[Windows] Introduce an accessibility plugin (flutter/engine#50975)

_This is the same pull request as https://github.com/flutter/engine/pull/50898. GitHub broke on the previous pull request so I re-created it_

This moves the logic to handle `flutter/accessibility` messages to a new type, `AccessibilityPlugin`. 

Notable changes:

1. Windows app no longer crashes if it receives accessibility events it does not support
2. Windows app no longer crashes if it receives accessibility events while in headless mode

@yaakovschectman After playing around with this, I ended up using a different pattern than what what I suggested on https://github.com/flutter/engine/pull/50598#discussion_r1488728089. This message handler is simple enough that splitting into a child/base types felt like unnecessary boilerplate. The key thing is separating messaging and implementation logic, which was achieved through the `SetUp` method. Let me know what you think, and sorry for all my flip-flopping on this topic! 😅

This is preparation for: https://github.com/flutter/flutter/issues/143765

Sample app for manual testing: https://github.com/flutter/flutter/issues/113059

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -29967,6 +29967,8 @@ ORIGIN: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_view.h + .
 ORIGIN: ../../../flutter/shell/platform/linux/public/flutter_linux/flutter_linux.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/accessibility_bridge_windows.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/accessibility_bridge_windows.h + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/windows/accessibility_plugin.cc + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/windows/accessibility_plugin.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/client_wrapper/flutter_engine.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/client_wrapper/flutter_view_controller.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/client_wrapper/include/flutter/dart_project.h + ../../../flutter/LICENSE
@@ -32840,6 +32842,8 @@ FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/fl_view.h
 FILE: ../../../flutter/shell/platform/linux/public/flutter_linux/flutter_linux.h
 FILE: ../../../flutter/shell/platform/windows/accessibility_bridge_windows.cc
 FILE: ../../../flutter/shell/platform/windows/accessibility_bridge_windows.h
+FILE: ../../../flutter/shell/platform/windows/accessibility_plugin.cc
+FILE: ../../../flutter/shell/platform/windows/accessibility_plugin.h
 FILE: ../../../flutter/shell/platform/windows/client_wrapper/flutter_engine.cc
 FILE: ../../../flutter/shell/platform/windows/client_wrapper/flutter_view_controller.cc
 FILE: ../../../flutter/shell/platform/windows/client_wrapper/include/flutter/dart_project.h

engine/src/flutter/shell/platform/windows/BUILD.gn

@@ -41,6 +41,8 @@ source_set("flutter_windows_source") {
   sources = [
     "accessibility_bridge_windows.cc",
     "accessibility_bridge_windows.h",
+    "accessibility_plugin.cc",
+    "accessibility_plugin.h",
     "compositor.h",
     "compositor_opengl.cc",
     "compositor_opengl.h",

