Hide `Subscription` internals

.. and introduce `stream::channel` helper

6 files changed, 205 insertions, 177 deletions

diff --git a/futures/src/backend/native/async_std.rs b/futures/src/backend/native/async_std.rs
index b7da5e90..86714f45 100644
--- a/futures/src/backend/native/async_std.rs
+++ b/futures/src/backend/native/async_std.rs
@@ -27,7 +27,7 @@ pub mod time {
     pub fn every(
         duration: std::time::Duration,
     ) -> Subscription<std::time::Instant> {
-        Subscription::from_recipe(Every(duration))
+        subscription::from_recipe(Every(duration))
     }
 
     #[derive(Debug)]
diff --git a/futures/src/backend/native/smol.rs b/futures/src/backend/native/smol.rs
index aaf1518c..8d448e7f 100644
--- a/futures/src/backend/native/smol.rs
+++ b/futures/src/backend/native/smol.rs
@@ -26,7 +26,7 @@ pub mod time {
     pub fn every(
         duration: std::time::Duration,
     ) -> Subscription<std::time::Instant> {
-        Subscription::from_recipe(Every(duration))
+        subscription::from_recipe(Every(duration))
     }
 
     #[derive(Debug)]
diff --git a/futures/src/backend/native/tokio.rs b/futures/src/backend/native/tokio.rs
index df91d798..9dc3593d 100644
--- a/futures/src/backend/native/tokio.rs
+++ b/futures/src/backend/native/tokio.rs
@@ -31,7 +31,7 @@ pub mod time {
     pub fn every(
         duration: std::time::Duration,
     ) -> Subscription<std::time::Instant> {
-        Subscription::from_recipe(Every(duration))
+        subscription::from_recipe(Every(duration))
     }
 
     #[derive(Debug)]
diff --git a/futures/src/lib.rs b/futures/src/lib.rs
index a874a618..31738823 100644
--- a/futures/src/lib.rs
+++ b/futures/src/lib.rs
@@ -15,6 +15,7 @@ pub mod backend;
 pub mod event;
 pub mod executor;
 pub mod keyboard;
+pub mod stream;
 pub mod subscription;
 
 pub use executor::Executor;
diff --git a/futures/src/stream.rs b/futures/src/stream.rs
new file mode 100644
index 00000000..2ec505f1
--- /dev/null
+++ b/futures/src/stream.rs
@@ -0,0 +1,26 @@
+//! Create asynchronous streams of data.
+use futures::channel::mpsc;
+use futures::never::Never;
+use futures::stream::{self, Stream, StreamExt};
+
+use std::future::Future;
+
+/// Creates a new [`Stream`] that produces the items sent from a [`Future`]
+/// to the [`mpsc::Sender`] provided to the closure.
+///
+/// This is a more ergonomic [`stream::unfold`], which allows you to go
+/// from the "world of futures" to the "world of streams" by simply looping
+/// and publishing to an async channel from inside a [`Future`].
+pub fn channel<T, F>(
+    size: usize,
+    f: impl FnOnce(mpsc::Sender<T>) -> F,
+) -> impl Stream<Item = T>
+where
+    F: Future<Output = Never>,
+{
+    let (sender, receiver) = mpsc::channel(size);
+
+    let runner = stream::once(f(sender)).map(|_| unreachable!());
+
+    stream::select(receiver, runner)
+}
diff --git a/futures/src/subscription.rs b/futures/src/subscription.rs
index 5ec39582..1a0d454d 100644
--- a/futures/src/subscription.rs
+++ b/futures/src/subscription.rs
@@ -5,11 +5,9 @@ pub use tracker::Tracker;
 
 use crate::core::event;
 use crate::core::window;
-use crate::futures::{Future, Stream};
+use crate::futures::Stream;
 use crate::{BoxStream, MaybeSend};
 
-use futures::channel::mpsc;
-use futures::never::Never;
 use std::any::TypeId;
 use std::hash::Hash;
 
@@ -61,20 +59,66 @@ pub type Hasher = rustc_hash::FxHasher;
 
