[engine] support combined UI/Platform thread for iOS/Android.

common/settings.h

@@ -229,6 +229,10 @@ struct Settings {
   bool enable_impeller = false;
 #endif
 
+  // If true, the UI thread is the platform thread on supported
+  // platforms.
+  bool merged_platform_ui_thread = false;
+
   // Log a warning during shell initialization if Impeller is not enabled.
   bool warn_on_impeller_opt_out = false;
 

shell/common/shell.cc

@@ -944,11 +944,12 @@ void Shell::OnPlatformViewDestroyed() {
 
   // 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();
-    }
-  });
+  fml::TaskRunner::RunNowOrPostTask(task_runners_.GetUITaskRunner(),
+                                    [engine = engine_->GetWeakPtr()]() {
+                                      if (engine) {
+                                        engine->NotifyDestroyed();
+                                      }
+                                    });
 
   // Note:
   // This is a synchronous operation because certain platforms depend on
@@ -1006,11 +1007,12 @@ void Shell::OnPlatformViewScheduleFrame() {
   FML_DCHECK(is_set_up_);
   FML_DCHECK(task_runners_.GetPlatformTaskRunner()->RunsTasksOnCurrentThread());
 
-  task_runners_.GetUITaskRunner()->PostTask([engine = engine_->GetWeakPtr()]() {
-    if (engine) {
-      engine->ScheduleFrame();
-    }
-  });
+  fml::TaskRunner::RunNowOrPostTask(task_runners_.GetUITaskRunner(),
+                                    [engine = engine_->GetWeakPtr()]() {
+                                      if (engine) {
+                                        engine->ScheduleFrame();
+                                      }
+                                    });
 }
 
 // |PlatformView::Delegate|
@@ -1038,7 +1040,8 @@ void Shell::OnPlatformViewSetViewportMetrics(int64_t view_id,
         }
       });
 
-  task_runners_.GetUITaskRunner()->PostTask(
+  fml::TaskRunner::RunNowOrPostTask(
+      task_runners_.GetUITaskRunner(),
       [engine = engine_->GetWeakPtr(), view_id, metrics]() {
         if (engine) {
           engine->SetViewportMetrics(view_id, metrics);
@@ -1078,8 +1081,10 @@ void Shell::OnPlatformViewDispatchPlatformMessage(
 
   // The static leak checker gets confused by the use of fml::MakeCopyable.
   // NOLINTNEXTLINE(clang-analyzer-cplusplus.NewDeleteLeaks)
-  task_runners_.GetUITaskRunner()->PostTask(fml::MakeCopyable(
-      [engine = engine_->GetWeakPtr(), message = std::move(message)]() mutable {
+  fml::TaskRunner::RunNowOrPostTask(
+      task_runners_.GetUITaskRunner(),
+      fml::MakeCopyable([engine = engine_->GetWeakPtr(),
+                         message = std::move(message)]() mutable {
         if (engine) {
           engine->DispatchPlatformMessage(std::move(message));
         }
@@ -1095,7 +1100,8 @@ void Shell::OnPlatformViewDispatchPointerDataPacket(
   TRACE_FLOW_BEGIN("flutter", "PointerEvent", next_pointer_flow_id_);
   FML_DCHECK(is_set_up_);
   FML_DCHECK(task_runners_.GetPlatformTaskRunner()->RunsTasksOnCurrentThread());
-  task_runners_.GetUITaskRunner()->PostTask(
+  fml::TaskRunner::RunNowOrPostTask(
+      task_runners_.GetUITaskRunner(),
       fml::MakeCopyable([engine = weak_engine_, packet = std::move(packet),
                          flow_id = next_pointer_flow_id_]() mutable {
         if (engine) {
@@ -1112,7 +1118,8 @@ void Shell::OnPlatformViewDispatchSemanticsAction(int32_t node_id,
   FML_DCHECK(is_set_up_);
   FML_DCHECK(task_runners_.GetPlatformTaskRunner()->RunsTasksOnCurrentThread());
 
-  task_runners_.GetUITaskRunner()->PostTask(
+  fml::TaskRunner::RunNowOrPostTask(
+      task_runners_.GetUITaskRunner(),
       fml::MakeCopyable([engine = engine_->GetWeakPtr(), node_id, action,
                          args = std::move(args)]() mutable {
         if (engine) {
@@ -1126,25 +1133,25 @@ void Shell::OnPlatformViewSetSemanticsEnabled(bool enabled) {
   FML_DCHECK(is_set_up_);
   FML_DCHECK(task_runners_.GetPlatformTaskRunner()->RunsTasksOnCurrentThread());
 