engine/src/flutter/shell/platform/windows/accessibility_plugin.cc

@@ -0,0 +1,108 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "flutter/shell/platform/windows/accessibility_plugin.h"
+
+#include <variant>
+
+#include "flutter/fml/logging.h"
+#include "flutter/fml/platform/win/wstring_conversion.h"
+#include "flutter/shell/platform/common/client_wrapper/include/flutter/standard_message_codec.h"
+#include "flutter/shell/platform/windows/flutter_windows_engine.h"
+#include "flutter/shell/platform/windows/flutter_windows_view.h"
+
+namespace flutter {
+
+namespace {
+
+static constexpr char kAccessibilityChannelName[] = "flutter/accessibility";
+static constexpr char kTypeKey[] = "type";
+static constexpr char kDataKey[] = "data";
+static constexpr char kMessageKey[] = "message";
+static constexpr char kAnnounceValue[] = "announce";
+
+// Handles messages like:
+// {"type": "announce", "data": {"message": "Hello"}}
+void HandleMessage(AccessibilityPlugin* plugin, const EncodableValue& message) {
+  const auto* map = std::get_if<EncodableMap>(&message);
+  if (!map) {
+    FML_LOG(ERROR) << "Accessibility message must be a map.";
+    return;
+  }
+  const auto& type_itr = map->find(EncodableValue{kTypeKey});
+  const auto& data_itr = map->find(EncodableValue{kDataKey});
+  if (type_itr == map->end()) {
+    FML_LOG(ERROR) << "Accessibility message must have a 'type' property.";
+    return;
+  }
+  if (data_itr == map->end()) {
+    FML_LOG(ERROR) << "Accessibility message must have a 'data' property.";
+    return;
+  }
+  const auto* type = std::get_if<std::string>(&type_itr->second);
+  const auto* data = std::get_if<EncodableMap>(&data_itr->second);
+  if (!type) {
+    FML_LOG(ERROR) << "Accessibility message 'type' property must be a string.";
+    return;
+  }
+  if (!data) {
+    FML_LOG(ERROR) << "Accessibility message 'data' property must be a map.";
+    return;
+  }
+
+  if (type->compare(kAnnounceValue) == 0) {
+    const auto& message_itr = data->find(EncodableValue{kMessageKey});
+    if (message_itr == data->end()) {
+      return;
+    }
+    const auto* message = std::get_if<std::string>(&message_itr->second);
+    if (!message) {
+      return;
+    }
+
+    plugin->Announce(*message);
+  } else {
+    FML_LOG(ERROR) << "Accessibility message type '" << *type
+                   << "' is not supported.";
+  }
+}
+
+}  // namespace
+
+AccessibilityPlugin::AccessibilityPlugin(FlutterWindowsEngine* engine)
+    : engine_(engine) {}
+
+void AccessibilityPlugin::SetUp(BinaryMessenger* binary_messenger,
+                                AccessibilityPlugin* plugin) {
+  BasicMessageChannel<> channel{binary_messenger, kAccessibilityChannelName,
+                                &StandardMessageCodec::GetInstance()};
+
+  channel.SetMessageHandler(
+      [plugin](const EncodableValue& message,
+               const MessageReply<EncodableValue>& reply) {
+        HandleMessage(plugin, message);
+
+        // The accessibility channel does not support error handling.
+        // Always return an empty response even on failure.
+        reply(EncodableValue{std::monostate{}});
+      });
+}
+
+void AccessibilityPlugin::Announce(const std::string_view message) {
+  if (!engine_->semantics_enabled()) {
+    return;
+  }
+
+  // TODO(loicsharma): Remove implicit view assumption.
+  // https://github.com/flutter/flutter/issues/142845
+  auto view = engine_->view(kImplicitViewId);
+  if (!view) {
+    return;
+  }
+
+  std::wstring wide_text = fml::Utf8ToWideString(message);
+  view->AnnounceAlert(wide_text);
+}
+
+}  // namespace flutter

engine/src/flutter/shell/platform/windows/accessibility_plugin.h

@@ -0,0 +1,41 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef FLUTTER_SHELL_PLATFORM_WINDOWS_ACCESSIBILITY_PLUGIN_H_
+#define FLUTTER_SHELL_PLATFORM_WINDOWS_ACCESSIBILITY_PLUGIN_H_
+
+#include <string_view>
+
+#include "flutter/fml/macros.h"
+#include "flutter/shell/platform/common/client_wrapper/include/flutter/binary_messenger.h"
+
+namespace flutter {
+
+class FlutterWindowsEngine;
+
+// Handles messages on the flutter/accessibility channel.
+//
+// See:
+// https://api.flutter.dev/flutter/semantics/SemanticsService-class.html
+class AccessibilityPlugin {
+ public:
+  explicit AccessibilityPlugin(FlutterWindowsEngine* engine);
+
+  // Begin handling accessibility messages on the `binary_messenger`.
+  static void SetUp(BinaryMessenger* binary_messenger,
+                    AccessibilityPlugin* plugin);
+
+  // Announce a message through the assistive technology.
+  virtual void Announce(const std::string_view message);
+
+ private:
+  // The engine that owns this plugin.
+  FlutterWindowsEngine* engine_ = nullptr;
+
+  FML_DISALLOW_COPY_AND_ASSIGN(AccessibilityPlugin);
+};
+
+}  // namespace flutter
+
+#endif  // FLUTTER_SHELL_PLATFORM_WINDOWS_ACCESSIBILITY_PLUGIN_H_

