11 files changed, 452 insertions, 20 deletions

diff --git a/core/src/command.rs b/futures/src/command.rs
index e7885fb8..e7885fb8 100644
--- a/core/src/command.rs
+++ b/futures/src/command.rs
diff --git a/futures/src/executor.rs b/futures/src/executor.rs
new file mode 100644
index 00000000..c2b9cc72
--- /dev/null
+++ b/futures/src/executor.rs
@@ -0,0 +1,55 @@
+//! Choose your preferred executor to power a runtime.
+mod null;
+
+#[cfg(feature = "thread-pool")]
+mod thread_pool;
+
+#[cfg(feature = "tokio")]
+mod tokio;
+
+#[cfg(feature = "async-std")]
+mod async_std;
+
+#[cfg(target_arch = "wasm32")]
+mod wasm_bindgen;
+
+pub use null::Null;
+
+#[cfg(feature = "thread-pool")]
+pub use thread_pool::ThreadPool;
+
+#[cfg(feature = "tokio")]
+pub use self::tokio::Tokio;
+
+#[cfg(feature = "async-std")]
+pub use self::async_std::AsyncStd;
+
+#[cfg(target_arch = "wasm32")]
+pub use wasm_bindgen::WasmBindgen;
+
+use futures::Future;
+
+/// A type that can run futures.
+pub trait Executor: Sized {
+    /// Creates a new [`Executor`].
+    ///
+    /// [`Executor`]: trait.Executor.html
+    fn new() -> Result<Self, futures::io::Error>
+    where
+        Self: Sized;
+
+    /// Spawns a future in the [`Executor`].
+    ///
+    /// [`Executor`]: trait.Executor.html
+    fn spawn(&self, future: impl Future<Output = ()> + Send + 'static);
+
+    /// Runs the given closure inside the [`Executor`].
+    ///
+    /// Some executors, like `tokio`, require some global state to be in place
+    /// before creating futures. This method can be leveraged to set up this
+    /// global state, call a function, restore the state, and obtain the result
+    /// of the call.
+    fn enter<R>(&self, f: impl FnOnce() -> R) -> R {
+        f()
+    }
+}
diff --git a/futures/src/executor/async_std.rs b/futures/src/executor/async_std.rs
new file mode 100644
index 00000000..27949e31
--- /dev/null
+++ b/futures/src/executor/async_std.rs
@@ -0,0 +1,17 @@
+use crate::Executor;
+
+use futures::Future;
+
+/// An `async-std` runtime.
+#[derive(Debug)]
+pub struct AsyncStd;
+
+impl Executor for AsyncStd {
+    fn new() -> Result<Self, futures::io::Error> {
+        Ok(Self)
+    }
+
+    fn spawn(&self, future: impl Future<Output = ()> + Send + 'static) {
+        let _ = async_std::task::spawn(future);
+    }
+}
diff --git a/futures/src/executor/null.rs b/futures/src/executor/null.rs
new file mode 100644
index 00000000..6d5cf982
--- /dev/null
+++ b/futures/src/executor/null.rs
@@ -0,0 +1,15 @@
+use crate::Executor;
+
+use futures::Future;
+
+/// An executor that drops all the futures, instead of spawning them.
+#[derive(Debug)]
+pub struct Null;
+
+impl Executor for Null {
+    fn new() -> Result<Self, futures::io::Error> {
+        Ok(Self)
+    }
+
+    fn spawn(&self, _future: impl Future<Output = ()> + Send + 'static) {}
+}
diff --git a/futures/src/executor/thread_pool.rs b/futures/src/executor/thread_pool.rs
new file mode 100644
index 00000000..1ec5bf69
--- /dev/null
+++ b/futures/src/executor/thread_pool.rs
@@ -0,0 +1,16 @@
+use crate::Executor;
+
+use futures::Future;
+
+/// A thread pool runtime for futures.
+pub type ThreadPool = futures::executor::ThreadPool;
+
+impl Executor for futures::executor::ThreadPool {
+    fn new() -> Result<Self, futures::io::Error> {
+        futures::executor::ThreadPool::new()
+    }
+
+    fn spawn(&self, future: impl Future<Output = ()> + Send + 'static) {
+        self.spawn_ok(future);
+    }
+}
diff --git a/futures/src/executor/tokio.rs b/futures/src/executor/tokio.rs
new file mode 100644
index 00000000..20802ceb
--- /dev/null
+++ b/futures/src/executor/tokio.rs
@@ -0,0 +1,20 @@
+use crate::Executor;
+
+use futures::Future;
+
+/// A `tokio` runtime.
+pub type Tokio = tokio::runtime::Runtime;
+
+impl Executor for Tokio {
+    fn new() -> Result<Self, futures::io::Error> {
+        tokio::runtime::Runtime::new()
+    }
+
+    fn spawn(&self, future: impl Future<Output = ()> + Send + 'static) {
+        let _ = tokio::runtime::Runtime::spawn(self, future);
+    }
+
+    fn enter<R>(&self, f: impl FnOnce() -> R) -> R {
+        tokio::runtime::Runtime::enter(self, f)
+    }
+}
diff --git a/futures/src/executor/wasm_bindgen.rs b/futures/src/executor/wasm_bindgen.rs
new file mode 100644
index 00000000..69b7c7e2
--- /dev/null
+++ b/futures/src/executor/wasm_bindgen.rs
@@ -0,0 +1,18 @@
+use crate::Executor;
+
+/// A `wasm-bindgen-futures` runtime.
+#[derive(Debug)]
+pub struct WasmBindgen;
+
+impl Executor for WasmBindgen {
+    fn new() -> Result<Self, futures::io::Error> {
+        Ok(Self)
+    }
+
+    fn spawn(
+        &self,
+        future: impl futures::Future<Output = ()> + Send + 'static,
+    ) {
+        wasm_bindgen_futures::spawn_local(future);
+    }
+}
diff --git a/futures/src/lib.rs b/futures/src/lib.rs
new file mode 100644
