Add Platform View Manager to Windows shell (flutter/engine#50598)

Create a manager for platform views accessible to the compositor, handle
the methods that the framework will send.

Addresses https://github.com/flutter/flutter/issues/143375

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] I signed the [CLA].
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[test-exempt]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[C++, Objective-C, Java style guides]:
https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
[testing the engine]:
https://github.com/flutter/flutter/wiki/Testing-the-engine
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/wiki/Chat

---------

Co-authored-by: Loïc Sharma <737941+loic-sharma@users.noreply.github.com>

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -30012,6 +30012,7 @@ ORIGIN: ../../../flutter/shell/platform/windows/flutter_window.h + ../../../flut
 ORIGIN: ../../../flutter/shell/platform/windows/flutter_windows.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/flutter_windows_engine.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/flutter_windows_engine.h + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/windows/flutter_windows_internal.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/flutter_windows_texture_registrar.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/flutter_windows_texture_registrar.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/flutter_windows_view.cc + ../../../flutter/LICENSE
@@ -30030,6 +30031,10 @@ ORIGIN: ../../../flutter/shell/platform/windows/keyboard_utils.cc + ../../../flu
 ORIGIN: ../../../flutter/shell/platform/windows/keyboard_utils.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/platform_handler.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/platform_handler.h + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/windows/platform_view_manager.cc + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/windows/platform_view_manager.h + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/windows/platform_view_plugin.cc + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/windows/platform_view_plugin.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/public/flutter_windows.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/sequential_id_generator.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/windows/sequential_id_generator.h + ../../../flutter/LICENSE
@@ -32873,6 +32878,7 @@ FILE: ../../../flutter/shell/platform/windows/flutter_window.h
 FILE: ../../../flutter/shell/platform/windows/flutter_windows.cc
 FILE: ../../../flutter/shell/platform/windows/flutter_windows_engine.cc
 FILE: ../../../flutter/shell/platform/windows/flutter_windows_engine.h
+FILE: ../../../flutter/shell/platform/windows/flutter_windows_internal.h
 FILE: ../../../flutter/shell/platform/windows/flutter_windows_texture_registrar.cc
 FILE: ../../../flutter/shell/platform/windows/flutter_windows_texture_registrar.h
 FILE: ../../../flutter/shell/platform/windows/flutter_windows_view.cc
@@ -32891,6 +32897,10 @@ FILE: ../../../flutter/shell/platform/windows/keyboard_utils.cc
 FILE: ../../../flutter/shell/platform/windows/keyboard_utils.h
 FILE: ../../../flutter/shell/platform/windows/platform_handler.cc
 FILE: ../../../flutter/shell/platform/windows/platform_handler.h
+FILE: ../../../flutter/shell/platform/windows/platform_view_manager.cc
+FILE: ../../../flutter/shell/platform/windows/platform_view_manager.h
+FILE: ../../../flutter/shell/platform/windows/platform_view_plugin.cc
+FILE: ../../../flutter/shell/platform/windows/platform_view_plugin.h
 FILE: ../../../flutter/shell/platform/windows/public/flutter_windows.h
 FILE: ../../../flutter/shell/platform/windows/sequential_id_generator.cc
 FILE: ../../../flutter/shell/platform/windows/sequential_id_generator.h

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

@@ -8,6 +8,7 @@ import("//flutter/shell/platform/glfw/config.gni")
 import("//flutter/testing/testing.gni")
 
 _public_headers = [ "public/flutter_windows.h" ]
+_internal_headers = [ "flutter_windows_internal.h" ]
 
 config("relative_angle_headers") {
   include_dirs = [ "//third_party/angle/include" ]
@@ -24,7 +25,7 @@ config("relative_flutter_windows_headers") {
 # The headers are a separate source set since the client wrapper is allowed
 # to depend on the public headers, but none of the rest of the code.
 source_set("flutter_windows_headers") {
-  public = _public_headers
+  public = _public_headers + _internal_headers
 
   public_deps = [ "//flutter/shell/platform/common:common_cpp_library_headers" ]
 
