//! Listen to external events in your application.
use crate::event::{self, Event};
use crate::Hasher;

use iced_futures::futures::{self, Future, Stream};
use iced_futures::{BoxStream, MaybeSend};

use std::hash::Hash;

/// A request to listen to external events.
///
/// Besides performing async actions on demand with [`Command`], most
/// applications also need to listen to external events passively.
///
/// A [`Subscription`] is normally provided to some runtime, like a [`Command`],
/// and it will generate events as long as the user keeps requesting it.
///
/// For instance, you can use a [`Subscription`] to listen to a WebSocket
/// connection, keyboard presses, mouse events, time ticks, etc.
///
/// [`Command`]: crate::Command
pub type Subscription<T> =
    iced_futures::Subscription<Hasher, (Event, event::Status), T>;

/// A stream of runtime events.
///
/// It is the input of a [`Subscription`] in the native runtime.
pub type EventStream = BoxStream<(Event, event::Status)>;

/// A native [`Subscription`] tracker.
pub type Tracker =
    iced_futures::subscription::Tracker<Hasher, (Event, event::Status)>;

pub use iced_futures::subscription::Recipe;

/// Returns a [`Subscription`] to all the runtime events.
///
/// This subscription will notify your application of any [`Event`] that was
/// not captured by any widget.
pub fn events() -> Subscription<Event> {
    events_with(|event, status| match status {
        event::Status::Ignored => Some(event),
        event::Status::Captured => None,
    })
}

/// Returns a [`Subscription`] that filters all the runtime events with the
/// provided function, producing messages accordingly.
///
/// This subscription will call the provided function for every [`Event`]
/// handled by the runtime. If the function:
///
/// - Returns `None`, the [`Event`] will be discarded.
/// - Returns `Some` message, the `Message` will be produced.
pub fn events_with<Message>(
    f: fn(Event, event::Status) -> Option<Message>,
) -> Subscription<Message>
where
    Message: 'static + MaybeSend,
{
    Subscription::from_recipe(Runner {
        id: f,
        spawn: move |events| {
            use futures::future;
            use futures::stream::StreamExt;

            events.filter_map(move |(event, status)| {
                future::ready(f(event, status))
            })
        },
    })
}

/// Returns a [`Subscription`] that will create and asynchronously run the
/// given [`Stream`].
///
/// The `id` will be used to uniquely identify the [`Subscription`].
pub fn run<I, S, Message>(id: I, stream: S) -> Subscription<Message>
where
    I: Hash + 'static,
    S: Stream<Item = Message> + MaybeSend + 'static,
    Message: 'static,
{
    Subscription::from_recipe(Runner {
        id,
        spawn: move |_| stream,
    })
}

/// Returns a [`Subscription`] that will create and asynchronously run a
/// [`Stream`] that will call the provided closure to produce every `Message`.
///
/// The `id` will be used to uniquely identify the [`Subscription`].
///
/// # Creating an asynchronous worker with bidirectional communication
/// You can leverage this helper to create a [`Subscription`] that spawns
/// an asynchronous worker in the background and establish a channel of
/// communication with an `iced` application.
///
/// You can achieve this by creating an `mpsc` channel inside the closure
/// and returning the `Sender` as a `Message` for the `Application`:
///
/// ```
/// use iced_native::subscription::{self, Subscription};
/// use iced_native::futures::channel::mpsc;
///
/// pub enum Event {
///     Ready(mpsc::Sender<Input>),
///     WorkFinished,
///     // ...
/// }
///
/// enum Input {
///     DoSomeWork,
///     // ...
/// }
///
/// enum State {
///     Starting,
///     Ready(mpsc::Receiver<Input>),
/// }
///
/// fn some_worker() -> Subscription<Event> {
///     struct SomeWorker;
///
///     subscription::unfold(std::any::TypeId::of::<SomeWorker>(), State::Starting, |state| async move {
///         match state {
///             State::Starting => {
///                 // Create channel
///                 let (sender, receiver) = mpsc::channel(100);
///
///                 (Some(Event::Ready(sender)), State::Ready(receiver))
///             }
///             State::Ready(mut receiver) => {
///                 use iced_native::futures::StreamExt;
///
///                 // Read next input sent from `Application`
///                 let input = receiver.select_next_some().await;
///
///                 match input {
///                     Input::DoSomeWork => {
///                         // Do some async work...
///
///                         // Finally, we can optionally return a message to tell the
///                         // `Application` the work is done
///                         (Some(Event::WorkFinished), State::Ready(receiver))
///                     }
///                 }
///             }
///         }
///     })
/// }
/// ```
///
/// Check out the [`websocket`] example, which showcases this pattern to maintain a WebSocket
/// connection open.
///
/// [`websocket`]: https://github.com/iced-rs/iced/tree/0.4/examples/websocket
pub fn unfold<I, T, Fut, Message>(
    id: I,
    initial: T,
    mut f: impl FnMut(T) -> Fut + MaybeSend + Sync + 'static,
) -> Subscription<Message>
where
    I: Hash + 'static,
    T: MaybeSend + 'static,
    Fut: Future<Output = (Option<Message>, T)> + MaybeSend + 'static,
    Message: 'static + MaybeSend,
{
    use futures::future::{self, FutureExt};
    use futures::stream::StreamExt;

    run(
        id,
        futures::stream::unfold(initial, move |state| f(state).map(Some))
            .filter_map(future::ready),
    )
}

struct Runner<I, F, S, Message>
where
    F: FnOnce(EventStream) -> S,
    S: Stream<Item = Message>,
{
    id: I,
    spawn: F,
}

impl<I, S, F, Message> Recipe<Hasher, (Event, event::Status)>
    for Runner<I, F, S, Message>
where
    I: Hash + 'static,
    F: FnOnce(EventStream) -> S,
    S: Stream<Item = Message> + MaybeSend + 'static,
{
    type Output = Message;

    fn hash(&self, state: &mut Hasher) {
        std::any::TypeId::of::<I>().hash(state);
        self.id.hash(state);
    }

    fn stream(self: Box<Self>, input: EventStream) -> BoxStream<Self::Output> {
        iced_futures::boxed_stream((self.spawn)(input))
    }
}