index 00000000..4872df10
--- /dev/null
+++ b/futures/src/lib.rs
@@ -0,0 +1,18 @@
+//! Asynchronous tasks for GUI programming, inspired by Elm.
+#![deny(missing_docs)]
+#![deny(missing_debug_implementations)]
+#![deny(unused_results)]
+#![deny(unsafe_code)]
+#![deny(rust_2018_idioms)]
+pub use futures;
+
+mod command;
+mod runtime;
+
+pub mod executor;
+pub mod subscription;
+
+pub use command::Command;
+pub use executor::Executor;
+pub use runtime::Runtime;
+pub use subscription::Subscription;
diff --git a/futures/src/runtime.rs b/futures/src/runtime.rs
new file mode 100644
index 00000000..9fd9899a
--- /dev/null
+++ b/futures/src/runtime.rs
@@ -0,0 +1,119 @@
+//! Run commands and keep track of subscriptions.
+use crate::{subscription, Command, Executor, Subscription};
+
+use futures::Sink;
+use std::marker::PhantomData;
+
+/// A batteries-included runtime of commands and subscriptions.
+///
+/// If you have an [`Executor`], a [`Runtime`] can be leveraged to run any
+/// [`Command`] or [`Subscription`] and get notified of the results!
+///
+/// [`Runtime`]: struct.Runtime.html
+/// [`Executor`]: executor/trait.Executor.html
+/// [`Command`]: struct.Command.html
+/// [`Subscription`]: subscription/struct.Subscription.html
+#[derive(Debug)]
+pub struct Runtime<Hasher, Event, Executor, Sender, Message> {
+    executor: Executor,
+    sender: Sender,
+    subscriptions: subscription::Tracker<Hasher, Event>,
+    _message: PhantomData<Message>,
+}
+
+impl<Hasher, Event, Executor, Sender, Message>
+    Runtime<Hasher, Event, Executor, Sender, Message>
+where
+    Hasher: std::hash::Hasher + Default,
+    Event: Send + Clone + 'static,
+    Executor: self::Executor,
+    Sender: Sink<Message, Error = core::convert::Infallible>
+        + Unpin
+        + Send
+        + Clone
+        + 'static,
+    Message: Send + 'static,
+{
+    /// Creates a new empty [`Runtime`].
+    ///
+    /// You need to provide:
+    /// - an [`Executor`] to spawn futures
+    /// - a `Sender` implementing `Sink` to receive the results
+    ///
+    /// [`Runtime`]: struct.Runtime.html
+    pub fn new(executor: Executor, sender: Sender) -> Self {
+        Self {
+            executor,
+            sender,
+            subscriptions: subscription::Tracker::new(),
+            _message: PhantomData,
+        }
+    }
+
+    /// Runs the given closure inside the [`Executor`] of the [`Runtime`].
+    ///
+    /// See [`Executor::enter`] to learn more.
+    ///
+    /// [`Executor`]: executor/trait.Executor.html
+    /// [`Runtime`]: struct.Runtime.html
+    /// [`Executor::enter`]: executor/trait.Executor.html#method.enter
+    pub fn enter<R>(&self, f: impl FnOnce() -> R) -> R {
+        self.executor.enter(f)
+    }
+
+    /// Spawns a [`Command`] in the [`Runtime`].
+    ///
+    /// The resulting `Message` will be forwarded to the `Sender` of the
+    /// [`Runtime`].
+    ///
+    /// [`Command`]: struct.Command.html
+    /// [`Runtime`]: struct.Runtime.html
+    pub fn spawn(&mut self, command: Command<Message>) {
+        use futures::{FutureExt, SinkExt};
+
+        let futures = command.futures();
+
+        for future in futures {
+            let mut sender = self.sender.clone();
+
+            self.executor.spawn(future.then(|message| {
+                async move {
+                    let _ = sender.send(message).await;
+
+                    ()
+                }
+            }));
+        }
+    }
+
+    /// Tracks a [`Subscription`] in the [`Runtime`].
+    ///
+    /// It will spawn new streams or close old ones as necessary! See
+    /// [`Tracker::update`] to learn more about this!
+    ///
+    /// [`Subscription`]: subscription/struct.Subscription.html
+    /// [`Runtime`]: struct.Runtime.html
+    /// [`Tracker::update`]: subscription/struct.Tracker.html#method.update
+    pub fn track(
+        &mut self,
+        subscription: Subscription<Hasher, Event, Message>,
+    ) {
+        let futures =
+            self.subscriptions.update(subscription, self.sender.clone());
+
+        for future in futures {
+            self.executor.spawn(future);
+        }
+    }
+
+    /// Broadcasts an event to all the subscriptions currently alive in the
+    /// [`Runtime`].
+    ///
+    /// See [`Tracker::broadcast`] to learn more.
+    ///
+    /// [`Runtime`]: struct.Runtime.html
+    /// [`Tracker::broadcast`]: subscription/struct.Tracker.html#method.broadcast
+    pub fn broadcast(&mut self, event: Event) {
+        self.subscriptions.broadcast(event);
+    }
+}
diff --git a/core/src/subscription.rs b/futures/src/subscription.rs
index d9e7e388..b68444cd 100644
--- a/core/src/subscription.rs
+++ b/futures/src/subscription.rs
@@ -1,4 +1,9 @@
 //! Listen to external events in your application.
+mod tracker;
+
+pub use tracker::Tracker;
+
+use futures::stream::BoxStream;
 
