[engine] add back opt out for merged threads. (#55952)

Add back opt out for merged threads for customer money.

Author: Jonah Williams

common/settings.h

@@ -363,6 +363,10 @@ struct Settings {
   ///
   /// This is used by the runOnPlatformThread API.
   bool enable_platform_isolates = false;
+
+  // If true, the UI thread is the platform thread on supported
+  // platforms.
+  bool merged_platform_ui_thread = true;
 };
 
 }  // namespace flutter

shell/common/switches.cc

@@ -529,6 +529,9 @@ Settings SettingsFromCommandLine(const fml::CommandLine& command_line) {
   settings.disable_surface_control = command_line.HasOption(
       FlagForSwitch(Switch::DisableAndroidSurfaceControl));
 
+  settings.merged_platform_ui_thread = !command_line.HasOption(
+      FlagForSwitch(Switch::DisableMergedPlatformUIThread));
+
   return settings;
 }
 

shell/common/switches.h

@@ -294,8 +294,8 @@ 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",
+DEF_SWITCH(DisableMergedPlatformUIThread,
+           "no-enable-merged-platform-ui-thread",
            "Merge the ui thread and platform thread.")
 DEF_SWITCH(DisableAndroidSurfaceControl,
            "disable-surface-control",

shell/platform/android/android_shell_holder.cc

@@ -90,6 +90,9 @@ AndroidShellHolder::AndroidShellHolder(
   auto thread_label = std::to_string(thread_host_count++);
 
   auto mask = ThreadHost::Type::kRaster | ThreadHost::Type::kIo;
+  if (!settings.merged_platform_ui_thread) {
+    mask |= ThreadHost::Type::kUi;
+  }
 
   flutter::ThreadHost::ThreadHostConfig host_config(
       thread_label, mask, AndroidPlatformThreadConfigSetter);
@@ -136,7 +139,11 @@ AndroidShellHolder::AndroidShellHolder(
   fml::RefPtr<fml::TaskRunner> platform_runner =
       fml::MessageLoop::GetCurrent().GetTaskRunner();
   raster_runner = thread_host_->raster_thread->GetTaskRunner();
-  ui_runner = platform_runner;
+  if (settings.merged_platform_ui_thread) {
+    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_unittests.cc

@@ -174,5 +174,19 @@ TEST(AndroidShellHolder, CreateWithMergedPlatformAndUIThread) {
       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/loader/FlutterLoader.java

@@ -47,8 +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";
+  private static final String DISABLE_MERGED_PLATFORM_UI_THREAD_KEY =
+      "io.flutter.embedding.android.DisableMergedPlatformUIThread";
   private static final String DISABLE_SURFACE_CONTROL =
       "io.flutter.embedding.android.DisableSurfaceControl";
 
@@ -363,17 +363,16 @@ public void ensureInitializationComplete(
         if (metaData.getBoolean(IMPELLER_VULKAN_GPU_TRACING_DATA_KEY, false)) {
           shellArgs.add("--enable-vulkan-gpu-tracing");
         }
-        if (metaData.getBoolean(DISABLE_SURFACE_CONTROL, false)) {
-          shellArgs.add("--disable-surface-control");
-        }
-        if (metaData.containsKey(ENABLED_MERGED_PLATFORM_UI_THREAD_KEY)) {
-          if (metaData.getBoolean(ENABLED_MERGED_PLATFORM_UI_THREAD_KEY)) {
-            shellArgs.add("--enable-merged-platform-ui-thread");
-          } else {
+        if (metaData.containsKey(DISABLE_MERGED_PLATFORM_UI_THREAD_KEY)) {
+          if (metaData.getBoolean(DISABLE_MERGED_PLATFORM_UI_THREAD_KEY)) {
             shellArgs.add("--no-enable-merged-platform-ui-thread");
           }
         }
 
+        if (metaData.getBoolean(DISABLE_SURFACE_CONTROL, false)) {
+          shellArgs.add("--disable-surface-control");
+        }
+
         String backend = metaData.getString(IMPELLER_BACKEND_META_DATA_KEY);
         if (backend != null) {
           shellArgs.add("--impeller-backend=" + backend);