@@ -98,6 +99,10 @@ source_set("flutter_windows_source") {
     "keyboard_utils.h",
     "platform_handler.cc",
     "platform_handler.h",
+    "platform_view_manager.cc",
+    "platform_view_manager.h",
+    "platform_view_plugin.cc",
+    "platform_view_plugin.h",
     "sequential_id_generator.cc",
     "sequential_id_generator.h",
     "settings_plugin.cc",
@@ -218,6 +223,7 @@ executable("flutter_windows_unittests") {
     "testing/flutter_windows_engine_builder.cc",
     "testing/flutter_windows_engine_builder.h",
     "testing/mock_direct_manipulation.h",
+    "testing/mock_platform_view_manager.h",
     "testing/mock_text_input_manager.cc",
     "testing/mock_text_input_manager.h",
     "testing/mock_window.cc",

engine/src/flutter/shell/platform/windows/client_wrapper/testing/stub_flutter_windows_api.cc

@@ -176,6 +176,16 @@ bool FlutterDesktopEngineProcessExternalWindowMessage(
   return false;
 }
 
+void FlutterDesktopEngineRegisterPlatformViewType(
+    FlutterDesktopEngineRef engine,
+    const char* view_type_name,
+    FlutterPlatformViewTypeEntry view_type) {
+  if (s_stub_implementation) {
+    s_stub_implementation->EngineRegisterPlatformViewType(view_type_name,
+                                                          view_type);
+  }
+}
+
 FlutterDesktopViewRef FlutterDesktopPluginRegistrarGetView(
     FlutterDesktopPluginRegistrarRef controller) {
   // The stub ignores this, so just return an arbitrary non-zero value.

engine/src/flutter/shell/platform/windows/client_wrapper/testing/stub_flutter_windows_api.h

@@ -7,6 +7,7 @@
 
 #include <memory>
 
+#include "flutter/shell/platform/windows/flutter_windows_internal.h"
 #include "flutter/shell/platform/windows/public/flutter_windows.h"
 
 namespace flutter {
@@ -72,6 +73,11 @@ class StubFlutterWindowsApi {
   // Called for FlutterDesktopEngineReloadSystemFonts.
   virtual void EngineReloadSystemFonts() {}
 
+  // Called for FlutterDesktopEngineRegisterPlatformViewType.
+  virtual void EngineRegisterPlatformViewType(
+      const char* view_type_name,
+      FlutterPlatformViewTypeEntry view_type) {}
+
   // Called for FlutterDesktopViewGetHWND.
   virtual HWND ViewGetHWND() { return reinterpret_cast<HWND>(1); }
 

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

@@ -4,7 +4,7 @@
 
 import 'dart:async';
 import 'dart:io' as io;
-import 'dart:typed_data' show ByteData, Uint8List;
+import 'dart:typed_data' show ByteData, Endian, Uint8List;
 import 'dart:ui' as ui;
 import 'dart:convert';
 
@@ -171,6 +171,37 @@ void enableLifecycleToFrom() async {
   });
 }
 
+@pragma('vm:entry-point')
+void sendCreatePlatformViewMethod() async {
+  // The platform view 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 valueString = 7;
+  const int valueMap = 13;
+  const int valueInt32 = 3;
+  const String method = 'create';
+  const String typeKey = 'viewType';
+  const String typeValue = 'type';
+  const String idKey = 'id';
+  final List<int> data = <int>[
+    // Method name
+    valueString, method.length, ...utf8.encode(method),
+    // Method arguments: {'type': 'type':, 'id': 0}
+    valueMap, 2,
+    valueString, typeKey.length, ...utf8.encode(typeKey),
+    valueString, typeValue.length, ...utf8.encode(typeValue),
+    valueString, idKey.length, ...utf8.encode(idKey),
+    valueInt32, 0, 0, 0, 0,
+  ];
+
+  final Completer<ByteData?> completed = Completer<ByteData?>();
+  final ByteData bytes = ByteData.sublistView(Uint8List.fromList(data));
+  ui.PlatformDispatcher.instance.sendPlatformMessage('flutter/platform_views', bytes, (ByteData? response) {
+    completed.complete(response);
+  });
+  await completed.future;
+}
+
 @pragma('vm:entry-point')
 void customEntrypoint() {}
 

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

@@ -245,6 +245,14 @@ bool FlutterDesktopEngineProcessExternalWindowMessage(
   return lresult.has_value();
 }
 
