Implement window::frames subscription

... and use it in the `solar_system` example 🎉

native/src/subscription.rs

@@ -59,8 +59,11 @@ pub fn events_with<Message>(
 where
     Message: 'static + MaybeSend,
 {
+    #[derive(Hash)]
+    struct EventsWith;
+
     Subscription::from_recipe(Runner {
-        id: f,
+        id: (EventsWith, f),
         spawn: move |events| {
             use futures::future;
             use futures::stream::StreamExt;
@@ -75,6 +78,28 @@ where
     })
 }
 
+pub(crate) fn raw_events<Message>(
+    f: fn(Event, event::Status) -> Option<Message>,
+) -> Subscription<Message>
+where
+    Message: 'static + MaybeSend,
+{
+    #[derive(Hash)]
+    struct RawEvents;
+
+    Subscription::from_recipe(Runner {
+        id: (RawEvents, 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`].
 ///

native/src/window.rs

@@ -8,3 +8,18 @@ pub use action::Action;
 pub use event::Event;
 pub use mode::Mode;
 pub use user_attention::UserAttention;
+
+use crate::subscription::{self, Subscription};
+
+use std::time::Instant;
+
+/// Subscribes to the frames of the window of the running application.
+///
+/// The resulting [`Subscription`] will produce items at a rate equal to the
+/// framerate of the monitor of said window.
+pub fn frames() -> Subscription<Instant> {
+    subscription::raw_events(|event, _status| match event {
+        crate::Event::Window(Event::RedrawRequested(at)) => Some(at),
+        _ => None,
+    })
+}