Add a second per-frame callback that is invoked after the microtask queue is drained (#3632)

See https://github.com/flutter/flutter/issues/7626

lib/ui/hooks.dart

@@ -83,6 +83,11 @@ void _beginFrame(int microseconds) {
     window.onBeginFrame(new Duration(microseconds: microseconds));
 }
 
+void _drawFrame() {
+  if (window.onDrawFrame != null)
+    window.onDrawFrame();
+}
+
 // If this value changes, update the encoding code in the following files:
 //
 //  * pointer_data.cc

lib/ui/window.dart

@@ -149,6 +149,12 @@ class Window {
   /// last time this callback was invoked.
   FrameCallback onBeginFrame;
 
+  /// A callback that is invoked for each frame after onBeginFrame has
+  /// completed and after the microtask queue has been drained.  This can be
+  /// used to implement a second phase of frame rendering that happens
+  /// after any deferred work queued by the onBeginFrame phase.
+  VoidCallback onDrawFrame;
+
   /// A callback that is invoked when pointer data is available.
   PointerDataPacketCallback onPointerDataPacket;
 

lib/ui/window/window.cc

@@ -10,6 +10,7 @@
 #include "lib/tonic/converter/dart_converter.h"
 #include "lib/tonic/dart_args.h"
 #include "lib/tonic/dart_library_natives.h"
+#include "lib/tonic/dart_microtask_queue.h"
 #include "lib/tonic/logging/dart_invoke.h"
 #include "lib/tonic/typed_data/dart_byte_data.h"
 
@@ -221,6 +222,10 @@ void Window::BeginFrame(ftl::TimePoint frameTime) {
                   {
                       Dart_NewInteger(microseconds),
                   });
+
+  tonic::DartMicrotaskQueue::RunMicrotasks();
+
+  DartInvokeField(library_.value(), "_drawFrame", {});
 }
 
 void Window::CompletePlatformMessageEmptyResponse(int response_id) {

runtime/dart_vm_entry_points.txt

@@ -5,6 +5,7 @@ dart:isolate,::,_getIsolateScheduleImmediateClosure
 dart:isolate,::,_setupHooks
 dart:isolate,::,_startMainIsolate
 dart:ui,::,_beginFrame
+dart:ui,::,_drawFrame
 dart:ui,::,_dispatchPlatformMessage
 dart:ui,::,_dispatchPointerDataPacket
 dart:ui,::,_dispatchSemanticsAction