-  task_runners_.GetUITaskRunner()->PostTask(
-      [engine = engine_->GetWeakPtr(), enabled] {
-        if (engine) {
-          engine->SetSemanticsEnabled(enabled);
-        }
-      });
+  fml::TaskRunner::RunNowOrPostTask(task_runners_.GetUITaskRunner(),
+                                    [engine = engine_->GetWeakPtr(), enabled] {
+                                      if (engine) {
+                                        engine->SetSemanticsEnabled(enabled);
+                                      }
+                                    });
 }
 
 // |PlatformView::Delegate|
 void Shell::OnPlatformViewSetAccessibilityFeatures(int32_t flags) {
   FML_DCHECK(is_set_up_);
   FML_DCHECK(task_runners_.GetPlatformTaskRunner()->RunsTasksOnCurrentThread());
 
-  task_runners_.GetUITaskRunner()->PostTask(
-      [engine = engine_->GetWeakPtr(), flags] {
-        if (engine) {
-          engine->SetAccessibilityFeatures(flags);
-        }
-      });
+  fml::TaskRunner::RunNowOrPostTask(task_runners_.GetUITaskRunner(),
+                                    [engine = engine_->GetWeakPtr(), flags] {
+                                      if (engine) {
+                                        engine->SetAccessibilityFeatures(flags);
+                                      }
+                                    });
 }
 
 // |PlatformView::Delegate|
@@ -1205,11 +1212,12 @@ void Shell::OnPlatformViewMarkTextureFrameAvailable(int64_t texture_id) {
       });
 
   // Schedule a new frame without having to rebuild the layer tree.
-  task_runners_.GetUITaskRunner()->PostTask([engine = engine_->GetWeakPtr()]() {
-    if (engine) {
-      engine->ScheduleFrame(false);
-    }
-  });
+  fml::TaskRunner::RunNowOrPostTask(task_runners_.GetUITaskRunner(),
+                                    [engine = engine_->GetWeakPtr()]() {
+                                      if (engine) {
+                                        engine->ScheduleFrame(false);
+                                      }
+                                    });
 }
 
 // |PlatformView::Delegate|
@@ -1317,7 +1325,8 @@ void Shell::OnEngineUpdateSemantics(SemanticsNodeUpdates update,
   FML_DCHECK(is_set_up_);
   FML_DCHECK(task_runners_.GetUITaskRunner()->RunsTasksOnCurrentThread());
 
-  task_runners_.GetPlatformTaskRunner()->PostTask(
+  task_runners_.GetPlatformTaskRunner()->RunNowOrPostTask(
+      task_runners_.GetPlatformTaskRunner(),
       [view = platform_view_->GetWeakPtr(), update = std::move(update),
        actions = std::move(actions)] {
         if (view) {
@@ -2142,15 +2151,16 @@ void Shell::OnPlatformViewAddView(int64_t view_id,
       << "Unexpected request to add the implicit view #"
       << kFlutterImplicitViewId << ". This view should never be added.";
 
-  task_runners_.GetUITaskRunner()->PostTask([engine = engine_->GetWeakPtr(),  //
-                                             viewport_metrics,                //
-                                             view_id,                         //
-                                             callback = std::move(callback)   //
+  task_runners_.GetUITaskRunner()->RunNowOrPostTask(
+      task_runners_.GetUITaskRunner(), [engine = engine_->GetWeakPtr(),  //
+                                        viewport_metrics,                //
+                                        view_id,                         //
+                                        callback = std::move(callback)   //
   ] {
-    if (engine) {
-      engine->AddView(view_id, viewport_metrics, callback);
-    }
-  });
+        if (engine) {
+          engine->AddView(view_id, viewport_metrics, callback);
+        }
+      });
 }
 
 void Shell::OnPlatformViewRemoveView(int64_t view_id,
@@ -2163,7 +2173,8 @@ void Shell::OnPlatformViewRemoveView(int64_t view_id,
       << kFlutterImplicitViewId << ". This view should never be removed.";
 
