Web backend refactor and documentation (#1415)

The current implementation of the event loop runner has some significant
problems. It can't handle multiple events being emitted at once (for
example, when a keyboard event causes a key input, a text input, and a
modifier change.) It's also relatively easy to introduce bugs for the
different possible control flow states.

The new model separates intentionally emitting a NewEvents (poll
completed, wait completed, init) and emitting a normal event, as well as
providing a method for emitting multiple events in a single call.

src/platform_impl/web/event_loop/runner.rs

@@ -8,6 +8,7 @@ use std::{
     cell::RefCell,
     clone::Clone,
     collections::{HashSet, VecDeque},
+    iter,
     rc::Rc,
 };
 
@@ -60,7 +61,7 @@ impl<T: 'static> Shared<T> {
         event_handler: Box<dyn FnMut(Event<'static, T>, &mut root::ControlFlow)>,
     ) {
         self.0.runner.replace(Some(Runner::new(event_handler)));
-        self.send_event(Event::NewEvents(StartCause::Init));
+        self.init();
 
         let close_instance = self.clone();
         backend::on_unload(move || close_instance.handle_unload());
@@ -79,55 +80,98 @@ impl<T: 'static> Shared<T> {
         self.0.redraw_pending.borrow_mut().insert(id);
     }
 
-    // Add an event to the event loop runner
+    pub fn init(&self) {
+        let start_cause = Event::NewEvents(StartCause::Init);
+        self.run_until_cleared(iter::once(start_cause));
+    }
+
+    // Run the polling logic for the Poll ControlFlow, which involves clearing the queue
+    pub fn poll(&self) {
+        let start_cause = Event::NewEvents(StartCause::Poll);
+        self.run_until_cleared(iter::once(start_cause));
+    }
+
+    // Run the logic for waking from a WaitUntil, which involves clearing the queue
+    // Generally there shouldn't be events built up when this is called
+    pub fn resume_time_reached(&self, start: Instant, requested_resume: Instant) {
+        let start_cause = Event::NewEvents(StartCause::ResumeTimeReached {
+            start,
+            requested_resume,
+        });
+        self.run_until_cleared(iter::once(start_cause));
+    }
+
+    // Add an event to the event loop runner, from the user or an event handler
     //
     // It will determine if the event should be immediately sent to the user or buffered for later
     pub fn send_event(&self, event: Event<'static, T>) {
+        self.send_events(iter::once(event));
+    }
+
+    // Add a series of events to the event loop runner
+    //
+    // It will determine if the event should be immediately sent to the user or buffered for later
+    pub fn send_events(&self, events: impl Iterator<Item = Event<'static, T>>) {
         // If the event loop is closed, it should discard any new events
         if self.is_closed() {
             return;
         }
-
-        // Determine if event handling is in process, and then release the borrow on the runner
-        let (start_cause, event_is_start) = match *self.0.runner.borrow() {
-            Some(ref runner) if !runner.is_busy => {
-                if let Event::NewEvents(cause) = event {
-                    (cause, true)
-                } else {
-                    (
-                        match runner.state {
-                            State::Init => StartCause::Init,
-                            State::Poll { .. } => StartCause::Poll,
-                            State::Wait { start } => StartCause::WaitCancelled {
-                                start,
-                                requested_resume: None,
-                            },
-                            State::WaitUntil { start, end, .. } => StartCause::WaitCancelled {
-                                start,
-                                requested_resume: Some(end),
-                            },
-                            State::Exit => {
-                                return;
-                            }
-                        },
-                        false,
-                    )
-                }
+        // If we can run the event processing right now, or need to queue this and wait for later
+        let mut process_immediately = true;
+        if let Some(ref runner) = &*self.0.runner.borrow() {
+            // If we're currently polling, queue this and wait for the poll() method to be called
+            if let State::Poll { .. } = runner.state {
+                process_immediately = false;
             }
-            _ => {
-                // Events are currently being handled, so queue this one and don't try to
-                // double-process the event queue
-                self.0.events.borrow_mut().push_back(event);
-                return;
+            // If the runner is busy, queue this and wait for it to process it later
+            if runner.is_busy {
+                process_immediately = false;
