[Embedder API] Add next frame callback

shell/common/rasterizer.cc

@@ -641,7 +641,7 @@ RasterStatus Rasterizer::DrawToSurfaceUnsafe(
     frame_timings_recorder.RecordRasterStart(fml::TimePoint::Now());
 
     std::unique_ptr<FrameDamage> damage;
-    // when leaf layer tracing is enabled we wish to repaing the whole frame for
+    // when leaf layer tracing is enabled we wish to repaint the whole frame for
     // accurate performance metrics.
     if (frame->framebuffer_info().supports_partial_repaint &&
         !layer_tree.is_leaf_layer_tracing_enabled()) {

shell/platform/embedder/embedder.cc

@@ -2683,6 +2683,36 @@ FlutterEngineResult FlutterEngineScheduleFrame(FLUTTER_API_SYMBOL(FlutterEngine)
                                   "Could not schedule frame.");
 }
 
+FlutterEngineResult FlutterEngineSetNextFrameCallback(
+    FLUTTER_API_SYMBOL(FlutterEngine) engine,
+    VoidCallback callback,
+    void* user_data) {
+  if (engine == nullptr) {
+    return LOG_EMBEDDER_ERROR(kInvalidArguments, "Invalid engine handle.");
+  }
+
+  if (callback == nullptr) {
+    return LOG_EMBEDDER_ERROR(kInvalidArguments,
+                              "Next frame callback was null.");
+  }
+
+  flutter::EmbedderEngine* embedder_engine =
+      reinterpret_cast<flutter::EmbedderEngine*>(engine);
+
+  fml::WeakPtr<flutter::PlatformView> weak_platform_view =
+      embedder_engine->GetShell().GetPlatformView();
+
+  if (!weak_platform_view) {
+    return LOG_EMBEDDER_ERROR(kInternalInconsistency,
+                              "Platform view unavailable.");
+  }
+
+  weak_platform_view->SetNextFrameCallback(
+      [callback, user_data]() { callback(user_data); });
+
+  return kSuccess;
+}
+
 FlutterEngineResult FlutterEngineGetProcAddresses(
     FlutterEngineProcTable* table) {
   if (!table) {
@@ -2734,6 +2764,7 @@ FlutterEngineResult FlutterEngineGetProcAddresses(
            FlutterEnginePostCallbackOnAllNativeThreads);
   SET_PROC(NotifyDisplayUpdate, FlutterEngineNotifyDisplayUpdate);
   SET_PROC(ScheduleFrame, FlutterEngineScheduleFrame);
+  SET_PROC(SetNextFrameCallback, FlutterEngineSetNextFrameCallback);
 #undef SET_PROC
 
   return kSuccess;

shell/platform/embedder/embedder.h

@@ -2510,6 +2510,26 @@ FLUTTER_EXPORT
 FlutterEngineResult FlutterEngineScheduleFrame(FLUTTER_API_SYMBOL(FlutterEngine)
                                                    engine);
 
+//------------------------------------------------------------------------------
+/// @brief      Schedule a callback to be called after the next frame is drawn.
+///             This must be called from the platform thread. The callback is
+///             executed only once from the raster thread; embedders must
+///             re-thread if necessary. Performing blocking calls
+///             in this callback may introduce application jank.
+///
+/// @param[in]  engine     A running engine instance.
+/// @param[in]  callback   The callback to execute.
+/// @param[in]  user_data  A baton passed by the engine to the callback. This
+///                        baton is not interpreted by the engine in any way.
+///
+/// @return     The result of the call.
+///
+FLUTTER_EXPORT
+FlutterEngineResult FlutterEngineSetNextFrameCallback(
+    FLUTTER_API_SYMBOL(FlutterEngine) engine,
+    VoidCallback callback,
+    void* user_data);
+
 #endif  // !FLUTTER_ENGINE_NO_PROTOTYPES
 
 // Typedefs for the function pointers in FlutterEngineProcTable.
@@ -2628,6 +2648,10 @@ typedef FlutterEngineResult (*FlutterEngineNotifyDisplayUpdateFnPtr)(
     size_t display_count);
 typedef FlutterEngineResult (*FlutterEngineScheduleFrameFnPtr)(
     FLUTTER_API_SYMBOL(FlutterEngine) engine);
+typedef FlutterEngineResult (*FlutterEngineSetNextFrameCallbackFnPtr)(
+    FLUTTER_API_SYMBOL(FlutterEngine) engine,
+    VoidCallback callback,
+    void* user_data);
 
 /// Function-pointer-based versions of the APIs above.
 typedef struct {
@@ -2673,6 +2697,7 @@ typedef struct {
       PostCallbackOnAllNativeThreads;
   FlutterEngineNotifyDisplayUpdateFnPtr NotifyDisplayUpdate;
   FlutterEngineScheduleFrameFnPtr ScheduleFrame;
+  FlutterEngineSetNextFrameCallbackFnPtr SetNextFrameCallback;
 } FlutterEngineProcTable;
 
 //------------------------------------------------------------------------------

shell/platform/embedder/tests/embedder_unittests.cc

@@ -2126,6 +2126,44 @@ TEST_F(EmbedderTest, CanScheduleFrame) {
   check_latch.Wait();
 }
 
+TEST_F(EmbedderTest, CanSetNextFrameCallback) {
+  auto& context = GetEmbedderContext(EmbedderTestContextType::kSoftwareContext);
+  EmbedderConfigBuilder builder(context);
+  builder.SetSoftwareRendererConfig();
+  builder.SetDartEntrypoint("draw_solid_red");
+
+  auto engine = builder.LaunchEngine();
+  ASSERT_TRUE(engine.is_valid());
+
+  // Register the callback that is executed once the next frame is drawn.
+  fml::AutoResetWaitableEvent callback_latch;
+  VoidCallback callback = [](void* user_data) {
+    fml::AutoResetWaitableEvent* callback_latch =
+        static_cast<fml::AutoResetWaitableEvent*>(user_data);
+
+    callback_latch->Signal();
+  };
+
+  auto result = FlutterEngineSetNextFrameCallback(engine.get(), callback,
+                                                  &callback_latch);
+  ASSERT_EQ(result, kSuccess);
+
+  // Send a window metrics events so frames may be scheduled.
+  FlutterWindowMetricsEvent event = {};
+  event.struct_size = sizeof(event);
+  event.width = 800;
+  event.height = 600;
+  event.pixel_ratio = 1.0;
+  event.physical_view_inset_top = 0.0;
+  event.physical_view_inset_right = 0.0;
+  event.physical_view_inset_bottom = 0.0;
+  event.physical_view_inset_left = 0.0;
+  ASSERT_EQ(FlutterEngineSendWindowMetricsEvent(engine.get(), &event),
+            kSuccess);
+
+  callback_latch.Wait();
+}
+
 #if defined(FML_OS_MACOSX)
 
 static void MockThreadConfigSetter(const fml::Thread::ThreadConfig& config) {

shell/platform/windows/client_wrapper/include/flutter/flutter_engine.h

@@ -22,7 +22,8 @@ namespace flutter {
 //
 // In the future, this will be the API surface used for all interactions with
 // the engine, rather than having them duplicated on FlutterViewController.
-// For now it is only used in the rare where you need a headless Flutter engine.
+// For now it is only used in the rare case where you need a headless Flutter
+// engine.
 class FlutterEngine : public PluginRegistry {
  public:
   // Creates a new engine for running the given project.