   expected_frame_sizes_.erase(view_id);
-  task_runners_.GetUITaskRunner()->PostTask(
+  task_runners_.GetUITaskRunner()->RunNowOrPostTask(
+      task_runners_.GetUITaskRunner(),
       [&task_runners = task_runners_,           //
        engine = engine_->GetWeakPtr(),          //
        rasterizer = rasterizer_->GetWeakPtr(),  //
@@ -2303,13 +2314,13 @@ void Shell::OnDisplayUpdates(std::vector<std::unique_ptr<Display>> displays) {
   for (const auto& display : displays) {
     display_data.push_back(display->GetDisplayData());
   }
-  task_runners_.GetUITaskRunner()->PostTask(
-      [engine = engine_->GetWeakPtr(),
-       display_data = std::move(display_data)]() {
-        if (engine) {
-          engine->SetDisplays(display_data);
-        }
-      });
+  fml::TaskRunner::RunNowOrPostTask(task_runners_.GetUITaskRunner(),
+                                    [engine = engine_->GetWeakPtr(),
+                                     display_data = std::move(display_data)]() {
+                                      if (engine) {
+                                        engine->SetDisplays(display_data);
+                                      }
+                                    });
 
   display_manager_->HandleDisplayUpdates(std::move(displays));
 }

shell/common/switches.cc

@@ -361,6 +361,9 @@ Settings SettingsFromCommandLine(const fml::CommandLine& command_line) {
   settings.verbose_logging =
       command_line.HasOption(FlagForSwitch(Switch::VerboseLogging));
 
+  settings.merged_platform_ui_thread = command_line.HasOption(
+      FlagForSwitch(Switch::EnableMergedPlatformUIThread));
+
   command_line.GetOptionValue(FlagForSwitch(Switch::FlutterAssetsDir),
                               &settings.assets_path);
 

shell/common/switches.h

@@ -294,6 +294,9 @@ DEF_SWITCH(EnableEmbedderAPI,
 DEF_SWITCH(EnablePlatformIsolates,
            "enable-platform-isolates",
            "Enable support for isolates that run on the platform thread.")
+DEF_SWITCH(EnableMergedPlatformUIThread,
+           "enable-merged-platform-ui-thread",
+           "Merge the ui thread and platform thread.")
 DEF_SWITCHES_END
 
 void PrintUsage(const std::string& executable_name);

shell/platform/android/android_shell_holder.cc

@@ -94,10 +94,12 @@ AndroidShellHolder::AndroidShellHolder(
 
   flutter::ThreadHost::ThreadHostConfig host_config(
       thread_label, mask, AndroidPlatformThreadConfigSetter);
-  host_config.ui_config = fml::Thread::ThreadConfig(
-      flutter::ThreadHost::ThreadHostConfig::MakeThreadName(
-          flutter::ThreadHost::Type::kUi, thread_label),
-      fml::Thread::ThreadPriority::kDisplay);
+  if (!settings.merged_platform_ui_thread) {
+    host_config.ui_config = fml::Thread::ThreadConfig(
+        flutter::ThreadHost::ThreadHostConfig::MakeThreadName(
+            flutter::ThreadHost::Type::kUi, thread_label),
+        fml::Thread::ThreadPriority::kDisplay);
+  }
   host_config.raster_config = fml::Thread::ThreadConfig(
       flutter::ThreadHost::ThreadHostConfig::MakeThreadName(
           flutter::ThreadHost::Type::kRaster, thread_label),
@@ -137,7 +139,13 @@ AndroidShellHolder::AndroidShellHolder(
   fml::RefPtr<fml::TaskRunner> platform_runner =
       fml::MessageLoop::GetCurrent().GetTaskRunner();
   raster_runner = thread_host_->raster_thread->GetTaskRunner();
-  ui_runner = thread_host_->ui_thread->GetTaskRunner();
+  if (settings.merged_platform_ui_thread) {
+    FML_LOG(IMPORTANT)
+        << "Warning: Using highly experimental merged thread mode.";
+    ui_runner = platform_runner;
+  } else {
+    ui_runner = thread_host_->ui_thread->GetTaskRunner();
+  }
   io_runner = thread_host_->io_thread->GetTaskRunner();
 
   flutter::TaskRunners task_runners(thread_label,     // label

shell/platform/android/android_shell_holder.h

@@ -105,6 +105,9 @@ class AndroidShellHolder {
 
   void UpdateDisplayMetrics();
 
