Add scheduleWarmUpFrame (#50570)

This PR adds `PlatformDispatcher.scheduleWarmUpFrame`.

This PR is needed for the follow up changes:
* The framework will switch to using this function to render warmup
frames in flutter/flutter#143290.
* Then the engine will finally be able to switch to multiview pipeline
with no regression on startup timing in
#49950.

For why the warm up frame must involve the engine to render, see
flutter/flutter#142851.


## Pre-launch Checklist

- [ ] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [ ] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [ ] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I signed the [CLA].
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
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https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
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[testing the engine]:
https://github.com/flutter/flutter/wiki/Testing-the-engine
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/wiki/Chat

lib/ui/dart_ui.cc

@@ -98,6 +98,7 @@ typedef CanvasPath Path;
   V(NativeStringAttribute::initSpellOutStringAttribute)            \
   V(PlatformConfigurationNativeApi::DefaultRouteName)              \
   V(PlatformConfigurationNativeApi::ScheduleFrame)                 \
+  V(PlatformConfigurationNativeApi::EndWarmUpFrame)                \
   V(PlatformConfigurationNativeApi::Render)                        \
   V(PlatformConfigurationNativeApi::UpdateSemantics)               \
   V(PlatformConfigurationNativeApi::SetNeedsReportTimings)         \

lib/ui/platform_dispatcher.dart

@@ -801,11 +801,47 @@ class PlatformDispatcher {
   ///
   ///  * [SchedulerBinding], the Flutter framework class which manages the
   ///    scheduling of frames.
+  ///  * [scheduleWarmUpFrame], which should only be used to schedule warm up
+  ///    frames.
   void scheduleFrame() => _scheduleFrame();
 
   @Native<Void Function()>(symbol: 'PlatformConfigurationNativeApi::ScheduleFrame')
   external static void _scheduleFrame();
 
+  /// Schedule a frame to run as soon as possible, rather than waiting for the
+  /// engine to request a frame in response to a system "Vsync" signal.
+  ///
+  /// The application can call this method as soon as it starts up so that the
+  /// first frame (which is likely to be quite expensive) can start a few extra
+  /// milliseconds earlier. Using it in other situations might lead to
+  /// unintended results, such as screen tearing. Depending on platforms and
+  /// situations, the warm up frame might or might not be actually rendered onto
+  /// the screen.
+  ///
+  /// For more introduction to the warm up frame, see
+  /// [SchedulerBinding.scheduleWarmUpFrame].
+  ///
+  /// This method uses the provided callbacks as the begin frame callback and
+  /// the draw frame callback instead of [onBeginFrame] and [onDrawFrame].
+  ///
+  /// See also:
+  ///
+  ///  * [SchedulerBinding.scheduleWarmUpFrame], which uses this method, and
+  ///    introduces the warm up frame in more details.
+  ///  * [scheduleFrame], which schedules the frame at the next appropriate
+  ///    opportunity and should be used to render regular frames.
+  void scheduleWarmUpFrame({required VoidCallback beginFrame, required VoidCallback drawFrame}) {
+    // We use timers here to ensure that microtasks flush in between.
+    Timer.run(beginFrame);
+    Timer.run(() {
+      drawFrame();
+      _endWarmUpFrame();
+    });
+  }
+
+  @Native<Void Function()>(symbol: 'PlatformConfigurationNativeApi::EndWarmUpFrame')
+  external static void _endWarmUpFrame();
+
   /// Additional accessibility features that may be enabled by the platform.
   AccessibilityFeatures get accessibilityFeatures => _configuration.accessibilityFeatures;
 

lib/ui/window/platform_configuration.cc

@@ -589,6 +589,11 @@ void PlatformConfigurationNativeApi::ScheduleFrame() {
   UIDartState::Current()->platform_configuration()->client()->ScheduleFrame();
 }
 