engine/src/flutter/shell/platform/windows/fixtures/main.dart

@@ -59,36 +59,65 @@ void sendAccessibilityAnnouncement() async {
     await semanticsChanged;
   }
 
-  // Serializers for data types are in the framework, so this will be hardcoded.
+  // Standard message codec magic number identifiers.
+  // See: https://github.com/flutter/flutter/blob/ee94fe262b63b0761e8e1f889ae52322fef068d2/packages/flutter/lib/src/services/message_codecs.dart#L262
   const int valueMap = 13, valueString = 7;
-  // Corresponds to:
-  // Map<String, Object> data =
-  // {"type": "announce", "data": {"message": ""}};
+
+  // Corresponds to: {"type": "announce", "data": {"message": "hello"}}
+  // See: https://github.com/flutter/flutter/blob/b781da9b5822de1461a769c3b245075359f5464d/packages/flutter/lib/src/semantics/semantics_event.dart#L86
   final Uint8List data = Uint8List.fromList([
-    valueMap, // _valueMap
-    2, // Size
-    // key: "type"
-    valueString,
-    'type'.length,
-    ...'type'.codeUnits,
-    // value: "announce"
-    valueString,
-    'announce'.length,
-    ...'announce'.codeUnits,
-    // key: "data"
-    valueString,
-    'data'.length,
-    ...'data'.codeUnits,
-    // value: map
-    valueMap, // _valueMap
-    1, // Size
-    // key: "message"
-    valueString,
-    'message'.length,
-    ...'message'.codeUnits,
-    // value: ""
-    valueString,
-    0, // Length of empty string == 0.
+    // Map with 2 entries
+    valueMap, 2,
+    // Map key: "type"
+    valueString, 'type'.length, ...'type'.codeUnits,
+    // Map value: "announce"
+    valueString, 'announce'.length, ...'announce'.codeUnits,
+    // Map key: "data"
+    valueString, 'data'.length, ...'data'.codeUnits,
+    // Map value: map with 1 entry
+    valueMap, 1,
+    // Map key: "message"
+    valueString, 'message'.length, ...'message'.codeUnits,
+    // Map value: "hello"
+    valueString, 'hello'.length, ...'hello'.codeUnits,
+  ]);
+  final ByteData byteData = data.buffer.asByteData();
+
+  ui.PlatformDispatcher.instance.sendPlatformMessage(
+    'flutter/accessibility',
+    byteData,
+    (ByteData? _) => signal(),
+  );
+}
+
+@pragma('vm:entry-point')
+void sendAccessibilityTooltipEvent() async {
+  // Wait until semantics are enabled.
+  if (!ui.PlatformDispatcher.instance.semanticsEnabled) {
+    await semanticsChanged;
+  }
+
+  // Standard message codec magic number identifiers.
+  // See: https://github.com/flutter/flutter/blob/ee94fe262b63b0761e8e1f889ae52322fef068d2/packages/flutter/lib/src/services/message_codecs.dart#L262
+  const int valueMap = 13, valueString = 7;
+
+  // Corresponds to: {"type": "tooltip", "data": {"message": "hello"}}
+  // See: https://github.com/flutter/flutter/blob/b781da9b5822de1461a769c3b245075359f5464d/packages/flutter/lib/src/semantics/semantics_event.dart#L120
+  final Uint8List data = Uint8List.fromList([
+    // Map with 2 entries
+    valueMap, 2,
+    // Map key: "type"
+    valueString, 'type'.length, ...'type'.codeUnits,
+    // Map value: "tooltip"
+    valueString, 'tooltip'.length, ...'tooltip'.codeUnits,
+    // Map key: "data"
+    valueString, 'data'.length, ...'data'.codeUnits,
+    // Map value: map with 1 entry
+    valueMap, 1,
+    // Map key: "message"
+    valueString, 'message'.length, ...'message'.codeUnits,
+    // Map value: "hello"
+    valueString, 'hello'.length, ...'hello'.codeUnits,
   ]);
   final ByteData byteData = data.buffer.asByteData();
 

