Reverts "[engine] more consistently flush dart event loop, run vsync callback immediately (#56738)" (flutter/engine#56767)

Reverts: flutter/engine#56738
Initiated by: jonahwilliams
Reason for reverting: speculative revert for framework failures.
Original PR Author: jonahwilliams

Reviewed By: {jason-simmons}

This change reverts the following previous change:
Changes the following shell callbacks to flush the dart event loop:

* OnPlatformViewSetViewportMetrics
* OnPlatformViewDispatchPointerDataPacket
* OnPlatformViewDispatchPlatformMessage
* OnPlatformViewSetSemanticsEnabled
* OnPlatformViewSetAccessibilityFeatures

Using a new TaskRunner API RunNowAndFlushMessages. If the task runner can run tasks on the current thread, this will immediately invoke a callback and then post an empty task to the event loop to ensure dart listeners fire.

This also updates the vsync waiter to use RunNowOrPostTask, so that we start vsync events as early as possible.

engine/src/flutter/fml/task_runner.cc

@@ -52,7 +52,6 @@ bool TaskRunner::RunsTasksOnCurrentThread() {
                                                 loop_queue_id);
 }
 
-// static
 void TaskRunner::RunNowOrPostTask(const fml::RefPtr<fml::TaskRunner>& runner,
                                   const fml::closure& task) {
   FML_DCHECK(runner);
@@ -63,20 +62,4 @@ void TaskRunner::RunNowOrPostTask(const fml::RefPtr<fml::TaskRunner>& runner,
   }
 }
 
-// static
-void TaskRunner::RunNowAndFlushMessages(
-    const fml::RefPtr<fml::TaskRunner>& runner,
-    const fml::closure& task) {
-  FML_DCHECK(runner);
-  if (runner->RunsTasksOnCurrentThread()) {
-    task();
-    // Post an empty task to make the UI message loop run its task observers.
-    // The observers will execute any Dart microtasks queued by the platform
-    // message handler.
-    runner->PostTask([] {});
-  } else {
-    runner->PostTask(task);
-  }
-}
-
 }  // namespace fml

engine/src/flutter/fml/task_runner.h

@@ -62,14 +62,6 @@ class TaskRunner : public fml::RefCountedThreadSafe<TaskRunner>,
   static void RunNowOrPostTask(const fml::RefPtr<fml::TaskRunner>& runner,
                                const fml::closure& task);
 
-  /// Like RunNowOrPostTask, except that if the task can be immediately
-  /// executed, an empty task will still be posted to the runner afterwards.
-  ///
-  /// This is used to ensure that messages posted to Dart from the platform
-  /// thread always flush the Dart event loop.
-  static void RunNowAndFlushMessages(const fml::RefPtr<fml::TaskRunner>& runner,
-                                     const fml::closure& task);
-
  protected:
   explicit TaskRunner(fml::RefPtr<MessageLoopImpl> loop);
 

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

@@ -640,13 +640,3 @@ void testSemanticsActions() {
     });
   };
 }
-
-@pragma('vm:entry-point')
-void testPointerActions() {
-  PlatformDispatcher.instance.onPointerDataPacket = (PointerDataPacket pointer) async {
-    await null;
-    Future<void>.value().then((_) {
-      notifyNative();
-    });
-  };
-}

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

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "fml/task_runner.h"
 #define RAPIDJSON_HAS_STDSTRING 1
 #include "flutter/shell/common/shell.h"
 
@@ -1037,7 +1036,7 @@ void Shell::OnPlatformViewSetViewportMetrics(int64_t view_id,
         }
       });
 
-  fml::TaskRunner::RunNowAndFlushMessages(
+  fml::TaskRunner::RunNowOrPostTask(
       task_runners_.GetUITaskRunner(),
       [engine = engine_->GetWeakPtr(), view_id, metrics]() {
         if (engine) {
@@ -1076,10 +1075,17 @@ void Shell::OnPlatformViewDispatchPlatformMessage(
   }
 #endif  // FLUTTER_RUNTIME_MODE == FLUTTER_RUNTIME_MODE_DEBUG
 
