[macos] Wireup platform views on macOS

* The plan is to only support platform views when using Metal rendering backend.
* This PR is based on the work done in: flutter#22905
* There are some threading issues to be addressed still to make this feature usable.

ci/licenses_golden/licenses_flutter

@@ -1220,6 +1220,7 @@ FILE: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterAppD
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterDartProject.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterEngine.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterMacOS.h
+FILE: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterPlatformViews.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterPluginMacOS.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterPluginRegistrarMacOS.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterViewController.h
@@ -1281,6 +1282,11 @@ FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterOpenG
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterPlatformNodeDelegateMac.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterPlatformNodeDelegateMac.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterPlatformNodeDelegateMacTest.mm
+FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterPlatformViewController.mm
+FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterPlatformViewControllerTest.mm
+FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterPlatformViewController_Internal.h
+FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterPlatformViewMock.h
+FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterPlatformViewMock.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterRenderer.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterRenderingBackend.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterRenderingBackend.mm

shell/platform/darwin/macos/BUILD.gn

@@ -40,6 +40,7 @@ _flutter_framework_headers = [
   "framework/Headers/FlutterDartProject.h",
   "framework/Headers/FlutterEngine.h",
   "framework/Headers/FlutterMacOS.h",
+  "framework/Headers/FlutterPlatformViews.h",
   "framework/Headers/FlutterPluginMacOS.h",
   "framework/Headers/FlutterPluginRegistrarMacOS.h",
   "framework/Headers/FlutterViewController.h",
@@ -95,6 +96,8 @@ source_set("flutter_framework_source") {
     "framework/Source/FlutterOpenGLRenderer.mm",
     "framework/Source/FlutterPlatformNodeDelegateMac.h",
     "framework/Source/FlutterPlatformNodeDelegateMac.mm",
+    "framework/Source/FlutterPlatformViewController.h",
+    "framework/Source/FlutterPlatformViewController.mm",
     "framework/Source/FlutterRenderer.h",
     "framework/Source/FlutterRenderingBackend.h",
     "framework/Source/FlutterRenderingBackend.mm",
@@ -186,11 +189,14 @@ executable("flutter_desktop_darwin_unittests") {
     "framework/Source/FlutterMetalSurfaceManagerTest.mm",
     "framework/Source/FlutterOpenGLRendererTest.mm",
     "framework/Source/FlutterPlatformNodeDelegateMacTest.mm",
+    "framework/Source/FlutterPlatformViewControllerTest.mm",
     "framework/Source/FlutterTextInputPluginTest.mm",
     "framework/Source/FlutterTextInputSemanticsObjectTest.mm",
     "framework/Source/FlutterViewControllerTest.mm",
     "framework/Source/FlutterViewControllerTestUtils.h",
     "framework/Source/FlutterViewControllerTestUtils.mm",
+    "framework/Source/TestFlutterPlatformView.h",
+    "framework/Source/TestFlutterPlatformView.mm",
   ]
 
   cflags_objcc = flutter_cflags_objcc_arc

shell/platform/darwin/macos/framework/Headers/FlutterPlatformViews.h

@@ -0,0 +1,43 @@
+// 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_FLUTTERPLATFORMVIEWS_H_
+#define FLUTTER_FLUTTERPLATFORMVIEWS_H_
+
+#import <AppKit/AppKit.h>
+
+#import "FlutterCodecs.h"
+#import "FlutterMacros.h"
+
+@protocol FlutterPlatformViewFactory <NSObject>
+
+/**
+ * Create a Platform View which is an `NSView`.
+ *
+ * Implemented by MacOS plugin code to return an `NSView` to be inserted into the Flutter view
+ * hierarchy.
+ *
+ * The implementation of this method should create a new `NSView` and return it.
+ *
+ * @param frame The rectangle for the newly created view measured in points.
+ * @param viewId A unique identifier for this view.
+ * @param args Parameters for creating the view sent from the Dart side of the
+ * Flutter app. If `createArgsCodec` is not implemented, or if no creation arguments were sent from
+ * the Dart code, this will be null. Otherwise this will be the value sent from the Dart code as
+ * decoded by `createArgsCodec`.
+ */
+- (nonnull NSView*)createWithFrame:(CGRect)frame
+                    viewIdentifier:(int64_t)viewId
+                         arguments:(nullable id)args;
+
+/**
+ * Returns the `FlutterMessageCodec` for decoding the args parameter of `createWithFrame`.
+ *
+ * Only needs to be implemented if `createWithFrame` needs an arguments parameter.
+ */
+@optional
+- (nullable NSObject<FlutterMessageCodec>*)createArgsCodec;
+@end
+
+#endif  // FLUTTER_FLUTTERPLATFORMVIEWS_H_

shell/platform/darwin/macos/framework/Headers/FlutterPluginRegistrarMacOS.h

@@ -7,6 +7,7 @@
 #import "FlutterBinaryMessenger.h"
 #import "FlutterChannels.h"
 #import "FlutterMacros.h"
+#import "FlutterPlatformViews.h"
 #import "FlutterPluginMacOS.h"
 #import "FlutterTexture.h"
 
@@ -48,6 +49,18 @@ FLUTTER_DARWIN_EXPORT
 - (void)addMethodCallDelegate:(nonnull id<FlutterPlugin>)delegate
                       channel:(nonnull FlutterMethodChannel*)channel;
 