+void FlutterDesktopEngineRegisterPlatformViewType(
+    FlutterDesktopEngineRef engine,
+    const char* view_type_name,
+    FlutterPlatformViewTypeEntry view_type) {
+  // TODO(schectman): forward to platform view manager.
+  // https://github.com/flutter/flutter/issues/143375
+}
+
 FlutterDesktopViewRef FlutterDesktopPluginRegistrarGetView(
     FlutterDesktopPluginRegistrarRef registrar) {
   // TODO(loicsharma): Add |FlutterDesktopPluginRegistrarGetViewById| and

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

@@ -383,11 +383,17 @@ bool FlutterWindowsEngine::Run(std::string_view entrypoint) {
 
   args.custom_task_runners = &custom_task_runners;
 
+  if (!platform_view_plugin_) {
+    platform_view_plugin_ = std::make_unique<PlatformViewPlugin>(
+        messenger_wrapper_.get(), task_runner_.get());
+  }
   if (egl_manager_) {
     auto resolver = [](const char* name) -> void* {
       return reinterpret_cast<void*>(::eglGetProcAddress(name));
     };
 
+    // TODO(schectman) Pass the platform view manager to the compositor
+    // constructors: https://github.com/flutter/flutter/issues/143375
     compositor_ = std::make_unique<CompositorOpenGL>(this, resolver);
   } else {
     compositor_ = std::make_unique<CompositorSoftware>(this);

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

@@ -32,6 +32,7 @@
 #include "flutter/shell/platform/windows/keyboard_handler_base.h"
 #include "flutter/shell/platform/windows/keyboard_key_embedder_handler.h"
 #include "flutter/shell/platform/windows/platform_handler.h"
+#include "flutter/shell/platform/windows/platform_view_plugin.h"
 #include "flutter/shell/platform/windows/public/flutter_windows.h"
 #include "flutter/shell/platform/windows/settings_plugin.h"
 #include "flutter/shell/platform/windows/task_runner.h"
@@ -144,6 +145,8 @@ class FlutterWindowsEngine {
 
   TaskRunner* task_runner() { return task_runner_.get(); }
 
+  BinaryMessenger* messenger_wrapper() { return messenger_wrapper_.get(); }
+
   FlutterWindowsTextureRegistrar* texture_registrar() {
     return texture_registrar_.get();
   }
@@ -434,6 +437,8 @@ class FlutterWindowsEngine {
 
   std::shared_ptr<egl::ProcTable> gl_;
 
+  std::unique_ptr<PlatformViewPlugin> platform_view_plugin_;
+
   FML_DISALLOW_COPY_AND_ASSIGN(FlutterWindowsEngine);
 };
 

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

@@ -12,6 +12,7 @@
 #include "flutter/shell/platform/windows/testing/egl/mock_manager.h"
 #include "flutter/shell/platform/windows/testing/engine_modifier.h"
 #include "flutter/shell/platform/windows/testing/flutter_windows_engine_builder.h"
+#include "flutter/shell/platform/windows/testing/mock_platform_view_manager.h"
 #include "flutter/shell/platform/windows/testing/mock_window_binding_handler.h"
 #include "flutter/shell/platform/windows/testing/mock_windows_proc_table.h"
 #include "flutter/shell/platform/windows/testing/test_keyboard.h"
@@ -1170,5 +1171,30 @@ TEST_F(FlutterWindowsEngineTest, ChannelListenedTo) {
   }
 }
 
+TEST_F(FlutterWindowsEngineTest, ReceivePlatformViewMessage) {
+  FlutterWindowsEngineBuilder builder{GetContext()};
+  builder.SetDartEntrypoint("sendCreatePlatformViewMethod");
+  auto engine = builder.Build();
+
+  EngineModifier modifier{engine.get()};
+  modifier.embedder_api().RunsAOTCompiledDartCode = []() { return false; };
+
+  bool received_call = false;
+
+  auto manager = std::make_unique<MockPlatformViewManager>(engine.get());
+  EXPECT_CALL(*manager, AddPlatformView)
+      .WillOnce([&](PlatformViewId id, std::string_view type_name) {
+        received_call = true;
+        return true;
+      });
+  modifier.SetPlatformViewPlugin(std::move(manager));
+
+  engine->Run();
+
+  while (!received_call) {
+    engine->task_runner()->ProcessTasks();
+  }
+}
+
 }  // namespace testing
 }  // namespace flutter