 /// A request to listen to external events.
 ///
@@ -11,16 +16,16 @@
 /// For instance, you can use a [`Subscription`] to listen to a WebSocket
 /// connection, keyboard presses, mouse events, time ticks, etc.
 ///
-/// This type is normally aliased by runtimes with a specific `Input` and/or
+/// This type is normally aliased by runtimes with a specific `Event` and/or
 /// `Hasher`.
 ///
 /// [`Command`]: ../struct.Command.html
 /// [`Subscription`]: struct.Subscription.html
-pub struct Subscription<Hasher, Input, Output> {
-    recipes: Vec<Box<dyn Recipe<Hasher, Input, Output = Output>>>,
+pub struct Subscription<Hasher, Event, Output> {
+    recipes: Vec<Box<dyn Recipe<Hasher, Event, Output = Output>>>,
 }
 
-impl<H, I, O> Subscription<H, I, O>
+impl<H, E, O> Subscription<H, E, O>
 where
     H: std::hash::Hasher,
 {
@@ -38,7 +43,7 @@ where
     /// [`Subscription`]: struct.Subscription.html
     /// [`Recipe`]: trait.Recipe.html
     pub fn from_recipe(
-        recipe: impl Recipe<H, I, Output = O> + 'static,
+        recipe: impl Recipe<H, E, Output = O> + 'static,
     ) -> Self {
         Self {
             recipes: vec![Box::new(recipe)],
@@ -50,7 +55,7 @@ where
     ///
     /// [`Subscription`]: struct.Subscription.html
     pub fn batch(
-        subscriptions: impl IntoIterator<Item = Subscription<H, I, O>>,
+        subscriptions: impl IntoIterator<Item = Subscription<H, E, O>>,
     ) -> Self {
         Self {
             recipes: subscriptions
@@ -63,7 +68,7 @@ where
     /// Returns the different recipes of the [`Subscription`].
     ///
     /// [`Subscription`]: struct.Subscription.html
-    pub fn recipes(self) -> Vec<Box<dyn Recipe<H, I, Output = O>>> {
+    pub fn recipes(self) -> Vec<Box<dyn Recipe<H, E, Output = O>>> {
         self.recipes
     }
 