             }
+        } else {
+            // The runner still hasn't been attached: queue this event and wait for it to be
+            process_immediately = false;
+        }
+        if !process_immediately {
+            // Queue these events to look at later
+            self.0.events.borrow_mut().extend(events);
+            return;
+        }
+        // At this point, we know this is a fresh set of events
+        // Now we determine why new events are incoming, and handle the events
+        let start_cause = if let Some(runner) = &*self.0.runner.borrow() {
+            match runner.state {
+                State::Init => StartCause::Init,
+                State::Poll { .. } => StartCause::Poll,
+                State::Wait { start } => StartCause::WaitCancelled {
+                    start,
+                    requested_resume: None,
+                },
+                State::WaitUntil { start, end, .. } => StartCause::WaitCancelled {
+                    start,
+                    requested_resume: Some(end),
+                },
+                State::Exit => {
+                    // If we're in the exit state, don't do event processing
+                    return;
+                }
+            }
+        } else {
+            unreachable!("The runner cannot process events when it is not attached");
         };
+        // Take the start event, then the events provided to this function, and run an iteration of
+        // the event loop
+        let start_event = Event::NewEvents(start_cause);
+        let events = iter::once(start_event).chain(events);
+        self.run_until_cleared(events);
+    }
+
+    // Given the set of new events, run the event loop until the main events and redraw events are
+    // cleared
+    //
+    // This will also process any events that have been queued or that are queued during processing
+    fn run_until_cleared(&self, events: impl Iterator<Item = Event<'static, T>>) {
         let mut control = self.current_control_flow();
-        // Handle starting a new batch of events
-        //
-        // The user is informed via Event::NewEvents that there is a batch of events to process
-        // However, there is only one of these per batch of events
-        self.handle_event(Event::NewEvents(start_cause), &mut control);
-        if !event_is_start {
+        for event in events {
             self.handle_event(event, &mut control);
         }
         self.handle_event(Event::MainEventsCleared, &mut control);
@@ -196,10 +240,7 @@ impl<T: 'static> Shared<T> {
             root::ControlFlow::Poll => {
                 let cloned = self.clone();
                 State::Poll {
-                    timeout: backend::Timeout::new(
-                        move || cloned.send_event(Event::NewEvents(StartCause::Poll)),
-                        Duration::from_millis(0),
-                    ),
+                    timeout: backend::Timeout::new(move || cloned.poll(), Duration::from_millis(0)),
                 }
             }
             root::ControlFlow::Wait => State::Wait {
@@ -220,7 +261,7 @@ impl<T: 'static> Shared<T> {
                     start,
                     end,
                     timeout: backend::Timeout::new(
-                        move || cloned.send_event(Event::NewEvents(StartCause::Poll)),
+                        move || cloned.resume_time_reached(start, end),
                         delay,
                     ),
                 }

src/platform_impl/web/mod.rs

@@ -1,3 +1,22 @@
+// Brief introduction to the internals of the web backend:
+// Currently, the web backend supports both wasm-bindgen and stdweb as methods of binding to the
+// environment. Because they are both supporting the same underlying APIs, the actual web bindings
+// are cordoned off into backend abstractions, which present the thinnest unifying layer possible.
+//
+// When adding support for new events or interactions with the browser, first consult trusted
+// documentation (such as MDN) to ensure it is well-standardised and supported across many browsers.
+// Once you have decided on the relevant web APIs, add support to both backends.
+//
+// The backend is used by the rest of the module to implement Winit's business logic, which forms
+// the rest of the code. 'device', 'error', 'monitor', and 'window' define web-specific structures
+// for winit's cross-platform structures. They are all relatively simple translations.
+//
+// The event_loop module handles listening for and processing events. 'Proxy' implements
+// EventLoopProxy and 'WindowTarget' implements EventLoopWindowTarget. WindowTarget also handles
+// registering the event handlers. The 'Execution' struct in the 'runner' module handles taking
+// incoming events (from the registered handlers) and ensuring they are passed to the user in a
+// compliant way.
+
 mod device;
 mod error;
 mod event_loop;