Draft first version of event subscriptions :tada:

11 files changed, 182 insertions, 12 deletions

diff --git a/Cargo.toml b/Cargo.toml
index 888a1c07..a07da082 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -41,6 +41,8 @@ serde_json = "1.0"
 directories = "2.0"
 reqwest = "0.9"
 rand = "0.7"
+chrono = "0.4"
+futures = "0.3"
 
 [target.'cfg(target_arch = "wasm32")'.dev-dependencies]
 wasm-bindgen = "0.2.51"
diff --git a/core/Cargo.toml b/core/Cargo.toml
index c623ba78..0a8fd8ef 100644
--- a/core/Cargo.toml
+++ b/core/Cargo.toml
@@ -10,6 +10,8 @@ repository = "https://github.com/hecrj/iced"
 [features]
 # Exposes a future-based `Command` type
 command = ["futures"]
+# Exposes a future-based `Subscription` type
+subscription = ["futures"]
 
 [dependencies]
 futures = { version = "0.3", optional = true }
diff --git a/core/src/lib.rs b/core/src/lib.rs
index 65304e8b..6f13c310 100644
--- a/core/src/lib.rs
+++ b/core/src/lib.rs
@@ -9,7 +9,7 @@
 //! [Iced]: https://github.com/hecrj/iced
 //! [`iced_native`]: https://github.com/hecrj/iced/tree/master/native
 //! [`iced_web`]: https://github.com/hecrj/iced/tree/master/web
-#![deny(missing_docs)]
+//#![deny(missing_docs)]
 #![deny(missing_debug_implementations)]
 #![deny(unused_results)]
 #![deny(unsafe_code)]
@@ -38,3 +38,9 @@ mod command;
 
 #[cfg(feature = "command")]
 pub use command::Command;
+
+#[cfg(feature = "subscription")]
+pub mod subscription;
+
+#[cfg(feature = "subscription")]
+pub use subscription::Subscription;
diff --git a/core/src/subscription.rs b/core/src/subscription.rs
new file mode 100644
index 00000000..1e6695d6
--- /dev/null
+++ b/core/src/subscription.rs
@@ -0,0 +1,61 @@
+//! Generate events asynchronously for you application.
+
+/// An event subscription.
+pub struct Subscription<T> {
+    definitions: Vec<Box<dyn Definition<Message = T>>>,
+}
+
+impl<T> Subscription<T> {
+    pub fn none() -> Self {
+        Self {
+            definitions: Vec::new(),
+        }
+    }
+
+    pub fn batch(subscriptions: impl Iterator<Item = Subscription<T>>) -> Self {
+        Self {
+            definitions: subscriptions
+                .flat_map(|subscription| subscription.definitions)
+                .collect(),
+        }
+    }
+
+    pub fn definitions(self) -> Vec<Box<dyn Definition<Message = T>>> {
+        self.definitions
+    }
+}
+
+impl<T, A> From<A> for Subscription<T>
+where
+    A: Definition<Message = T> + 'static,
+{
+    fn from(definition: A) -> Self {
+        Self {
+            definitions: vec![Box::new(definition)],
+        }
+    }
+}
+
+/// The definition of an event subscription.
+pub trait Definition {
+    type Message;
+
+    fn id(&self) -> u64;
+
+    fn stream(
+        &self,
+    ) -> (
+        futures::stream::BoxStream<'static, Self::Message>,
+        Box<dyn Handle>,
+    );
+}
+
+pub trait Handle {
+    fn cancel(&mut self);
+}
+
+impl<T> std::fmt::Debug for Subscription<T> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("Command").finish()
+    }
+}
diff --git a/native/Cargo.toml b/native/Cargo.toml
index 7993676e..c5f9c7ac 100644
--- a/native/Cargo.toml
+++ b/native/Cargo.toml
@@ -8,6 +8,6 @@ license = "MIT"
 repository = "https://github.com/hecrj/iced"
 