@@ -73,10 +78,10 @@ where
     pub fn map<A>(
         mut self,
         f: impl Fn(O) -> A + Send + Sync + 'static,
-    ) -> Subscription<H, I, A>
+    ) -> Subscription<H, E, A>
     where
         H: 'static,
-        I: 'static,
+        E: 'static,
         O: 'static,
         A: 'static,
     {
@@ -88,7 +93,7 @@ where
                 .drain(..)
                 .map(|recipe| {
                     Box::new(Map::new(recipe, function.clone()))
-                        as Box<dyn Recipe<H, I, Output = A>>
+                        as Box<dyn Recipe<H, E, Output = A>>
                 })
                 .collect(),
         }
@@ -109,7 +114,7 @@ impl<I, O, H> std::fmt::Debug for Subscription<I, O, H> {
 ///
 /// [`Subscription`]: struct.Subscription.html
 /// [`Recipe`]: trait.Recipe.html
-pub trait Recipe<Hasher: std::hash::Hasher, Input> {
+pub trait Recipe<Hasher: std::hash::Hasher, Event> {
     /// The events that will be produced by a [`Subscription`] with this
     /// [`Recipe`].
     ///
@@ -128,31 +133,32 @@ pub trait Recipe<Hasher: std::hash::Hasher, Input> {
     /// Executes the [`Recipe`] and produces the stream of events of its
     /// [`Subscription`].
     ///
-    /// It receives some generic `Input`, which is normally defined by runtimes.
+    /// It receives some stream of generic events, which is normally defined by
+    /// shells.
     ///
     /// [`Subscription`]: struct.Subscription.html
     /// [`Recipe`]: trait.Recipe.html
     fn stream(
         self: Box<Self>,
-        input: Input,
-    ) -> futures::stream::BoxStream<'static, Self::Output>;
+        input: BoxStream<'static, Event>,
+    ) -> BoxStream<'static, Self::Output>;
 }
 
-struct Map<Hasher, Input, A, B> {
-    recipe: Box<dyn Recipe<Hasher, Input, Output = A>>,
+struct Map<Hasher, Event, A, B> {
+    recipe: Box<dyn Recipe<Hasher, Event, Output = A>>,
     mapper: std::sync::Arc<dyn Fn(A) -> B + Send + Sync>,
 }
 
-impl<H, I, A, B> Map<H, I, A, B> {
+impl<H, E, A, B> Map<H, E, A, B> {
     fn new(
-        recipe: Box<dyn Recipe<H, I, Output = A>>,
+        recipe: Box<dyn Recipe<H, E, Output = A>>,
         mapper: std::sync::Arc<dyn Fn(A) -> B + Send + Sync + 'static>,
     ) -> Self {
         Map { recipe, mapper }
     }
 }
 
-impl<H, I, A, B> Recipe<H, I> for Map<H, I, A, B>
+impl<H, E, A, B> Recipe<H, E> for Map<H, E, A, B>
 where
     A: 'static,
     B: 'static,
@@ -169,7 +175,7 @@ where
 
     fn stream(
         self: Box<Self>,
-        input: I,
+        input: BoxStream<'static, E>,
     ) -> futures::stream::BoxStream<'static, Self::Output> {
         use futures::StreamExt;
 
