsync(_))));
        assert!(matches!(def.write, Some(WriteFn::ExclusiveAsync(_))));

        let def = AttrDef::<AsyncModel>::new("Label")
            .read_async(AsyncModel::read_label)
            .write_async(AsyncModel::check_label);
        assert_eq!(def.kind, Some(AttrDataKind::String));
        assert!(matches!(def.write, Some(WriteFn::SharedAsync(_))));

        let def = AttrDef::<AsyncModel>::new("Ticks").read_exclusive_async(AsyncModel::tick);
        assert!(matches!(def.read, Some(ReadFn::ExclusiveAsync(_))));

        let def = AttrDef::<AsyncModel>::new("Raw")
            .kind(AttrDataKind::Long)
            .scalar()
            .read_async_with(|model: &AsyncModel, _dev| Box::pin(async move {
                Ok(AttrRead {
                    value: Some(model.count.into()),
                    w_value: None,
                    quality: AttrQuality::Valid,
                    time: SystemTime::now(),
                })
            }))
            .write_exclusive_async_with(|model: &mut AsyncModel, _dev, value| Box::pin(async move {
                model.count = i32::try_from(value)?;
                Ok(())
            }));
        assert!(matches!(def.read, Some(ReadFn::SharedAsync(_))));
        assert!(matches!(def.write, Some(WriteFn::ExclusiveAsync(_))));

    }

    #[test]
    #[should_panic(expected = "the data kind is already defined as String")]
    fn incompatible_attr_async_kind() {
        let _ = AttrDef::<AsyncModel>::new("Label")
            .read_async(AsyncModel::read_label)
            .write_exclusive_async(AsyncModel::set_count);
    }

    /// Check that the async state and status functions are stored with the access they
    /// asked for, a later definition replacing an earlier one.
    #[test]
    fn class_async_handlers() {

        let def = ClassDef::<AsyncModel>::new("Motor", |_dev| Ok(AsyncModel { count: 0, label: String::new() }))
            .state_async(AsyncModel::state)
            .status_exclusive_async(AsyncModel::status);
        assert!(matches!(def.state, Some(StateFn::SharedAsync(_))));
        assert!(matches!(def.status, Some(StatusFn::ExclusiveAsync(_))));

        let def = def
            .state_exclusive_async_with(|_model: &mut AsyncModel, _dev| Box::pin(async { Ok(DevState::Moving) }))
            .status_async_with(|model: &AsyncModel, _dev| Box::pin(async move { Ok(Cow::Borrowed(model.label.as_str())) }));
        assert!(matches!(def.state, Some(StateFn::ExclusiveAsync(_))));
        assert!(matches!(def.status, Some(StatusFn::SharedAsync(_))));

    }

}