engine/src/flutter/shell/platform/windows/flutter_windows_engine.cc

@@ -183,14 +183,6 @@ FlutterWindowsEngine::FlutterWindowsEngine(
       std::make_unique<BinaryMessengerImpl>(messenger_->ToRef());
   message_dispatcher_ =
       std::make_unique<IncomingMessageDispatcher>(messenger_->ToRef());
-  message_dispatcher_->SetMessageCallback(
-      kAccessibilityChannelName,
-      [](FlutterDesktopMessengerRef messenger,
-         const FlutterDesktopMessage* message, void* data) {
-        FlutterWindowsEngine* engine = static_cast<FlutterWindowsEngine*>(data);
-        engine->HandleAccessibilityMessage(messenger, message);
-      },
-      static_cast<void*>(this));
 
   texture_registrar_ =
       std::make_unique<FlutterWindowsTextureRegistrar>(this, gl_);
@@ -219,6 +211,11 @@ FlutterWindowsEngine::FlutterWindowsEngine(
   // https://github.com/flutter/flutter/issues/71099
   internal_plugin_registrar_ =
       std::make_unique<PluginRegistrar>(plugin_registrar_.get());
+
+  accessibility_plugin_ = std::make_unique<AccessibilityPlugin>(this);
+  AccessibilityPlugin::SetUp(messenger_wrapper_.get(),
+                             accessibility_plugin_.get());
+
   cursor_handler_ =
       std::make_unique<CursorHandler>(messenger_wrapper_.get(), this);
   platform_handler_ =
@@ -765,7 +762,9 @@ void FlutterWindowsEngine::UpdateSemanticsEnabled(bool enabled) {
   if (engine_ && semantics_enabled_ != enabled) {
     semantics_enabled_ = enabled;
     embedder_api_.UpdateSemanticsEnabled(engine_, enabled);
-    view_->UpdateSemanticsEnabled(enabled);
+    if (view_) {
+      view_->UpdateSemanticsEnabled(enabled);
+    }
   }
 }
 
@@ -813,27 +812,6 @@ void FlutterWindowsEngine::SendAccessibilityFeatures() {
       engine_, static_cast<FlutterAccessibilityFeature>(flags));
 }
 
-void FlutterWindowsEngine::HandleAccessibilityMessage(
-    FlutterDesktopMessengerRef messenger,
-    const FlutterDesktopMessage* message) {
-  const auto& codec = StandardMessageCodec::GetInstance();
-  auto data = codec.DecodeMessage(message->message, message->message_size);
-  EncodableMap map = std::get<EncodableMap>(*data);
-  std::string type = std::get<std::string>(map.at(EncodableValue("type")));
-  if (type.compare("announce") == 0) {
-    if (semantics_enabled_) {
-      EncodableMap data_map =
-          std::get<EncodableMap>(map.at(EncodableValue("data")));
-      std::string text =
-          std::get<std::string>(data_map.at(EncodableValue("message")));
-      std::wstring wide_text = fml::Utf8ToWideString(text);
-      view_->AnnounceAlert(wide_text);
-    }
-  }
-  SendPlatformMessageResponse(message->response_handle,
-                              reinterpret_cast<const uint8_t*>(""), 0);
-}
-
 void FlutterWindowsEngine::RequestApplicationQuit(HWND hwnd,
                                                   WPARAM wparam,
                                                   LPARAM lparam,