diff --git a/futures/src/subscription/tracker.rs b/futures/src/subscription/tracker.rs
new file mode 100644
index 00000000..c8a1ee18
--- /dev/null
+++ b/futures/src/subscription/tracker.rs
@@ -0,0 +1,148 @@
+use crate::Subscription;
+
+use futures::{future::BoxFuture, sink::Sink};
+use std::collections::HashMap;
+use std::marker::PhantomData;
+
+/// A registry of subscription streams.
+///
+/// If you have an application that continuously returns a [`Subscription`],
+/// you can use a [`Tracker`] to keep track of the different recipes and keep
+/// its executions alive.
+#[derive(Debug)]
+pub struct Tracker<Hasher, Event> {
+    subscriptions: HashMap<u64, Execution<Event>>,
+    _hasher: PhantomData<Hasher>,
+}
+
+#[derive(Debug)]
+pub struct Execution<Event> {
+    _cancel: futures::channel::oneshot::Sender<()>,
+    listener: Option<futures::channel::mpsc::Sender<Event>>,
+}
+
+impl<Hasher, Event> Tracker<Hasher, Event>
+where
+    Hasher: std::hash::Hasher + Default,
+    Event: 'static + Send + Clone,
+{
+    /// Creates a new empty [`Tracker`].
+    ///
+    /// [`Tracker`]: struct.Tracker.html
+    pub fn new() -> Self {
+        Self {
+            subscriptions: HashMap::new(),
+            _hasher: PhantomData,
+        }
+    }
+
+    /// Updates the [`Tracker`] with the given [`Subscription`].
+    ///
+    /// A [`Subscription`] can cause new streams to be spawned or old streams
+    /// to be closed.
+    ///
+    /// The [`Tracker`] keeps track of these streams between calls to this
+    /// method:
+    ///
+    /// - If the provided [`Subscription`] contains a new [`Recipe`] that is
+    /// currently not being run, it will spawn a new stream and keep it alive.
+    /// - On the other hand, if a [`Recipe`] is currently in execution and the
+    /// provided [`Subscription`] does not contain it anymore, then the
+    /// [`Tracker`] will close and drop the relevant stream.
+    ///
+    /// It returns a list of futures that need to be spawned to materialize
+    /// the [`Tracker`] changes.
+    ///
+    /// [`Tracker`]: struct.Tracker.html
+    /// [`Subscription`]: struct.Subscription.html
+    /// [`Recipe`]: trait.Recipe.html
+    pub fn update<Message, Receiver>(
+        &mut self,
+        subscription: Subscription<Hasher, Event, Message>,
+        receiver: Receiver,
+    ) -> Vec<BoxFuture<'static, ()>>
+    where
+        Message: 'static + Send,
+        Receiver: 'static
+            + Sink<Message, Error = core::convert::Infallible>
+            + Unpin
+            + Send
+            + Clone,
+    {
+        use futures::{future::FutureExt, stream::StreamExt};
+
+        let mut futures = Vec::new();
+
+        let recipes = subscription.recipes();
+        let mut alive = std::collections::HashSet::new();
+
+        for recipe in recipes {
+            let id = {
+                let mut hasher = Hasher::default();
+                recipe.hash(&mut hasher);
+
+                hasher.finish()
+            };
+
+            let _ = alive.insert(id);
+
+            if self.subscriptions.contains_key(&id) {
+                continue;
+            }
+
+            let (cancel, cancelled) = futures::channel::oneshot::channel();
+
+            // TODO: Use bus if/when it supports async
+            let (event_sender, event_receiver) =
+                futures::channel::mpsc::channel(100);
+
+            let stream = recipe.stream(event_receiver.boxed());
+
+            let future = futures::future::select(
+                cancelled,
+                stream.map(Ok).forward(receiver.clone()),
+            )
+            .map(|_| ());
+
+            let _ = self.subscriptions.insert(
+                id,
+                Execution {
+                    _cancel: cancel,
+                    listener: if event_sender.is_closed() {
+                        None
+                    } else {
+                        Some(event_sender)
+                    },
+                },
+            );
+
+            futures.push(future.boxed());
+        }
+
+        self.subscriptions.retain(|id, _| alive.contains(&id));
+
+        futures
+    }
+
+    /// Broadcasts an event to the subscriptions currently alive.
+    ///
+    /// A subscription's [`Recipe::stream`] always receives a stream of events
+    /// as input. This stream can be used by some subscription to listen to
+    /// shell events.
+    ///
+    /// This method publishes the given event to all the subscription streams
+    /// currently open.
+    pub fn broadcast(&mut self, event: Event) {
+        self.subscriptions
+            .values_mut()
+            .filter_map(|connection| connection.listener.as_mut())
+            .for_each(|listener| {
+                if let Err(error) = listener.try_send(event.clone()) {
+                    log::error!(
+                        "Error sending event to subscription: {:?}",
+                        error
+                    );
+                }
+            });
+    }
+}