+  if (task_runners_.GetUITaskRunner()->RunsTasksOnCurrentThread()) {
+    engine_->DispatchPlatformMessage(std::move(message));
+
+    // Post an empty task to make the UI message loop run its task observers.
+    // The observers will execute any Dart microtasks queued by the platform
+    // message handler.
+    task_runners_.GetUITaskRunner()->PostTask([] {});
+  } else {
     // The static leak checker gets confused by the use of fml::MakeCopyable.
     // NOLINTNEXTLINE(clang-analyzer-cplusplus.NewDeleteLeaks)
-  fml::TaskRunner::RunNowAndFlushMessages(
+    task_runners_.GetUITaskRunner()->PostTask(
-      task_runners_.GetUITaskRunner(),
         fml::MakeCopyable([engine = engine_->GetWeakPtr(),
                            message = std::move(message)]() mutable {
           if (engine) {
@@ -1087,6 +1093,7 @@ void Shell::OnPlatformViewDispatchPlatformMessage(
           }
         }));
   }
+}
 
 // |PlatformView::Delegate|
 void Shell::OnPlatformViewDispatchPointerDataPacket(
@@ -1097,7 +1104,7 @@ void Shell::OnPlatformViewDispatchPointerDataPacket(
   TRACE_FLOW_BEGIN("flutter", "PointerEvent", next_pointer_flow_id_);
   FML_DCHECK(is_set_up_);
   FML_DCHECK(task_runners_.GetPlatformTaskRunner()->RunsTasksOnCurrentThread());
-  fml::TaskRunner::RunNowAndFlushMessages(
+  fml::TaskRunner::RunNowOrPostTask(
       task_runners_.GetUITaskRunner(),
       fml::MakeCopyable([engine = weak_engine_, packet = std::move(packet),
                          flow_id = next_pointer_flow_id_]() mutable {
@@ -1115,8 +1122,7 @@ void Shell::OnPlatformViewDispatchSemanticsAction(int32_t node_id,
   FML_DCHECK(is_set_up_);
   FML_DCHECK(task_runners_.GetPlatformTaskRunner()->RunsTasksOnCurrentThread());
 
-  fml::TaskRunner::RunNowAndFlushMessages(
+  task_runners_.GetUITaskRunner()->PostTask(
-      task_runners_.GetUITaskRunner(),
       fml::MakeCopyable([engine = engine_->GetWeakPtr(), node_id, action,
                          args = std::move(args)]() mutable {
         if (engine) {
@@ -1130,8 +1136,7 @@ void Shell::OnPlatformViewSetSemanticsEnabled(bool enabled) {
   FML_DCHECK(is_set_up_);
   FML_DCHECK(task_runners_.GetPlatformTaskRunner()->RunsTasksOnCurrentThread());
 
-  fml::TaskRunner::RunNowAndFlushMessages(
+  fml::TaskRunner::RunNowOrPostTask(task_runners_.GetUITaskRunner(),
-      task_runners_.GetUITaskRunner(),
                                     [engine = engine_->GetWeakPtr(), enabled] {
                                       if (engine) {
                                         engine->SetSemanticsEnabled(enabled);
@@ -1144,8 +1149,8 @@ void Shell::OnPlatformViewSetAccessibilityFeatures(int32_t flags) {
   FML_DCHECK(is_set_up_);
   FML_DCHECK(task_runners_.GetPlatformTaskRunner()->RunsTasksOnCurrentThread());
 
-  fml::TaskRunner::RunNowAndFlushMessages(
+  fml::TaskRunner::RunNowOrPostTask(task_runners_.GetUITaskRunner(),
-      task_runners_.GetUITaskRunner(), [engine = engine_->GetWeakPtr(), flags] {
+                                    [engine = engine_->GetWeakPtr(), flags] {
                                       if (engine) {
                                         engine->SetAccessibilityFeatures(flags);
                                       }