 /// A request to listen to external events.
 ///
-/// Besides performing async actions on demand with `Command`, most
+/// Besides performing async actions on demand with `Task`, most
 /// applications also need to listen to external events passively.
 ///
-/// A [`Subscription`] is normally provided to some runtime, like a `Command`,
+/// A [`Subscription`] is normally provided to some runtime, like a `Task`,
 /// 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.
+///
+/// # The Lifetime of a [`Subscription`]
+/// Much like a [`Future`] or a [`Stream`], a [`Subscription`] does not produce any effects
+/// on its own. For a [`Subscription`] to run, it must be returned to the iced runtime—normally
+/// in the `subscription` function of an `application` or a `daemon`.
+///
+/// When a [`Subscription`] is provided to the runtime for the first time, the runtime will
+/// start running it asynchronously. Running a [`Subscription`] consists in building its underlying
+/// [`Stream`] and executing it in an async runtime.
+///
+/// Therefore, you can think of a [`Subscription`] as a "stream builder". It simply represents a way
+/// to build a certain [`Stream`] together with some way to _identify_ it.
+///
+/// Identification is important because when a specific [`Subscription`] stops being returned to the
+/// iced runtime, the runtime will kill its associated [`Stream`]. The runtime uses the identity of a
+/// [`Subscription`] to keep track of it.
+///
+/// This way, iced allows you to declaratively __subscribe__ to particular streams of data temporarily
+/// and whenever necessary.
+///
+/// ```
+/// # mod iced {
+/// #     pub mod time {
+/// #         pub use iced_futures::backend::default::time::every;
+/// #         pub use std::time::{Duration, Instant};
+/// #     }
+/// #
+/// #     pub use iced_futures::Subscription;
+/// # }
+/// use iced::time::{self, Duration, Instant};
+/// use iced::Subscription;
+///
+/// struct State {
+///     timer_enabled: bool,
+/// }
+///
+/// fn subscription(state: &State) -> Subscription<Instant> {
+///     if state.timer_enabled {
+///         time::every(Duration::from_secs(1))
+///     } else {
+///         Subscription::none()
+///     }
+/// }
+/// ```
+///
+/// [`Future`]: std::future::Future
 #[must_use = "`Subscription` must be returned to runtime to take effect"]
-pub struct Subscription<Message> {
-    recipes: Vec<Box<dyn Recipe<Output = Message>>>,
+pub struct Subscription<T> {
+    recipes: Vec<Box<dyn Recipe<Output = T>>>,
 }
 
-impl<Message> Subscription<Message> {
+impl<T> Subscription<T> {
     /// Returns an empty [`Subscription`] that will not produce any output.
     pub fn none() -> Self {
         Self {
@@ -82,19 +126,102 @@ impl<Message> Subscription<Message> {
         }
     }
 
-    /// Creates a [`Subscription`] from a [`Recipe`] describing it.
-    pub fn from_recipe(
-        recipe: impl Recipe<Output = Message> + 'static,
-    ) -> Self {
-        Self {
-            recipes: vec![Box::new(recipe)],
-        }
+    /// Returns a [`Subscription`] that will call the given function to create and
+    /// asynchronously run the given [`Stream`].
+    ///
+    /// # 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_futures::subscription::{self, Subscription};
+    /// use iced_futures::stream;
+    /// use iced_futures::futures::channel::mpsc;
+    /// use iced_futures::futures::sink::SinkExt;
+    /// use iced_futures::futures::Stream;
+    ///
+    /// pub enum Event {
+    ///     Ready(mpsc::Sender<Input>),
+    ///     WorkFinished,
+    ///     // ...
+    /// }
+    ///
+    /// enum Input {
+    ///     DoSomeWork,
+    ///     // ...
+    /// }
+    ///
+    /// fn some_worker() -> impl Stream<Item = Event> {
+    ///     stream::channel(100, |mut output| async move {
+    ///         // Create channel
+    ///         let (sender, mut receiver) = mpsc::channel(100);
+    ///
+    ///         // Send the sender back to the application
+    ///         output.send(Event::Ready(sender)).await;
+    ///
+    ///         loop {
+    ///             use iced_futures::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 produce a message to tell the
+    ///                     // `Application` the work is done
+    ///                     output.send(Event::WorkFinished).await;
+    ///                 }
+    ///             }
+    ///         }
+    ///     })
+    /// }
+    ///
+    /// fn subscription() -> Subscription<Event> {
+    ///     Subscription::run(some_worker)
+    /// }
+    /// ```
+    ///
+    /// Check out the [`websocket`] example, which showcases this pattern to maintain a `WebSocket`
+    /// connection open.
+    ///
+    /// [`websocket`]: https://github.com/iced-rs/iced/tree/0.12/examples/websocket
+    pub fn run<S>(builder: fn() -> S) -> Self
+    where
+        S: Stream<Item = T> + MaybeSend + 'static,
+        T: 'static,
+    {
+        from_recipe(Runner {
+            id: builder,
+            spawn: move |_| builder(),
+        })
+    }
+
+    /// 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_with_id<I, S>(id: I, stream: S) -> Subscription<T>
+    where
+        I: Hash + 'static,
+        S: Stream<Item = T> + MaybeSend + 'static,
+        T: 'static,
+    {
+        from_recipe(Runner {
+            id,
+            spawn: move |_| stream,
+        })
     }
 
