[Windows] Move keyboard initialization

shell/platform/windows/fixtures/main.dart

@@ -231,6 +231,47 @@ void sendCreatePlatformViewMethod() async {
   await completed.future;
 }
 
+@pragma('vm:entry-point')
+void sendGetKeyboardState() async {
+  // The keyboard method channel uses the standard method codec.
+  // See https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/services/message_codecs.dart#L262
+  // for the implementation of the encoding and magic number identifiers.
+  const int valueNull = 0;
+  const int valueString = 7;
+  const int valueMap = 13;
+
+  const String method = 'getKeyboardState';
+  final List<int> data = <int>[
+    // Method name
+    valueString, method.length, ...utf8.encode(method),
+    // Method arguments: null
+    valueNull, 2,
+  ];
+
+  final Completer<void> completer = Completer<void>();
+  final ByteData bytes = ByteData.sublistView(Uint8List.fromList(data));
+  ui.PlatformDispatcher.instance.sendPlatformMessage('flutter/keyboard', bytes, (ByteData? response) {
+    // For magic numbers for decoding a reply envelope, see:
+    // https://github.com/flutter/flutter/blob/67271f69f7f88a4edba6d8023099e3bd27a072d2/packages/flutter/lib/src/services/message_codecs.dart#L577-L587
+    const int replyEnvelopeSuccess = 0;
+
+    // Ensure the response is a success containing a map of keyboard states.
+    if (response == null) {
+      signalStringValue('Unexpected null response');
+    } else if (response.lengthInBytes < 2) {
+      signalStringValue('Unexpected response length of ${response.lengthInBytes} bytes');
+    } else if (response.getUint8(0) != replyEnvelopeSuccess) {
+      signalStringValue('Unexpected response envelope status: ${response.getUint8(0)}');
+    } else if (response.getUint8(1) != valueMap) {
+      signalStringValue('Unexpected response value magic number: ${response.getUint8(1)}');
+    } else {
+      signalStringValue('Success');
+    }
+    completer.complete();
+  });
+  await completer.future;
+}
+
 @pragma('vm:entry-point')
 void customEntrypoint() {}
 

shell/platform/windows/flutter_windows_engine.cc

@@ -473,6 +473,8 @@ bool FlutterWindowsEngine::Run(std::string_view entrypoint) {
   settings_plugin_->StartWatching();
   settings_plugin_->SendSettings();
 
+  InitializeKeyboard();
+
   return true;
 }
 
@@ -497,7 +499,6 @@ std::unique_ptr<FlutterWindowsView> FlutterWindowsEngine::CreateView(
       kImplicitViewId, this, std::move(window), windows_proc_table_);
 
   views_[kImplicitViewId] = view.get();
-  InitializeKeyboard();
 
   return std::move(view);
 }
@@ -675,10 +676,6 @@ void FlutterWindowsEngine::SendSystemLocales() {
 }
 
 void FlutterWindowsEngine::InitializeKeyboard() {
-  if (views_.empty()) {
-    FML_LOG(ERROR) << "Cannot initialize keyboard on Windows headless mode.";
-  }
-
   auto internal_plugin_messenger = internal_plugin_registrar_->messenger();
   KeyboardKeyEmbedderHandler::GetKeyStateHandler get_key_state = GetKeyState;
   KeyboardKeyEmbedderHandler::MapVirtualKeyToScanCode map_vk_to_scan =
@@ -773,9 +770,7 @@ void FlutterWindowsEngine::UpdateSemanticsEnabled(bool enabled) {
 
 void FlutterWindowsEngine::OnPreEngineRestart() {
   // Reset the keyboard's state on hot restart.
-  if (!views_.empty()) {
-    InitializeKeyboard();
-  }
+  InitializeKeyboard();
 }
 
 std::string FlutterWindowsEngine::GetExecutableName() const {

shell/platform/windows/flutter_windows_unittests.cc

@@ -477,5 +477,43 @@ TEST_F(WindowsTest, Lifecycle) {
                /* bRepaint*/ false);
 }
 
+TEST_F(WindowsTest, GetKeyboardStateHeadless) {
+  // Run the test on its own thread so that it can pump its event loop while
+  // this thread waits.
+  fml::AutoResetWaitableEvent latch;
+  auto platform_task_runner = CreateNewThread("test_platform_thread");
+  platform_task_runner->PostTask([&]() {
+    auto& context = GetContext();
+    WindowsConfigBuilder builder(context);
+    builder.SetDartEntrypoint("sendGetKeyboardState");
+
+    bool done = false;
+    context.AddNativeFunction(
+        "SignalStringValue",
+        CREATE_NATIVE_ENTRY([&](Dart_NativeArguments args) {
+          auto handle = Dart_GetNativeArgument(args, 0);
+          ASSERT_FALSE(Dart_IsError(handle));
+          auto value = tonic::DartConverter<std::string>::FromDart(handle);
+          EXPECT_EQ(value, "Success");
+          done = true;
+          latch.Signal();
+        }));
+
+    ViewControllerPtr controller{builder.Run()};
+    ASSERT_NE(controller, nullptr);
+
+    // Pump messages for the Windows platform task runner.
+    ::MSG msg;
+    while (!done) {
+      if (::GetMessage(&msg, nullptr, 0, 0)) {
+        ::TranslateMessage(&msg);
+        ::DispatchMessage(&msg);
+      }
+    }
+  });
+
+  latch.Wait();
+}
+
 }  // namespace testing
 }  // namespace flutter