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

@@ -4312,8 +4312,7 @@ TEST_F(ShellTest, NavigationMessageDispachedImmediately) {
   ASSERT_FALSE(DartVMRef::IsInstanceRunning());
 }
 
-// Verifies a semantics Action will flush the dart event loop.
+TEST_F(ShellTest, SemanticsActionsPostTask) {
-TEST_F(ShellTest, SemanticsActionsFlushMessageLoop) {
   Settings settings = CreateSettingsForFixture();
   ThreadHost thread_host("io.flutter.test." + GetCurrentTestName() + ".",
                          ThreadHost::Type::kPlatform);
@@ -4328,40 +4327,13 @@ TEST_F(ShellTest, SemanticsActionsFlushMessageLoop) {
   configuration.SetEntrypoint("testSemanticsActions");
 
   RunEngine(shell.get(), std::move(configuration));
-  fml::CountDownLatch latch(1);
-  AddNativeCallback(
-      // The Dart native function names aren't very consistent but this is
-      // just the native function name of the second vm entrypoint in the
-      // fixture.
-      "NotifyNative",
-      CREATE_NATIVE_ENTRY([&](auto args) { latch.CountDown(); }));
 
   task_runners.GetPlatformTaskRunner()->PostTask([&] {
     SendSemanticsAction(shell.get(), 0, SemanticsAction::kTap,
                         fml::MallocMapping(nullptr, 0));
   });
-  latch.Wait();
 
-  DestroyShell(std::move(shell), task_runners);
+  // Fulfill native function for the second Shell's entrypoint.
-  ASSERT_FALSE(DartVMRef::IsInstanceRunning());
-}
-
-// Verifies a pointer event will flush the dart event loop.
-TEST_F(ShellTest, PointerPacketFlushMessageLoop) {
-  Settings settings = CreateSettingsForFixture();
-  ThreadHost thread_host("io.flutter.test." + GetCurrentTestName() + ".",
-                         ThreadHost::Type::kPlatform);
-  auto task_runner = thread_host.platform_thread->GetTaskRunner();
-  TaskRunners task_runners("test", task_runner, task_runner, task_runner,
-                           task_runner);
-
-  EXPECT_EQ(task_runners.GetPlatformTaskRunner(),
-            task_runners.GetUITaskRunner());
-  auto shell = CreateShell(settings, task_runners);
-  auto configuration = RunConfiguration::InferFromSettings(settings);
-  configuration.SetEntrypoint("testPointerActions");
-
-  RunEngine(shell.get(), std::move(configuration));
   fml::CountDownLatch latch(1);
   AddNativeCallback(
       // The Dart native function names aren't very consistent but this is
@@ -4369,8 +4341,6 @@ TEST_F(ShellTest, PointerPacketFlushMessageLoop) {
       // fixture.
       "NotifyNative",
       CREATE_NATIVE_ENTRY([&](auto args) { latch.CountDown(); }));
-
-  DispatchFakePointerData(shell.get(), 23);
   latch.Wait();
 
   DestroyShell(std::move(shell), task_runners);

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

@@ -128,10 +128,7 @@ void VsyncWaiter::FireCallback(fml::TimePoint frame_start_time,
     fml::TaskQueueId ui_task_queue_id =
         task_runners_.GetUITaskRunner()->GetTaskQueueId();
 
-    // This task does not need to use RunNowAndFlushMessages as the
+    task_runners_.GetUITaskRunner()->PostTask(
-    // vsync callback will explicitly flush the Dart event loop.
-    fml::TaskRunner::RunNowOrPostTask(
-        task_runners_.GetUITaskRunner(),
         [ui_task_queue_id, callback, flow_identifier, frame_start_time,
          frame_target_time, pause_secondary_tasks]() {
           FML_TRACE_EVENT_WITH_FLOW_IDS(