Add call to Dart_NotifyDestroyed when the flutter view is destroyed. (flutter/engine#37539)

* Add calls to Dart_NotifyDestroyed when the flutter view is destroyed.

* Add unit test case.

* Format.

* Ensure the destroy task runs.

* Address code review comments.

engine/src/flutter/runtime/runtime_controller.cc

@@ -235,6 +235,19 @@ bool RuntimeController::NotifyIdle(fml::TimePoint deadline) {
   return true;
 }
 
+bool RuntimeController::NotifyDestroyed() {
+  std::shared_ptr<DartIsolate> root_isolate = root_isolate_.lock();
+  if (!root_isolate) {
+    return false;
+  }
+
+  tonic::DartState::Scope scope(root_isolate);
+
+  Dart_NotifyDestroyed();
+
+  return true;
+}
+
 bool RuntimeController::DispatchPlatformMessage(
     std::unique_ptr<PlatformMessage> message) {
   if (auto* platform_configuration = GetPlatformConfigurationIfAvailable()) {

engine/src/flutter/runtime/runtime_controller.h

@@ -360,6 +360,17 @@ class RuntimeController : public PlatformConfigurationClient {
   ///
   virtual bool NotifyIdle(fml::TimePoint deadline);
 
+  //----------------------------------------------------------------------------
+  /// @brief      Notify the Dart VM that the attached flutter view has been
+  ///             destroyed. This gives the Dart VM to perform some cleanup
+  ///             activities e.g: perform garbage collection to free up any
+  ///             unused memory.
+  ///
+  /// NotifyDestroyed is advisory. The VM may or may not perform any clean up
+  /// activities.
+  ///
+  virtual bool NotifyDestroyed();
+
   //----------------------------------------------------------------------------
   /// @brief      Returns if the root isolate is running. The isolate must be
   ///             transitioned to the running phase manually. The isolate can

engine/src/flutter/shell/common/engine.cc

@@ -266,6 +266,11 @@ void Engine::NotifyIdle(fml::TimePoint deadline) {
   runtime_controller_->NotifyIdle(deadline);
 }
 
+void Engine::NotifyDestroyed() {
+  TRACE_EVENT0("flutter", "Engine::NotifyDestroyed");
+  runtime_controller_->NotifyDestroyed();
+}
+
 std::optional<uint32_t> Engine::GetUIIsolateReturnCode() {
   return runtime_controller_->GetRootIsolateReturnCode();
 }

engine/src/flutter/shell/common/engine.h

@@ -554,6 +554,13 @@ class Engine final : public RuntimeDelegate, PointerDataDispatcher::Delegate {
   ///
   void NotifyIdle(fml::TimePoint deadline);
 
+  //----------------------------------------------------------------------------
+  /// @brief      Notifies the engine that the attached flutter view has been
+  ///             destroyed.
+  ///             This enables the engine to notify the Dart VM so it can do
+  ///             some cleanp activities.
+  void NotifyDestroyed();
+
   //----------------------------------------------------------------------------
   /// @brief      Dart code cannot fully measure the time it takes for a
   ///             specific frame to be rendered. This is because Dart code only

engine/src/flutter/shell/common/fixtures/shell_test.dart

@@ -212,6 +212,11 @@ void performanceModeImpactsNotifyIdle() {
   PlatformDispatcher.instance.requestDartPerformanceMode(DartPerformanceMode.balanced);
 }
 
+@pragma('vm:entry-point')
+void callNotifyDestroyed() {
+  notifyDestroyed();
+}
+
 @pragma('vm:external-name', 'NotifyMessage')
 external void notifyMessage(String string);
 
@@ -424,6 +429,8 @@ Future<void> runCallback(IsolateParam param) async {
 @pragma('vm:entry-point')
 @pragma('vm:external-name', 'NotifyNativeBool')
 external void notifyNativeBool(bool value);
+@pragma('vm:external-name', 'NotifyDestroyed')
+external void notifyDestroyed();
 
 @pragma('vm:entry-point')
 Future<void> testPluginUtilitiesCallbackHandle() async {

engine/src/flutter/shell/common/shell.cc

@@ -851,6 +851,14 @@ void Shell::OnPlatformViewDestroyed() {
   // This incorrect assumption can lead to deadlock.
   rasterizer_->DisableThreadMergerIfNeeded();
 
+  // Notify the Dart VM that the PlatformView has been destroyed and some
+  // cleanup activity can be done (e.g: garbage collect the Dart heap).
+  task_runners_.GetUITaskRunner()->PostTask([engine = engine_->GetWeakPtr()]() {
+    if (engine) {
+      engine->NotifyDestroyed();
+    }
+  });
+
   // Note:
   // This is a synchronous operation because certain platforms depend on
   // setup/suspension of all activities that may be interacting with the GPU in

engine/src/flutter/shell/common/shell_unittests.cc

@@ -3932,6 +3932,40 @@ TEST_F(ShellTest, NotifyIdleNotCalledInLatencyMode) {
   ASSERT_FALSE(DartVMRef::IsInstanceRunning());
 }
 
+TEST_F(ShellTest, NotifyDestroyed) {
+  ASSERT_FALSE(DartVMRef::IsInstanceRunning());
+  Settings settings = CreateSettingsForFixture();
+  ThreadHost thread_host("io.flutter.test." + GetCurrentTestName() + ".",
+                         ThreadHost::Type::Platform | ThreadHost::UI |
+                             ThreadHost::IO | ThreadHost::RASTER);
+  auto platform_task_runner = thread_host.platform_thread->GetTaskRunner();
+  TaskRunners task_runners("test", thread_host.platform_thread->GetTaskRunner(),
+                           thread_host.raster_thread->GetTaskRunner(),
+                           thread_host.ui_thread->GetTaskRunner(),
+                           thread_host.io_thread->GetTaskRunner());
+  auto shell = CreateShell(settings, task_runners);
+  ASSERT_TRUE(DartVMRef::IsInstanceRunning());
+  ASSERT_TRUE(ValidateShell(shell.get()));
+
+  fml::CountDownLatch latch(1);
+  AddNativeCallback("NotifyDestroyed", CREATE_NATIVE_ENTRY([&](auto args) {
+                      auto runtime_controller = const_cast<RuntimeController*>(
+                          shell->GetEngine()->GetRuntimeController());
+                      bool success = runtime_controller->NotifyDestroyed();
+                      EXPECT_TRUE(success);
+                      latch.CountDown();
+                    }));
+
+  auto configuration = RunConfiguration::InferFromSettings(settings);
+  configuration.SetEntrypoint("callNotifyDestroyed");
+  RunEngine(shell.get(), std::move(configuration));
+
+  latch.Wait();
+
+  DestroyShell(std::move(shell), task_runners);
+  ASSERT_FALSE(DartVMRef::IsInstanceRunning());
+}
+
 }  // namespace testing
 }  // namespace flutter