+void PlatformConfigurationNativeApi::EndWarmUpFrame() {
+  UIDartState::ThrowIfUIOperationsProhibited();
+  UIDartState::Current()->platform_configuration()->client()->EndWarmUpFrame();
+}
+
 void PlatformConfigurationNativeApi::UpdateSemantics(SemanticsUpdate* update) {
   UIDartState::ThrowIfUIOperationsProhibited();
   UIDartState::Current()->platform_configuration()->client()->UpdateSemantics(

lib/ui/window/platform_configuration.h

@@ -65,6 +65,13 @@ class PlatformConfigurationClient {
   ///
   virtual void ScheduleFrame() = 0;
 
+  //--------------------------------------------------------------------------
+  /// @brief    Called when a warm up frame has ended.
+  ///
+  ///           For more introduction, see `Animator::EndWarmUpFrame`.
+  ///
+  virtual void EndWarmUpFrame() = 0;
+
   //--------------------------------------------------------------------------
   /// @brief      Updates the client's rendering on the GPU with the newly
   ///             provided Scene.
@@ -557,6 +564,8 @@ class PlatformConfigurationNativeApi {
 
   static void ScheduleFrame();
 
+  static void EndWarmUpFrame();
+
   static void Render(int64_t view_id,
                      Scene* scene,
                      double width,

lib/web_ui/lib/platform_dispatcher.dart

@@ -90,6 +90,8 @@ abstract class PlatformDispatcher {
 
   void scheduleFrame();
 
+  void scheduleWarmUpFrame({required VoidCallback beginFrame, required VoidCallback drawFrame});
+
   AccessibilityFeatures get accessibilityFeatures;
 
   VoidCallback? get onAccessibilityFeaturesChanged;

lib/web_ui/lib/src/engine/initialization.dart

@@ -187,7 +187,8 @@ Future<void> initializeEngineServices({
           // TODO(yjbanov): technically Flutter flushes microtasks between
           //                onBeginFrame and onDrawFrame. We don't, which hasn't
           //                been an issue yet, but eventually we'll have to
-          //                implement it properly.
+          //                implement it properly. (Also see the to-do in
+          //                `EnginePlatformDispatcher.scheduleWarmUpFrame`).
           EnginePlatformDispatcher.instance.invokeOnDrawFrame();
         }
       });

lib/web_ui/lib/src/engine/platform_dispatcher.dart

@@ -782,6 +782,19 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
     scheduleFrameCallback!();
   }
 
+  @override
+  void scheduleWarmUpFrame({required ui.VoidCallback beginFrame, required ui.VoidCallback drawFrame}) {
+    Timer.run(beginFrame);
+    // We use timers here to ensure that microtasks flush in between.
+    //
+    // TODO(dkwingsmt): This logic was moved from the framework and is different
+    // from how Web renders a regular frame, which doesn't flush microtasks
+    // between the callbacks at all (see `initializeEngineServices`). We might
+    // want to change this. See the to-do in `initializeEngineServices` and
+    // https://github.com/flutter/engine/pull/50570#discussion_r1496671676
+    Timer.run(drawFrame);
+  }
+
   /// Updates the application's rendering on the GPU with the newly provided
   /// [Scene]. This function must be called within the scope of the
   /// [onBeginFrame] or [onDrawFrame] callbacks being invoked. If this function

lib/web_ui/test/engine/platform_dispatcher/platform_dispatcher_test.dart

@@ -417,6 +417,23 @@ void testMain() {
       dispatcher.dispose();
       expect(dispatcher.accessibilityPlaceholder.isConnected, isFalse);
     });
+
+    test('scheduleWarmupFrame should call both callbacks', () async {
+      bool beginFrameCalled = false;
+      final Completer<void> drawFrameCalled = Completer<void>();
+      dispatcher.scheduleWarmUpFrame(beginFrame: () {
+        expect(drawFrameCalled.isCompleted, false);
+        expect(beginFrameCalled, false);
+        beginFrameCalled = true;
+      }, drawFrame: () {
+        expect(beginFrameCalled, true);
+        expect(drawFrameCalled.isCompleted, false);
+        drawFrameCalled.complete();
+      });
+      await drawFrameCalled.future;
+      expect(beginFrameCalled, true);
+      expect(drawFrameCalled.isCompleted, true);
+    });
   });
 }
 

runtime/runtime_controller.cc

@@ -340,6 +340,11 @@ void RuntimeController::ScheduleFrame() {
   client_.ScheduleFrame();
 }
 