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

@@ -0,0 +1,47 @@
+// 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_FLUTTER_WINDOWS_INTERNAL_H_
+#define FLUTTER_SHELL_PLATFORM_WINDOWS_FLUTTER_WINDOWS_INTERNAL_H_
+
+#include "flutter/shell/platform/windows/public/flutter_windows.h"
+
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
+// Declare functions that are currently in-progress and shall be exposed to the
+// public facing API upon completion.
+
+typedef int64_t PlatformViewId;
+
+typedef struct {
+  size_t struct_size;
+  HWND parent_window;
+  const char* platform_view_type;
+  // user_data may hold any necessary additional information for creating a new
+  // platform view. For example, an instance of FlutterWindow.
+  void* user_data;
+  PlatformViewId platform_view_id;
+} FlutterPlatformViewCreationParameters;
+
+typedef HWND (*FlutterPlatformViewFactory)(
+    const FlutterPlatformViewCreationParameters*);
+
+typedef struct {
+  size_t struct_size;
+  FlutterPlatformViewFactory factory;
+  void* user_data;  // Arbitrary user data supplied to the creation struct.
+} FlutterPlatformViewTypeEntry;
+
+FLUTTER_EXPORT void FlutterDesktopEngineRegisterPlatformViewType(
+    FlutterDesktopEngineRef engine,
+    const char* view_type_name,
+    FlutterPlatformViewTypeEntry view_type);
+
+#if defined(__cplusplus)
+}
+#endif
+
+#endif  // FLUTTER_SHELL_PLATFORM_WINDOWS_FLUTTER_WINDOWS_INTERNAL_H_

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

@@ -0,0 +1,84 @@
+// 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/platform_view_manager.h"
+
+#include "flutter/shell/platform/common/client_wrapper/include/flutter/standard_method_codec.h"
+
+namespace flutter {
+
+namespace {
+constexpr char kChannelName[] = "flutter/platform_views";
+constexpr char kCreateMethod[] = "create";
+constexpr char kFocusMethod[] = "focus";
+constexpr char kViewTypeParameter[] = "viewType";
+constexpr char kIdParameter[] = "id";
+constexpr char kDirectionParameter[] = "direction";
+constexpr char kFocusParameter[] = "focus";
+}  // namespace
+
+PlatformViewManager::PlatformViewManager(BinaryMessenger* binary_messenger)
+    : channel_(std::make_unique<MethodChannel<EncodableValue>>(
+          binary_messenger,
+          kChannelName,
+          &StandardMethodCodec::GetInstance())) {
+  channel_->SetMethodCallHandler(
+      [this](const MethodCall<EncodableValue>& call,
+             std::unique_ptr<MethodResult<EncodableValue>> result) {
+        const auto& args = std::get<EncodableMap>(*call.arguments());
+        if (call.method_name() == kCreateMethod) {
+          const auto& type_itr = args.find(EncodableValue(kViewTypeParameter));
+          const auto& id_itr = args.find(EncodableValue(kIdParameter));
+          if (type_itr == args.end()) {
+            result->Error("AddPlatformView", "Parameter viewType is required");
+            return;
+          }
+          if (id_itr == args.end()) {
+            result->Error("AddPlatformView", "Parameter id is required");
+            return;
+          }
+          const auto& type = std::get<std::string>(type_itr->second);
+          const auto& id = std::get<std::int32_t>(id_itr->second);
+          if (AddPlatformView(id, type)) {
+            result->Success();
+          } else {
+            result->Error("AddPlatformView", "Failed to add platform view");
+          }
+          return;
+        } else if (call.method_name() == kFocusMethod) {
+          const auto& id_itr = args.find(EncodableValue(kIdParameter));
+          const auto& direction_itr =
+              args.find(EncodableValue(kDirectionParameter));
+          const auto& focus_itr = args.find(EncodableValue(kFocusParameter));
+          if (id_itr == args.end()) {
+            result->Error("FocusPlatformView", "Parameter id is required");
+            return;
+          }
+          if (direction_itr == args.end()) {
+            result->Error("FocusPlatformView",
+                          "Parameter direction is required");
+            return;
+          }
+          if (focus_itr == args.end()) {
+            result->Error("FocusPlatformView", "Parameter focus is required");
+            return;
+          }
+          const auto& id = std::get<std::int32_t>(id_itr->second);
+          const auto& direction = std::get<std::int32_t>(direction_itr->second);
+          const auto& focus = std::get<bool>(focus_itr->second);
+          if (FocusPlatformView(
+                  id, static_cast<FocusChangeDirection>(direction), focus)) {
+            result->Success();
+          } else {
+            result->Error("FocusPlatformView", "Failed to focus platform view");
+          }
+          return;
+        }
+        result->NotImplemented();
+      });
+}
+
+PlatformViewManager::~PlatformViewManager() {}
+
+}  // namespace flutter