 [dependencies]
-iced_core = { version = "0.1.0", path = "../core", features = ["command"] }
+iced_core = { version = "0.1.0", path = "../core", features = ["command", "subscription"] }
 twox-hash = "1.5"
 raw-window-handle = "0.3"
diff --git a/native/src/lib.rs b/native/src/lib.rs
index 45c3c699..9afb3bc9 100644
--- a/native/src/lib.rs
+++ b/native/src/lib.rs
@@ -52,8 +52,8 @@ mod size;
 mod user_interface;
 
 pub use iced_core::{
-    Align, Background, Color, Command, Font, HorizontalAlignment, Length,
-    Point, Rectangle, Vector, VerticalAlignment,
+    subscription, Align, Background, Color, Command, Font, HorizontalAlignment,
+    Length, Point, Rectangle, Subscription, Vector, VerticalAlignment,
 };
 
 pub use element::Element;
diff --git a/native/src/widget/checkbox.rs b/native/src/widget/checkbox.rs
index 9563291c..159cba84 100644
--- a/native/src/widget/checkbox.rs
+++ b/native/src/widget/checkbox.rs
@@ -31,6 +31,7 @@ pub struct Checkbox<Message> {
     on_toggle: Box<dyn Fn(bool) -> Message>,
     label: String,
     label_color: Option<Color>,
+    width: Length,
 }
 
 impl<Message> Checkbox<Message> {
@@ -53,6 +54,7 @@ impl<Message> Checkbox<Message> {
             on_toggle: Box::new(f),
             label: String::from(label),
             label_color: None,
+            width: Length::Fill,
         }
     }
 
@@ -63,6 +65,14 @@ impl<Message> Checkbox<Message> {
         self.label_color = Some(color.into());
         self
     }
+
+    /// Sets the width of the [`Checkbox`].
+    ///
+    /// [`Checkbox`]: struct.Checkbox.html
+    pub fn width(mut self, width: Length) -> Self {
+        self.width = width;
+        self
+    }
 }
 
 impl<Message, Renderer> Widget<Message, Renderer> for Checkbox<Message>
@@ -70,7 +80,7 @@ where
     Renderer: self::Renderer + text::Renderer + row::Renderer,
 {
     fn width(&self) -> Length {
-        Length::Fill
+        Length::Shrink
     }
 
     fn height(&self) -> Length {
@@ -85,6 +95,7 @@ where
         let size = self::Renderer::default_size(renderer);
 
         Row::<(), Renderer>::new()
+            .width(self.width)
             .spacing(15)
             .align_items(Align::Center)
             .push(
@@ -92,7 +103,7 @@ where
                     .width(Length::Units(size as u16))
                     .height(Length::Units(size as u16)),
             )
-            .push(Text::new(&self.label))
+            .push(Text::new(&self.label).width(self.width))
             .layout(renderer, limits)
     }
 
diff --git a/src/application.rs b/src/application.rs
index a4d20e68..95113344 100644
--- a/src/application.rs
+++ b/src/application.rs
@@ -1,4 +1,4 @@
-use crate::{Command, Element, Settings};
+use crate::{Command, Element, Settings, Subscription};
 
 /// An interactive cross-platform application.
 ///
@@ -117,6 +117,11 @@ pub trait Application: Sized {
     /// [`Command`]: struct.Command.html
     fn update(&mut self, message: Self::Message) -> Command<Self::Message>;
 
+    /// TODO
+    fn subscriptions(&self) -> Subscription<Self::Message> {
+        Subscription::none()
+    }
+
     /// Returns the widgets to display in the [`Application`].
     ///
     /// These widgets can produce __messages__ based on user interaction.
@@ -168,6 +173,10 @@ where
         self.0.update(message)
     }
 
+    fn subscriptions(&self) -> Subscription<Self::Message> {
+        self.0.subscriptions()
+    }
+
     fn view(&mut self) -> Element<'_, Self::Message> {
         self.0.view()
     }
diff --git a/src/native.rs b/src/native.rs
index 3537dd52..e3733955 100644
--- a/src/native.rs
+++ b/src/native.rs
@@ -1,6 +1,6 @@
 pub use iced_winit::{
-    Align, Background, Color, Command, Font, HorizontalAlignment, Length,
-    VerticalAlignment,
+    subscription, Align, Background, Color, Command, Font, HorizontalAlignment,
+    Length, Subscription, VerticalAlignment,
 };
 
 pub mod widget {
diff --git a/src/sandbox.rs b/src/sandbox.rs
index acf7f5e0..248aa152 100644
--- a/src/sandbox.rs
+++ b/src/sandbox.rs
@@ -1,4 +1,4 @@
-use crate::{Application, Command, Element, Settings};
+use crate::{Application, Command, Element, Settings, Subscription};
 
