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use core::cell::UnsafeCell;
use alloc::sync::{Arc, Weak};
use owner_monad::OwnerMut;
use super::{
handle_event, select, Context, Event, EventHandle, GenericSleep, Instant, Mutex, Selectable,
Task,
};
use crate::{error::Error, select};
/// Represents an ongoing operation which produces a result.
pub struct Promise<T: 'static = ()>(Arc<Mutex<PromiseData<T>>>);
impl<T: Send + Sync + 'static> Promise<T> {
/// Creates a new lightweight promise and an associated resolve function.
///
/// # Example
/// ```
/// let (promise, resolve) = Promise::<i32>::new();
/// Task::spawn(|| {
/// Task::delay(Duration::from_secs(1));
/// resolve(10);
/// })
/// .unwrap();
/// println!(
/// "n = {}",
/// select! {
/// n = promise.done() => n,
/// }
/// );
/// ```
pub fn new() -> (Self, impl FnOnce(T) + Send) {
let data = Arc::new(Mutex::new(PromiseData::Incomplete(Event::new())));
let promise = Self(data.clone());
let resolve = move |r: T| {
let mut l = data.lock();
if let Some(e) = l.event() {
e.notify();
*l = PromiseData::Complete(r.into());
}
};
(promise, resolve)
}
/// A [`Selectable`] event which occurs when the promise is resolved.
pub fn done(&'_ self) -> impl Selectable<&'_ T> + '_ {
struct PromiseSelect<'a, T: 'static> {
promise: &'a Promise<T>,
handle: EventHandle<PromiseHandle<T>>,
}
impl<'a, T> Selectable<&'a T> for PromiseSelect<'a, T> {
fn poll(self) -> Result<&'a T, Self> {
self.promise
.0
.lock()
.result()
// This is safe, since the promise can only be resolved once (thanks to the
// resolve function being FnOnce) and the promise's contract and API ensure that
// the result is never modified once set. The reference is safe since it is
// defined to last only as long as the `Context` object, which has an `Arc`
// reference to the object containing the result.
.map(|r| unsafe { &*UnsafeCell::<T>::raw_get(r) })
.ok_or(self)
}
#[inline]
fn sleep(&self) -> GenericSleep {
if self.handle.is_done() {
GenericSleep::Timestamp(Instant::from_millis(0))
} else {
GenericSleep::NotifyTake(None)
}
}
}
PromiseSelect {
promise: self,
handle: handle_event(PromiseHandle(Arc::downgrade(&self.0))),
}
}
#[inline]
/// Spawns a task to run the given function and returns a [`Promise`] that
/// resolves with the result when it returns. Panics on failure; see
/// [`Promise::try_spawn()`].
pub fn spawn(f: impl FnOnce() -> T + Send + 'static) -> Self {
Self::try_spawn(f).unwrap()
}
/// Spawns a task to run the given function and returns a [`Promise`] that
/// resolves with the result when it returns.
pub fn try_spawn(f: impl FnOnce() -> T + Send + 'static) -> Result<Self, Error> {
let (promise, resolve) = Self::new();
Task::spawn(|| resolve(f()))?;
Ok(promise)
}
#[inline]
/// Spawns a new promise which, upon the completion of `self`, runs `f`.
pub fn then<U: Send + Sync + 'static>(
&self,
f: impl FnOnce(&T) -> U + Send + 'static,
) -> Promise<U> {
let upstream = self.clone();
Promise::spawn(move || f(select(upstream.done())))
}
#[inline]
/// Spawns a new promise which, upon the completion of `self`, runs `f`. If
/// `ctx` is cancelled before `self` completes, the result is `default`.
pub fn then_or<U: Send + Sync + 'static>(
&self,
ctx: Context,
default: U,
f: impl FnOnce(&T) -> U + Send + 'static,
) -> Promise<U> {
let upstream = self.clone();
Promise::spawn(move || {
select! {
v = upstream.done() => f(v),
_ = ctx.done() => default,
}
})
}
#[inline]
/// Spawns a new promise which, upon the completion of `self`, runs `f`. If
/// `ctx` is cancelled before `self` completes, the result is `None`.
pub fn then_some<U: Send + Sync + 'static>(
&self,
ctx: Context,
f: impl FnOnce(&T) -> U + Send + 'static,
) -> Promise<Option<U>> {
self.then_or(ctx, None, |v| Some(f(v)))
}
}
impl<T: 'static> Clone for Promise<T> {
fn clone(&self) -> Self {
Self(self.0.clone())
}
}
enum PromiseData<T> {
Incomplete(Event),
Complete(UnsafeCell<T>),
}
impl<T> PromiseData<T> {
#[inline]
fn event(&mut self) -> Option<&mut Event> {
match self {
PromiseData::Incomplete(e) => Some(e),
PromiseData::Complete(_) => None,
}
}
#[inline]
fn result(&self) -> Option<&UnsafeCell<T>> {
match self {
PromiseData::Incomplete(_) => None,
PromiseData::Complete(r) => Some(r),
}
}
}
unsafe impl<T: Send> Send for PromiseData<T> {}
unsafe impl<T: Sync> Sync for PromiseData<T> {}
struct PromiseHandle<T>(Weak<Mutex<PromiseData<T>>>);
impl<T> OwnerMut<Event> for PromiseHandle<T> {
fn with<'a, U>(&'a mut self, f: impl FnOnce(&mut Event) -> U) -> Option<U>
where
Event: 'a,
{
Some(f(self.0.upgrade()?.as_ref().lock().event()?))
}
}