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

@@ -0,0 +1,52 @@
+// 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_PLATFORM_VIEW_MANAGER_H_
+#define FLUTTER_SHELL_PLATFORM_WINDOWS_PLATFORM_VIEW_MANAGER_H_
+
+#include <memory>
+
+#include "flutter/shell/platform/common/client_wrapper/include/flutter/method_channel.h"
+#include "flutter/shell/platform/windows/flutter_windows_internal.h"
+#include "flutter/shell/platform/windows/task_runner.h"
+
+namespace flutter {
+
+// Possible reasons for change of keyboard focus.
+enum class FocusChangeDirection {
+  kProgrammatic,  // Un-directed focus change.
+  kForward,       // Keyboard focus moves forwards, e.g. TAB key.
+  kBackward       // Keyboard focus moves backwards, e.g. Shift+TAB.
+};
+
+// The platform method handler for platform view related communication between
+// the engine and the framework. This base class is derived by a concrete class
+// (i.e. PlatformViewPlugin) to provide implementation of its abstract virtual
+// methods.
+class PlatformViewManager {
+ public:
+  PlatformViewManager(BinaryMessenger* binary_messenger);
+
+  virtual ~PlatformViewManager();
+
+  // Add a new platform view instance to be lazily instantiated when it is next
+  // composited. The manager will invoke Success when this method returns true,
+  // and invoke Error otherwise.
+  virtual bool AddPlatformView(PlatformViewId id,
+                               std::string_view type_name) = 0;
+
+  // The framework may invoke this method when keyboard focus must be given to
+  // the platform view. The manager will invoke Success when this method returns
+  // true, and invoke Error otherwise.
+  virtual bool FocusPlatformView(PlatformViewId id,
+                                 FocusChangeDirection direction,
+                                 bool focus) = 0;
+
+ private:
+  std::unique_ptr<MethodChannel<EncodableValue>> channel_;
+};
+
+}  // namespace flutter
+
+#endif  // FLUTTER_SHELL_PLATFORM_WINDOWS_PLATFORM_VIEW_MANAGER_H_

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

@@ -0,0 +1,47 @@
+// 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/platform_view_plugin.h"
+
+namespace flutter {
+
+PlatformViewPlugin::PlatformViewPlugin(BinaryMessenger* messenger,
+                                       TaskRunner* task_runner)
+    : PlatformViewManager(messenger), task_runner_(task_runner) {}
+
+PlatformViewPlugin::~PlatformViewPlugin() {}
+
+// TODO(schectman): Impelement platform view lookup.
+// https://github.com/flutter/flutter/issues/143375
+std::optional<HWND> PlatformViewPlugin::GetNativeHandleForId(
+    PlatformViewId id) const {
+  return std::nullopt;
+}
+
+// TODO(schectman): Impelement platform view type registration.
+// https://github.com/flutter/flutter/issues/143375
+void PlatformViewPlugin::RegisterPlatformViewType(
+    std::string_view type_name,
+    const FlutterPlatformViewTypeEntry& type) {}
+
+// TODO(schectman): Impelement platform view instantiation.
+// https://github.com/flutter/flutter/issues/143375
+void PlatformViewPlugin::InstantiatePlatformView(PlatformViewId id) {}
+
+// TODO(schectman): Impelement platform view addition.
+// https://github.com/flutter/flutter/issues/143375
+bool PlatformViewPlugin::AddPlatformView(PlatformViewId id,
+                                         std::string_view type_name) {
+  return true;
+}
+
+// TODO(schectman): Impelement platform view focus request.
+// https://github.com/flutter/flutter/issues/143375
+bool PlatformViewPlugin::FocusPlatformView(PlatformViewId id,
+                                           FocusChangeDirection direction,
+                                           bool focus) {
+  return true;
+}
+
+}  // namespace flutter