engine/src/flutter/shell/platform/windows/flutter_windows_engine.h

@@ -22,6 +22,7 @@
 #include "flutter/shell/platform/common/incoming_message_dispatcher.h"
 #include "flutter/shell/platform/embedder/embedder.h"
 #include "flutter/shell/platform/windows/accessibility_bridge_windows.h"
+#include "flutter/shell/platform/windows/accessibility_plugin.h"
 #include "flutter/shell/platform/windows/compositor.h"
 #include "flutter/shell/platform/windows/cursor_handler.h"
 #include "flutter/shell/platform/windows/egl/manager.h"
@@ -338,9 +339,6 @@ class FlutterWindowsEngine {
   // Send the currently enabled accessibility features to the engine.
   void SendAccessibilityFeatures();
 
-  void HandleAccessibilityMessage(FlutterDesktopMessengerRef messenger,
-                                  const FlutterDesktopMessage* message);
-
   // The handle to the embedder.h engine instance.
   FLUTTER_API_SYMBOL(FlutterEngine) engine_ = nullptr;
 
@@ -384,6 +382,9 @@ class FlutterWindowsEngine {
   // The plugin registrar managing internal plugins.
   std::unique_ptr<PluginRegistrar> internal_plugin_registrar_;
 
+  // Handler for accessibility events.
+  std::unique_ptr<AccessibilityPlugin> accessibility_plugin_;
+
   // Handler for cursor events.
   std::unique_ptr<CursorHandler> cursor_handler_;
 

engine/src/flutter/shell/platform/windows/flutter_windows_engine_unittests.cc

@@ -677,6 +677,54 @@ TEST_F(FlutterWindowsEngineTest, AccessibilityAnnouncement) {
   }
 }
 
+// Verify the app can send accessibility announcements while in headless mode.
+TEST_F(FlutterWindowsEngineTest, AccessibilityAnnouncementHeadless) {
+  auto& context = GetContext();
+  WindowsConfigBuilder builder{context};
+  builder.SetDartEntrypoint("sendAccessibilityAnnouncement");
+
+  bool done = false;
+  auto native_entry =
+      CREATE_NATIVE_ENTRY([&](Dart_NativeArguments args) { done = true; });
+  context.AddNativeFunction("Signal", native_entry);
+
+  EnginePtr engine{builder.RunHeadless()};
+  ASSERT_NE(engine, nullptr);
+
+  auto windows_engine = reinterpret_cast<FlutterWindowsEngine*>(engine.get());
+  windows_engine->UpdateSemanticsEnabled(true);
+
+  // Rely on timeout mechanism in CI.
+  while (!done) {
+    windows_engine->task_runner()->ProcessTasks();
+  }
+}
+
+// Verify the engine does not crash if it receives an accessibility event
+// it does not support yet.
+TEST_F(FlutterWindowsEngineTest, AccessibilityTooltip) {
+  auto& context = GetContext();
+  WindowsConfigBuilder builder{context};
+  builder.SetDartEntrypoint("sendAccessibilityTooltipEvent");
+
+  bool done = false;
+  auto native_entry =
+      CREATE_NATIVE_ENTRY([&](Dart_NativeArguments args) { done = true; });
+  context.AddNativeFunction("Signal", native_entry);
+
+  ViewControllerPtr controller{builder.Run()};
+  ASSERT_NE(controller, nullptr);
+
+  auto engine = FlutterDesktopViewControllerGetEngine(controller.get());
+  auto windows_engine = reinterpret_cast<FlutterWindowsEngine*>(engine);
+  windows_engine->UpdateSemanticsEnabled(true);
+
+  // Rely on timeout mechanism in CI.
+  while (!done) {
+    windows_engine->task_runner()->ProcessTasks();
+  }
+}
+
 class MockWindowsLifecycleManager : public WindowsLifecycleManager {
  public:
   MockWindowsLifecycleManager(FlutterWindowsEngine* engine)