[engine] reland: always post tasks for platform channel responses. (flutter/engine#55006)

Reland of https://github.com/flutter/engine/pull/54975

Fixes the initial route behavior for iOS. Previously the initial route setting would _always_ be posted as a task, which after merging the threads would fire after isolate creation, thus it would not actually update the initial route setting. Fixed Engine constructor so that it reads the initial route from the settings.

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

@@ -61,6 +61,7 @@ Engine::Engine(
       task_runners_(task_runners),
       weak_factory_(this) {
   pointer_data_dispatcher_ = dispatcher_maker(*this);
+  initial_route_ = settings_.route;
 }
 
 Engine::Engine(Delegate& delegate,

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

@@ -252,6 +252,27 @@ TEST_F(EngineTest, Create) {
   });
 }
 
+TEST_F(EngineTest, CreateWithRoute) {
+  PostUITaskSync([this] {
+    auto settings = settings_;
+    settings.route = "/testo";
+    auto engine = std::make_unique<Engine>(
+        /*delegate=*/delegate_,
+        /*dispatcher_maker=*/dispatcher_maker_,
+        /*image_decoder_task_runner=*/image_decoder_task_runner_,
+        /*task_runners=*/task_runners_,
+        /*settings=*/settings,
+        /*animator=*/std::move(animator_),
+        /*io_manager=*/io_manager_,
+        /*font_collection=*/std::make_shared<FontCollection>(),
+        /*runtime_controller=*/std::move(runtime_controller_),
+        /*gpu_disabled_switch=*/std::make_shared<fml::SyncSwitch>());
+
+    EXPECT_TRUE(engine);
+    EXPECT_EQ(engine->InitialRoute(), "/testo");
+  });
+}
+
 TEST_F(EngineTest, DispatchPlatformMessageUnknown) {
   PostUITaskSync([this] {
     MockRuntimeDelegate client;

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

@@ -1076,10 +1076,10 @@ void Shell::OnPlatformViewDispatchPlatformMessage(
 
   // The static leak checker gets confused by the use of fml::MakeCopyable.
   // NOLINTNEXTLINE(clang-analyzer-cplusplus.NewDeleteLeaks)
-  fml::TaskRunner::RunNowOrPostTask(
-      task_runners_.GetUITaskRunner(),
-      fml::MakeCopyable([engine = engine_->GetWeakPtr(),
-                         message = std::move(message)]() mutable {
+  // This logic must always explicitly post a task so that we are guaranteed
+  // to wake up the UI message loop to flush tasks.
+  task_runners_.GetUITaskRunner()->PostTask(fml::MakeCopyable(
+      [engine = engine_->GetWeakPtr(), message = std::move(message)]() mutable {
         if (engine) {
           engine->DispatchPlatformMessage(std::move(message));
         }

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterEngine.mm

@@ -602,13 +602,6 @@ static constexpr int kNumProfilerSamplesPerSec = 5;
       binaryMessenger:self.binaryMessenger
                 codec:[FlutterJSONMethodCodec sharedInstance]]);
 
-  if ([_initialRoute length] > 0) {
-    // Flutter isn't ready to receive this method call yet but the channel buffer will cache this.
-    [_navigationChannel invokeMethod:@"setInitialRoute" arguments:_initialRoute];
-    [_initialRoute release];
-    _initialRoute = nil;
-  }
-
   _restorationChannel.reset([[FlutterMethodChannel alloc]
          initWithName:@"flutter/restoration"
       binaryMessenger:self.binaryMessenger
@@ -904,6 +897,13 @@ static void SetEntryPoint(flutter::Settings* settings, NSString* entrypoint, NSS
   _isGpuDisabled =
       [UIApplication sharedApplication].applicationState == UIApplicationStateBackground;
 #endif
+  // Override the setting route, as the dart project or function may have specified
+  // different values. During construction, the Engine constuctor will read the
+  // value of settings.route to determine the initial route value.
+  if (self.initialRoute) {
+    settings.route = [self.initialRoute UTF8String];
+    self.initialRoute = nil;
+  }
 
   // Create the shell. This is a blocking operation.
   std::unique_ptr<flutter::Shell> shell = flutter::Shell::Create(

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterEngineTest.mm

@@ -201,17 +201,17 @@ FLUTTER_ASSERT_ARC
   // Run with an initial route.
   [engine runWithEntrypoint:FlutterDefaultDartEntrypoint initialRoute:@"test"];
 
-  // Now check that an encoded method call has been made on the binary messenger to set the
-  // initial route to "test".
+  // Initial route is set directly in the shell/engine and should not send a platform
+  // channel message as it will arrive too late.
   FlutterMethodCall* setInitialRouteMethodCall =
       [FlutterMethodCall methodCallWithMethodName:@"setInitialRoute" arguments:@"test"];
   NSData* encodedSetInitialRouteMethod =
       [[FlutterJSONMethodCodec sharedInstance] encodeMethodCall:setInitialRouteMethodCall];
-  OCMVerify([mockBinaryMessenger sendOnChannel:@"flutter/navigation"
+  OCMReject([mockBinaryMessenger sendOnChannel:@"flutter/navigation"
                                        message:encodedSetInitialRouteMethod]);
 }
 
-- (void)testInitialRouteSettingsSendsNavigationMessage {
+- (void)testInitialRouteSettingsDoesNotSendNavigationMessage {
   id mockBinaryMessenger = OCMClassMock([FlutterBinaryMessengerRelay class]);
 
   auto settings = FLTDefaultSettingsForBundle();
@@ -221,13 +221,13 @@ FLUTTER_ASSERT_ARC
   [engine setBinaryMessenger:mockBinaryMessenger];
   [engine run];
 
-  // Now check that an encoded method call has been made on the binary messenger to set the
-  // initial route to "test".
+  // Initial route is set directly in the shell/engine and should not send a platform
+  // channel message as it will arrive too late.
   FlutterMethodCall* setInitialRouteMethodCall =
       [FlutterMethodCall methodCallWithMethodName:@"setInitialRoute" arguments:@"test"];
   NSData* encodedSetInitialRouteMethod =
       [[FlutterJSONMethodCodec sharedInstance] encodeMethodCall:setInitialRouteMethodCall];
-  OCMVerify([mockBinaryMessenger sendOnChannel:@"flutter/navigation"
+  OCMReject([mockBinaryMessenger sendOnChannel:@"flutter/navigation"
                                        message:encodedSetInitialRouteMethod]);
 }