 /// A sandboxed [`Application`].
 ///
@@ -149,6 +149,10 @@ where
         Command::none()
     }
 
+    fn subscriptions(&self) -> Subscription<T::Message> {
+        Subscription::none()
+    }
+
     fn view(&mut self) -> Element<'_, T::Message> {
         T::view(self)
     }
diff --git a/winit/src/application.rs b/winit/src/application.rs
index 3772a667..959e142d 100644
--- a/winit/src/application.rs
+++ b/winit/src/application.rs
@@ -2,9 +2,10 @@ use crate::{
     conversion,
     input::{keyboard, mouse},
     renderer::{Target, Windowed},
-    Cache, Command, Container, Debug, Element, Event, Length, MouseCursor,
-    Settings, UserInterface,
+    subscription, Cache, Command, Container, Debug, Element, Event, Length,
+    MouseCursor, Settings, Subscription, UserInterface,
 };
+use std::collections::HashMap;
 
 /// An interactive, native cross-platform application.
 ///
@@ -57,6 +58,9 @@ pub trait Application: Sized {
     /// [`Command`]: struct.Command.html
     fn update(&mut self, message: Self::Message) -> Command<Self::Message>;
 
+    /// TODO
+    fn subscriptions(&self) -> Subscription<Self::Message>;
+
     /// Returns the widgets to display in the [`Application`].
     ///
     /// These widgets can produce __messages__ based on user interaction.
@@ -89,11 +93,15 @@ pub trait Application: Sized {
         let proxy = event_loop.create_proxy();
         let mut thread_pool =
             futures::executor::ThreadPool::new().expect("Create thread pool");
+        let mut alive_subscriptions = Subscriptions::new();
         let mut external_messages = Vec::new();
 
         let (mut application, init_command) = Self::new();
         spawn(init_command, &mut thread_pool, &proxy);
 
+        let subscriptions = application.subscriptions();
+        alive_subscriptions.update(subscriptions, &mut thread_pool, &proxy);
+
         let mut title = application.title();
 
         let window = {
@@ -204,6 +212,13 @@ pub trait Application: Sized {
                         debug.update_finished();
                     }
 
+                    let subscriptions = application.subscriptions();
+                    alive_subscriptions.update(
+                        subscriptions,
+                        &mut thread_pool,
+                        &proxy,
+                    );
+
                     // Update window title
                     let new_title = application.title();
 
@@ -404,6 +419,66 @@ fn spawn<Message: Send>(
     }
 }
 
+pub struct Subscriptions {
+    alive: HashMap<u64, Box<dyn subscription::Handle>>,
+}
+
+impl Subscriptions {
+    fn new() -> Self {
+        Self {
+            alive: HashMap::new(),
+        }
+    }
+
+    fn update<Message: Send>(
+        &mut self,
+        subscriptions: Subscription<Message>,
+        thread_pool: &mut futures::executor::ThreadPool,
+        proxy: &winit::event_loop::EventLoopProxy<Message>,
+    ) {
+        use futures::stream::StreamExt;
+
+        let definitions = subscriptions.definitions();
+        let mut alive = std::collections::HashSet::new();
+
+        for definition in definitions {
+            let id = definition.id();
+            let _ = alive.insert(id);
+
+            if !self.alive.contains_key(&id) {
+                let (stream, handle) = definition.stream();
+
+                let proxy =
+                    std::sync::Arc::new(std::sync::Mutex::new(proxy.clone()));
+
+                let future = stream.for_each(move |message| {
+                    proxy
+                        .lock()
+                        .expect("Acquire event loop proxy lock")
+                        .send_event(message)
+                        .expect("Send subscription result to event loop");
+
+                    futures::future::ready(())
+                });
+
+                thread_pool.spawn_ok(future);
+
+                let _ = self.alive.insert(id, handle);
+            }
+        }
+
+        self.alive.retain(|id, handle| {
+            let is_still_alive = alive.contains(&id);
+
+            if !is_still_alive {
+                handle.cancel();
+            }
+
+            is_still_alive
+        });
+    }
+}
+
 // As defined in: http://www.unicode.org/faq/private_use.html
 // TODO: Remove once https://github.com/rust-windowing/winit/pull/1254 lands
 fn is_private_use_character(c: char) -> bool {