+// |PlatformConfigurationClient|
+void RuntimeController::EndWarmUpFrame() {
+  client_.EndWarmUpFrame();
+}
+
 // |PlatformConfigurationClient|
 void RuntimeController::Render(Scene* scene, double width, double height) {
   // TODO(dkwingsmt): Currently only supports a single window.

runtime/runtime_controller.h

@@ -657,6 +657,9 @@ class RuntimeController : public PlatformConfigurationClient {
   // |PlatformConfigurationClient|
   void ScheduleFrame() override;
 
+  // |PlatformConfigurationClient|
+  void EndWarmUpFrame() override;
+
   // |PlatformConfigurationClient|
   void Render(Scene* scene, double width, double height) override;
 

runtime/runtime_delegate.h

@@ -25,6 +25,8 @@ class RuntimeDelegate {
 
   virtual void ScheduleFrame(bool regenerate_layer_trees = true) = 0;
 
+  virtual void EndWarmUpFrame() = 0;
+
   virtual void Render(std::unique_ptr<flutter::LayerTree> layer_tree,
                       float device_pixel_ratio) = 0;
 

shell/common/animator.cc

@@ -264,6 +264,12 @@ void Animator::AwaitVSync() {
   }
 }
 
+void Animator::EndWarmUpFrame() {
+  // Do nothing. The warm up frame does not need any additional work to end the
+  // frame for now. This will change once the pipeline supports multi-view.
+  // https://github.com/flutter/flutter/issues/142851
+}
+
 void Animator::ScheduleSecondaryVsyncCallback(uintptr_t id,
                                               const fml::closure& callback) {
   waiter_->ScheduleSecondaryCallback(id, callback);

shell/common/animator.h

@@ -53,6 +53,22 @@ class Animator final {
 
   void RequestFrame(bool regenerate_layer_trees = true);
 
+  //--------------------------------------------------------------------------
+  /// @brief    Tells the Animator that a warm up frame has ended.
+  ///
+  ///           In a warm up frame, `Animator::Render` is called out of vsync
+  ///           tasks, and Animator requires an explicit end-of-frame call to
+  ///           know when to send the layer trees to the pipeline.
+  ///
+  ///           This is different from regular frames, where Animator::Render is
+  ///           always called within a vsync task, and Animator can send
+  ///           the views at the end of the vsync task.
+  ///
+  ///           For more about warm up frames, see
+  ///           `PlatformDispatcher.scheduleWarmUpFrame`.
+  ///
+  void EndWarmUpFrame();
+
   //--------------------------------------------------------------------------
   /// @brief    Tells the Animator that this frame needs to render another view.
   ///

shell/common/engine.cc

@@ -462,6 +462,10 @@ void Engine::ScheduleFrame(bool regenerate_layer_trees) {
   animator_->RequestFrame(regenerate_layer_trees);
 }
 
+void Engine::EndWarmUpFrame() {
+  animator_->EndWarmUpFrame();
+}
+
 void Engine::Render(std::unique_ptr<flutter::LayerTree> layer_tree,
                     float device_pixel_ratio) {
   if (!layer_tree) {

shell/common/engine.h

@@ -837,6 +837,9 @@ class Engine final : public RuntimeDelegate, PointerDataDispatcher::Delegate {
   /// tree.
   void ScheduleFrame() { ScheduleFrame(true); }
 
+  // |RuntimeDelegate|
+  void EndWarmUpFrame() override;
+
   // |RuntimeDelegate|
   FontCollection& GetFontCollection() override;