[engine] dispatch platform channel messages through event loop, except navigation on start.

runtime/runtime_controller.cc

@@ -100,7 +100,7 @@ RuntimeController::~RuntimeController() {
   }
 }
 
-bool RuntimeController::IsRootIsolateRunning() {
+bool RuntimeController::IsRootIsolateRunning() const {
   std::shared_ptr<DartIsolate> root_isolate = root_isolate_.lock();
   if (root_isolate) {
     return root_isolate->GetPhase() == DartIsolate::Phase::Running;

runtime/runtime_controller.h

@@ -452,7 +452,7 @@ class RuntimeController : public PlatformConfigurationClient,
   ///
   /// @return     True if root isolate running, False otherwise.
   ///
-  virtual bool IsRootIsolateRunning();
+  virtual bool IsRootIsolateRunning() const;
 
   //----------------------------------------------------------------------------
   /// @brief      Dispatch the specified platform message to running root

shell/common/engine_unittests.cc

@@ -144,7 +144,7 @@ class MockRuntimeController : public RuntimeController {
   MockRuntimeController(RuntimeDelegate& client,
                         const TaskRunners& p_task_runners)
       : RuntimeController(client, p_task_runners) {}
-  MOCK_METHOD(bool, IsRootIsolateRunning, (), (override));
+  MOCK_METHOD(bool, IsRootIsolateRunning, (), (override, const));
   MOCK_METHOD(bool,
               DispatchPlatformMessage,
               (std::unique_ptr<PlatformMessage>),

shell/common/shell.cc

@@ -1074,16 +1074,38 @@ void Shell::OnPlatformViewDispatchPlatformMessage(
   }
 #endif  // FLUTTER_RUNTIME_MODE == FLUTTER_RUNTIME_MODE_DEBUG
 
-  // 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 {
-        if (engine) {
-          engine->DispatchPlatformMessage(std::move(message));
-        }
-      }));
+  // If the root isolate is not yet running this may be the navigation
+  // channel initial route and must be dispatched immediately so that
+  // it can be set before isolate creation.
+  static constexpr char kNavigationChannel[] = "flutter/navigation";
+  if (!engine_->GetRuntimeController()->IsRootIsolateRunning() &&
+      message->channel() == kNavigationChannel) {
+    // 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 {
+          if (engine) {
+            engine->DispatchPlatformMessage(std::move(message));
+          }
+        }));
+  } else {
+    // In all other cases, the message must be dispatched via a new task so
+    // that the completion of the platform channel response future is guaranteed
+    // to wake up the Dart event loop, even in cases where the platform and UI
+    // threads are the same.
+
+    // 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 {
+          if (engine) {
+            engine->DispatchPlatformMessage(std::move(message));
+          }
+        }));
+  }
 }
 
 // |PlatformView::Delegate|

shell/common/shell_unittests.cc

@@ -68,6 +68,33 @@ using ::testing::_;
 using ::testing::Return;
 
 namespace {
+
+std::unique_ptr<PlatformMessage> MakePlatformMessage(
+    const std::string& channel,
+    const std::map<std::string, std::string>& values,
+    const fml::RefPtr<PlatformMessageResponse>& response) {
+  rapidjson::Document document;
+  auto& allocator = document.GetAllocator();
+  document.SetObject();
+
+  for (const auto& pair : values) {
+    rapidjson::Value key(pair.first.c_str(), strlen(pair.first.c_str()),
+                         allocator);
+    rapidjson::Value value(pair.second.c_str(), strlen(pair.second.c_str()),
+                           allocator);
+    document.AddMember(key, value, allocator);
+  }
+
+  rapidjson::StringBuffer buffer;
+  rapidjson::Writer<rapidjson::StringBuffer> writer(buffer);
+  document.Accept(writer);
+  const uint8_t* data = reinterpret_cast<const uint8_t*>(buffer.GetString());
+
+  std::unique_ptr<PlatformMessage> message = std::make_unique<PlatformMessage>(
+      channel, fml::MallocMapping::Copy(data, buffer.GetSize()), response);
+  return message;
+}
+
 class MockPlatformViewDelegate : public PlatformView::Delegate {
   MOCK_METHOD(void,
               OnPlatformViewCreated,
@@ -4260,6 +4287,31 @@ TEST_F(ShellTest, PrintsErrorWhenPlatformMessageSentFromWrongThread) {
 #endif
 }
 
+TEST_F(ShellTest, NavigationMessageDispachedImmediately) {
+  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);
+  auto shell = CreateShell(settings, task_runners);
+
+  auto latch = std::make_shared<fml::CountDownLatch>(1u);
+  task_runner->PostTask([&]() {
+    auto message = MakePlatformMessage(
+        "flutter/navigation",
+        {{"method", "setInitialRoute"}, {"args", "/testo"}}, nullptr);
+    SendPlatformMessage(shell.get(), std::move(message));
+    EXPECT_EQ(shell->GetEngine()->InitialRoute(), "/testo");
+
+    latch->CountDown();
+  });
+  latch->Wait();
+
+  DestroyShell(std::move(shell), task_runners);
+  ASSERT_FALSE(DartVMRef::IsInstanceRunning());
+}
+
 TEST_F(ShellTest, DiesIfSoftwareRenderingAndImpellerAreEnabledDeathTest) {
 #if defined(OS_FUCHSIA)
   GTEST_SKIP() << "Fuchsia";