+  // Visible for testing.
+  const std::unique_ptr<Shell>& GetShellForTesting() const { return shell_; }
+
  private:
   const flutter::Settings settings_;
   const std::shared_ptr<PlatformViewAndroidJNI> jni_facade_;

shell/platform/android/android_shell_holder_unittests.cc

@@ -160,5 +160,34 @@ TEST(AndroidShellHolder, HandlePlatformMessage) {
   holder->GetPlatformMessageHandler()
       ->InvokePlatformMessageEmptyResponseCallback(response_id);
 }
+
+TEST(AndroidShellHolder, CreateWithMergedPlatformAndUIThread) {
+  Settings settings;
+  settings.merged_platform_ui_thread = true;
+  auto jni = std::make_shared<MockPlatformViewAndroidJNI>();
+  auto holder = std::make_unique<AndroidShellHolder>(settings, jni);
+  auto window = fml::MakeRefCounted<AndroidNativeWindow>(
+      nullptr, /*is_fake_window=*/true);
+  holder->GetPlatformView()->NotifyCreated(window);
+
+  EXPECT_EQ(
+      holder->GetShellForTesting()->GetTaskRunners().GetUITaskRunner(),
+      holder->GetShellForTesting()->GetTaskRunners().GetPlatformTaskRunner());
+}
+
+TEST(AndroidShellHolder, CreateWithUnMergedPlatformAndUIThread) {
+  Settings settings;
+  settings.merged_platform_ui_thread = false;
+  auto jni = std::make_shared<MockPlatformViewAndroidJNI>();
+  auto holder = std::make_unique<AndroidShellHolder>(settings, jni);
+  auto window = fml::MakeRefCounted<AndroidNativeWindow>(
+      nullptr, /*is_fake_window=*/true);
+  holder->GetPlatformView()->NotifyCreated(window);
+
+  EXPECT_NE(
+      holder->GetShellForTesting()->GetTaskRunners().GetUITaskRunner(),
+      holder->GetShellForTesting()->GetTaskRunners().GetPlatformTaskRunner());
+}
+
 }  // namespace testing
 }  // namespace flutter

shell/platform/android/io/flutter/embedding/engine/FlutterShellArgs.java

@@ -71,8 +71,6 @@ public static FlutterShellArgs fromIntent(@NonNull Intent intent) {
     // Before adding more entries to this list, consider that arbitrary
     // Android applications can generate intents with extra data and that
     // there are many security-sensitive args in the binary.
-    // TODO(mattcarroll): I left this warning as-is, but we should clarify what exactly this warning
-    // is warning against.
     ArrayList<String> args = new ArrayList<>();
 
     if (intent.getBooleanExtra(ARG_KEY_TRACE_STARTUP, false)) {

shell/platform/android/io/flutter/embedding/engine/loader/FlutterLoader.java

@@ -47,6 +47,8 @@ public class FlutterLoader {
       "io.flutter.embedding.android.EnableOpenGLGPUTracing";
   private static final String IMPELLER_VULKAN_GPU_TRACING_DATA_KEY =
       "io.flutter.embedding.android.EnableVulkanGPUTracing";
+  private static final String ENABLED_MERGED_PLATFORM_UI_THREAD_KEY =
+      "io.flutter.embedding.android.EnableMergedPlatformUIThread";
 
   /**
    * Set whether leave or clean up the VM after the last shell shuts down. It can be set from app's
@@ -352,6 +354,9 @@ public void ensureInitializationComplete(
         if (metaData.getBoolean(IMPELLER_VULKAN_GPU_TRACING_DATA_KEY, false)) {
           shellArgs.add("--enable-vulkan-gpu-tracing");
         }
+        if (metaData.getBoolean(ENABLED_MERGED_PLATFORM_UI_THREAD_KEY, false)) {
+          shellArgs.add("--enable-merged-platform-ui-thread");
+        }
         String backend = metaData.getString(IMPELLER_BACKEND_META_DATA_KEY);
         if (backend != null) {
           shellArgs.add("--impeller-backend=" + backend);

shell/platform/darwin/ios/framework/Source/FlutterDartProject.mm

@@ -212,6 +212,12 @@ static BOOL DoesHardwareSupportWideGamut() {
     settings.enable_dart_profiling = enableDartProfiling.boolValue;
   }
 
+  NSNumber* enableMergedPlatformUIThread =
+      [mainBundle objectForInfoDictionaryKey:@"FLTEnableMergedPlatformUIThread"];
+  if (enableMergedPlatformUIThread != nil) {
+    settings.merged_platform_ui_thread = enableMergedPlatformUIThread.boolValue;
+  }
+
   // Leak Dart VM settings, set whether leave or clean up the VM after the last shell shuts down.
   NSNumber* leakDartVM = [mainBundle objectForInfoDictionaryKey:@"FLTLeakDartVM"];
   // It will change the default leak_vm value in settings only if the key exists.