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

@@ -0,0 +1,67 @@
+// 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_PLATFORM_VIEW_PLUGIN_H_
+#define FLUTTER_SHELL_PLATFORM_WINDOWS_PLATFORM_VIEW_PLUGIN_H_
+
+#include "flutter/shell/platform/windows/platform_view_manager.h"
+
+#include <functional>
+#include <map>
+#include <optional>
+
+namespace flutter {
+
+// The concrete implementation of PlatformViewManager that keeps track of
+// existing platform view types and instances, and handles their instantiation.
+class PlatformViewPlugin : public PlatformViewManager {
+ public:
+  PlatformViewPlugin(BinaryMessenger* messenger, TaskRunner* task_runner);
+
+  ~PlatformViewPlugin();
+
+  // Find the HWND corresponding to a platform view id. Returns null if the id
+  // has no associated platform view.
+  std::optional<HWND> GetNativeHandleForId(PlatformViewId id) const;
+
+  // The runner-facing API calls this method to register a window type
+  // corresponding to a platform view identifier supplied to the widget tree.
+  void RegisterPlatformViewType(std::string_view type_name,
+                                const FlutterPlatformViewTypeEntry& type);
+
+  // | PlatformViewManager |
+  // type_name must correspond to a string that has already been registered
+  // with RegisterPlatformViewType.
+  bool AddPlatformView(PlatformViewId id, std::string_view type_name) override;
+
+  // Create a queued platform view instance after it has been added.
+  // id must correspond to an identifier that has already been added with
+  // AddPlatformView.
+  // This method will create the platform view within a task queued to the
+  // engine's TaskRunner, which will run on the UI thread.
+  void InstantiatePlatformView(PlatformViewId id);
+
+  // | PlatformViewManager |
+  // id must correspond to an identifier that has already been added with
+  // AddPlatformView.
+  bool FocusPlatformView(PlatformViewId id,
+                         FocusChangeDirection direction,
+                         bool focus) override;
+
+ private:
+  std::unordered_map<std::string, FlutterPlatformViewTypeEntry>
+      platform_view_types_;
+
+  std::unordered_map<PlatformViewId, HWND> platform_views_;
+
+  std::unordered_map<PlatformViewId, std::function<HWND()>>
+      pending_platform_views_;
+
+  // Pointer to the task runner of the associated engine.
+  TaskRunner* task_runner_;
+};
+
+}  // namespace flutter
+
+#endif  // FLUTTER_SHELL_PLATFORM_WINDOWS_PLATFORM_VIEW_PLUGIN_H_

engine/src/flutter/shell/platform/windows/testing/engine_modifier.h

@@ -76,6 +76,10 @@ class EngineModifier {
     engine_->lifecycle_manager_ = std::move(handler);
   }
 
+  void SetPlatformViewPlugin(std::unique_ptr<PlatformViewPlugin>&& manager) {
+    engine_->platform_view_plugin_ = std::move(manager);
+  }
+
  private:
   FlutterWindowsEngine* engine_;
 

engine/src/flutter/shell/platform/windows/testing/mock_platform_view_manager.h

@@ -0,0 +1,28 @@
+// 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_TESTING_MOCK_PLATFORM_VIEW_MANAGER_H_
+#define FLUTTER_SHELL_PLATFORM_WINDOWS_TESTING_MOCK_PLATFORM_VIEW_MANAGER_H_
+
+#include "flutter/shell/platform/windows/platform_view_plugin.h"
+
+#include "flutter/shell/platform/windows/flutter_windows_engine.h"
+#include "gmock/gmock.h"
+
+namespace flutter {
+
+class MockPlatformViewManager : public PlatformViewPlugin {
+ public:
+  MockPlatformViewManager(FlutterWindowsEngine* engine)
+      : PlatformViewPlugin(engine->messenger_wrapper(), engine->task_runner()) {
+  }
+
+  ~MockPlatformViewManager() {}
+
+  MOCK_METHOD(bool, AddPlatformView, (PlatformViewId id, std::string_view));
+};
+
+}  // namespace flutter
+
+#endif  // FLUTTER_SHELL_PLATFORM_WINDOWS_TESTING_MOCK_PLATFORM_VIEW_MANAGER_H_