     /// Batches all the provided subscriptions and returns the resulting
     /// [`Subscription`].
     pub fn batch(
-        subscriptions: impl IntoIterator<Item = Subscription<Message>>,
+        subscriptions: impl IntoIterator<Item = Subscription<T>>,
     ) -> Self {
         Self {
             recipes: subscriptions
@@ -104,18 +231,13 @@ impl<Message> Subscription<Message> {
         }
     }
 
-    /// Returns the different recipes of the [`Subscription`].
-    pub fn into_recipes(self) -> Vec<Box<dyn Recipe<Output = Message>>> {
-        self.recipes
-    }
-
     /// Adds a value to the [`Subscription`] context.
     ///
     /// The value will be part of the identity of a [`Subscription`].
-    pub fn with<T>(mut self, value: T) -> Subscription<(T, Message)>
+    pub fn with<A>(mut self, value: A) -> Subscription<(A, T)>
     where
-        Message: 'static,
-        T: std::hash::Hash + Clone + Send + Sync + 'static,
+        T: 'static,
+        A: std::hash::Hash + Clone + Send + Sync + 'static,
     {
         Subscription {
             recipes: self
@@ -123,7 +245,7 @@ impl<Message> Subscription<Message> {
                 .drain(..)
                 .map(|recipe| {
                     Box::new(With::new(recipe, value.clone()))
-                        as Box<dyn Recipe<Output = (T, Message)>>
+                        as Box<dyn Recipe<Output = (A, T)>>
                 })
                 .collect(),
         }
@@ -136,8 +258,8 @@ impl<Message> Subscription<Message> {
     /// will panic in debug mode otherwise.
     pub fn map<F, A>(mut self, f: F) -> Subscription<A>
     where
-        Message: 'static,
-        F: Fn(Message) -> A + MaybeSend + Clone + 'static,
+        T: 'static,
+        F: Fn(T) -> A + MaybeSend + Clone + 'static,
         A: 'static,
     {
         debug_assert!(
@@ -159,7 +281,23 @@ impl<Message> Subscription<Message> {
     }
 }
 
-impl<Message> std::fmt::Debug for Subscription<Message> {
+/// Creates a [`Subscription`] from a [`Recipe`] describing it.
+pub fn from_recipe<T>(
+    recipe: impl Recipe<Output = T> + 'static,
+) -> Subscription<T> {
+    Subscription {
+        recipes: vec![Box::new(recipe)],
+    }
+}
+
+/// Returns the different recipes of the [`Subscription`].
+pub fn into_recipes<T>(
+    subscription: Subscription<T>,
+) -> Vec<Box<dyn Recipe<Output = T>>> {
+    subscription.recipes
+}
+
+impl<T> std::fmt::Debug for Subscription<T> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         f.debug_struct("Subscription").finish()
     }
@@ -273,65 +411,13 @@ where
     }
 }
 
