Remove RedrawEventsCleared + MainEventsCleared, and added AboutToWait

The idea that redraw events are dispatched with a specific ordering
that makes it possible to specifically report when we have finished
dispatching redraw events isn't portable and the way in which we
dispatched RedrawEventsCleared was inconsistent across backends.

More generally speaking, there is no inherent relationship between
redrawing and event loop iterations. An event loop may wake up at any
frequency depending on what sources of input events are being listened
to but redrawing is generally throttled and in some way synchronized
with the display frequency.

Similarly there's no inherent relationship between a single event loop
iteration and the dispatching of any specific kind of "main" event.

An event loop wakes up when there are events to read (e.g. input
events or responses from a display server / compositor) and goes back
to waiting when there's nothing else to read.

There isn't really a special kind of "main" event that is dispatched
in order with respect to other events.

What we can do more portably is emit an event when the event loop
is about to block and wait for new events.

In practice this is very similar to how MainEventsCleared was
implemented except it wasn't the very last event previously since
redraw events could be dispatched afterwards.

The main backend where we don't strictly know when we're going to
wait for events is Web (since the real event loop is internal to
the browser). For now we emulate AboutToWait on Web similar to how
MainEventsCleared was dispatched.

In practice most applications almost certainly shouldn't care about
AboutToWait because the frequency of event loop iterations is
essentially arbitrary and usually irrelevant.

src/platform_impl/ios/app_state.rs

@@ -753,21 +753,18 @@ pub unsafe fn handle_main_events_cleared() {
     };
     drop(this);
 
-    // User events are always sent out at the end of the "MainEventLoop"
     handle_user_events();
-    handle_nonuser_event(EventWrapper::StaticEvent(Event::MainEventsCleared));
 
     let mut this = AppState::get_mut();
-    let mut redraw_events: Vec<EventWrapper> = this
+    let redraw_events: Vec<EventWrapper> = this
         .main_events_cleared_transition()
         .into_iter()
         .map(|window| EventWrapper::StaticEvent(Event::RedrawRequested(RootWindowId(window.id()))))
         .collect();
-
-    redraw_events.push(EventWrapper::StaticEvent(Event::RedrawEventsCleared));
     drop(this);
 
     handle_nonuser_events(redraw_events);
+    handle_nonuser_event(EventWrapper::StaticEvent(Event::AboutToWait));
 }
 
 // requires main thread

src/platform_impl/ios/event_loop.rs

@@ -237,10 +237,10 @@ fn setup_control_flow_observers() {
 
         // Core Animation registers its `CFRunLoopObserver` that performs drawing operations in
         // `CA::Transaction::ensure_implicit` with a priority of `0x1e8480`. We set the main_end
-        // priority to be 0, in order to send MainEventsCleared before RedrawRequested. This value was
+        // priority to be 0, in order to send AboutToWait before RedrawRequested. This value was
         // chosen conservatively to guard against apple using different priorities for their redraw
         // observers in different OS's or on different devices. If it so happens that it's too
-        // conservative, the main symptom would be non-redraw events coming in after `MainEventsCleared`.
+        // conservative, the main symptom would be non-redraw events coming in after `AboutToWait`.
         //
         // The value of `0x1e8480` was determined by inspecting stack traces and the associated
         // registers for every `CFRunLoopAddObserver` call on an iPad Air 2 running iOS 11.4.

src/platform_impl/ios/view.rs

@@ -42,14 +42,9 @@ declare_class!(
         fn draw_rect(&self, rect: CGRect) {
             let window = self.window().unwrap();
             unsafe {
-                app_state::handle_nonuser_events(
-                    std::iter::once(EventWrapper::StaticEvent(Event::RedrawRequested(
-                        RootWindowId(window.id()),
-                    )))
-                    .chain(std::iter::once(EventWrapper::StaticEvent(
-                        Event::RedrawEventsCleared,
-                    ))),
-                );
+                app_state::handle_nonuser_event(EventWrapper::StaticEvent(Event::RedrawRequested(
+                    RootWindowId(window.id()),
+                )));
             }
             let _: () = unsafe { msg_send![super(self), drawRect: rect] };
         }