-/// Returns a [`Subscription`] that will call the given function to create and
-/// asynchronously run the given [`Stream`].
-pub fn run<S, Message>(builder: fn() -> S) -> Subscription<Message>
-where
-    S: Stream<Item = Message> + MaybeSend + 'static,
-    Message: 'static,
-{
-    Subscription::from_recipe(Runner {
-        id: builder,
-        spawn: move |_| builder(),
-    })
-}
-
-/// 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_with_id<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`].
-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 = (Message, T)> + MaybeSend + 'static,
-    Message: 'static + MaybeSend,
-{
-    use futures::future::FutureExt;
-
-    run_with_id(
-        id,
-        futures::stream::unfold(initial, move |state| f(state).map(Some)),
-    )
-}
-
-pub(crate) fn filter_map<I, F, Message>(id: I, f: F) -> Subscription<Message>
+pub(crate) fn filter_map<I, F, T>(id: I, f: F) -> Subscription<T>
 where
     I: Hash + 'static,
-    F: Fn(Event) -> Option<Message> + MaybeSend + 'static,
-    Message: 'static + MaybeSend,
+    F: Fn(Event) -> Option<T> + MaybeSend + 'static,
+    T: 'static + MaybeSend,
 {
-    Subscription::from_recipe(Runner {
+    from_recipe(Runner {
         id,
         spawn: |events| {
             use futures::future;
@@ -342,107 +428,22 @@ where
     })
 }
 
-/// Creates a [`Subscription`] that publishes the events sent from a [`Future`]
-/// to an [`mpsc::Sender`] with the given bounds.
-///
-/// # 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_futures::subscription::{self, Subscription};
-/// use iced_futures::futures::channel::mpsc;
-/// use iced_futures::futures::sink::SinkExt;
-///
-/// pub enum Event {
-///     Ready(mpsc::Sender<Input>),
-///     WorkFinished,
-///     // ...
-/// }
-///
-/// enum Input {
-///     DoSomeWork,
-///     // ...
-/// }
-///
-/// fn some_worker() -> Subscription<Event> {
-///     struct SomeWorker;
-///
-///     subscription::channel(std::any::TypeId::of::<SomeWorker>(), 100, |mut output| async move {
-///         // Create channel
-///         let (sender, mut receiver) = mpsc::channel(100);
-///
-///         // Send the sender back to the application
-///         output.send(Event::Ready(sender)).await;
-///
-///         loop {
-///             use iced_futures::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 produce a message to tell the
-///                     // `Application` the work is done
-///                     output.send(Event::WorkFinished).await;
-///                 }
-///             }
-///         }
-///     })
-/// }
-/// ```
-///
-/// Check out the [`websocket`] example, which showcases this pattern to maintain a `WebSocket`
-/// connection open.
-///
-/// [`websocket`]: https://github.com/iced-rs/iced/tree/0.12/examples/websocket
-pub fn channel<I, Fut, Message>(
-    id: I,
-    size: usize,
-    f: impl FnOnce(mpsc::Sender<Message>) -> Fut + MaybeSend + 'static,
-) -> Subscription<Message>
-where
-    I: Hash + 'static,
-    Fut: Future<Output = Never> + MaybeSend + 'static,
-    Message: 'static + MaybeSend,
-{
-    use futures::stream::{self, StreamExt};
-
-    Subscription::from_recipe(Runner {
-        id,
-        spawn: move |_| {
-            let (sender, receiver) = mpsc::channel(size);
-
-            let runner = stream::once(f(sender)).map(|_| unreachable!());
-
-            stream::select(receiver, runner)
-        },
-    })
-}
-
-struct Runner<I, F, S, Message>
+struct Runner<I, F, S, T>
 where
     F: FnOnce(EventStream) -> S,
-    S: Stream<Item = Message>,
+    S: Stream<Item = T>,
 {
     id: I,
     spawn: F,
 }
 
-impl<I, S, F, Message> Recipe for Runner<I, F, S, Message>
+impl<I, F, S, T> Recipe for Runner<I, F, S, T>
 where
     I: Hash + 'static,
     F: FnOnce(EventStream) -> S,
-    S: Stream<Item = Message> + MaybeSend + 'static,
+    S: Stream<Item = T> + MaybeSend + 'static,
 {
-    type Output = Message;
+    type Output = T;
 
     fn hash(&self, state: &mut Hasher) {
         std::any::TypeId::of::<I>().hash(state);