+/**
+ * Registers a `FlutterPlatformViewFactory` for creation of platform views.
+ *
+ * Plugins expose `NSView` for embedding in Flutter apps by registering a view factory.
+ *
+ * @param factory The view factory that will be registered.
+ * @param factoryId A unique identifier for the factory, the Dart code of the Flutter app can use
+ *   this identifier to request creation of a `NSView` by the registered factory.
+ */
+- (void)registerViewFactory:(nonnull NSObject<FlutterPlatformViewFactory>*)factory
+                     withId:(nonnull NSString*)factoryId;
+
 @end
 
 /**

shell/platform/darwin/macos/framework/Headers/FlutterViewController.h

@@ -6,6 +6,7 @@
 
 #import "FlutterEngine.h"
 #import "FlutterMacros.h"
+#import "FlutterPlatformViews.h"
 #import "FlutterPluginRegistrarMacOS.h"
 
 /**

shell/platform/darwin/macos/framework/Source/FlutterEngine.mm

@@ -15,9 +15,10 @@
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterMetalCompositor.h"
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterMetalRenderer.h"
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterOpenGLRenderer.h"
+#import "flutter/shell/platform/darwin/macos/framework/Source/FlutterPlatformViewController.h"
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterRenderingBackend.h"
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterViewController_Internal.h"
-#include "flutter/shell/platform/embedder/embedder.h"
+#import "flutter/shell/platform/embedder/embedder.h"
 
 /**
  * Constructs and returns a FlutterLocale struct corresponding to |locale|, which must outlive
@@ -95,6 +96,11 @@ - (void)postMainThreadTask:(FlutterTask)task targetTimeInNanoseconds:(uint64_t)t
  */
 - (void)loadAOTData:(NSString*)assetsDir;
 
+/**
+ * Creates a platform view channel and sets up the method handler.
+ */
+- (void)setupPlatformViewChannel;
+
 @end
 
 #pragma mark -
@@ -145,6 +151,11 @@ - (void)addMethodCallDelegate:(nonnull id<FlutterPlugin>)delegate
   }];
 }
 
+- (void)registerViewFactory:(nonnull NSObject<FlutterPlatformViewFactory>*)factory
+                     withId:(nonnull NSString*)factoryId {
+  [[_flutterEngine platformViewController] registerViewFactory:factory withId:factoryId];
+}
+
 @end
 
 // Callbacks provided to the engine. See the called methods for documentation.
@@ -186,6 +197,14 @@ @implementation FlutterEngine {
 
   // FlutterCompositor is copied and used in embedder.cc.
   FlutterCompositor _compositor;
+
+  // Method channel for platform view functions. These functions include creating, disposing and
+  // mutating a platform view.
+  FlutterMethodChannel* _platformViewsChannel;
+
+  // Used to support creation and deletion of platform views and registering platform view
+  // factories. Lifecycle is tied to the engine.
+  FlutterPlatformViewController* _platformViewController;
 }
 
 - (instancetype)initWithName:(NSString*)labelPrefix project:(FlutterDartProject*)project {
@@ -219,6 +238,9 @@ - (instancetype)initWithName:(NSString*)labelPrefix
                              name:NSCurrentLocaleDidChangeNotification
                            object:nil];
 
+  _platformViewController = [[FlutterPlatformViewController alloc] init];
+  [self setupPlatformViewChannel];
+
   return self;
 }
 
@@ -383,8 +405,8 @@ - (FlutterCompositor*)createFlutterCompositor {
 
   if ([FlutterRenderingBackend renderUsingMetal]) {
     FlutterMetalRenderer* metalRenderer = reinterpret_cast<FlutterMetalRenderer*>(_renderer);
-    _macOSCompositor =
-        std::make_unique<flutter::FlutterMetalCompositor>(_viewController, metalRenderer.device);
+    _macOSCompositor = std::make_unique<flutter::FlutterMetalCompositor>(
+        _viewController, _platformViewController, metalRenderer.device);
     _macOSCompositor->SetPresentCallback([weakSelf](bool has_flutter_content) {
       if (has_flutter_content) {
         FlutterMetalRenderer* metalRenderer =
@@ -541,6 +563,10 @@ - (void)dispatchSemanticsAction:(FlutterSemanticsAction)action
   _embedderAPI.DispatchSemanticsAction(_engine, target, action, data.GetMapping(), data.GetSize());
 }
 
+- (FlutterPlatformViewController*)platformViewController {
+  return _platformViewController;
+}
+
 #pragma mark - Private methods
 
 - (void)sendUserLocales {
@@ -630,6 +656,18 @@ - (void)shutDownEngine {
   _engine = nullptr;
 }
 
+- (void)setupPlatformViewChannel {
+  _platformViewsChannel =
+      [FlutterMethodChannel methodChannelWithName:@"flutter/platform_views"
+                                  binaryMessenger:self.binaryMessenger
+                                            codec:[FlutterStandardMethodCodec sharedInstance]];
+
+  __weak FlutterEngine* weakSelf = self;
+  [_platformViewsChannel setMethodCallHandler:^(FlutterMethodCall* call, FlutterResult result) {
+    [[weakSelf platformViewController] handleMethodCall:call result:result];
+  }];
+}
+
 #pragma mark - FlutterBinaryMessenger
 
 - (void)sendOnChannel:(nonnull NSString*)channel message:(nullable NSData*)message {
@@ -781,20 +819,22 @@ - (void)updateSemanticsCustomActions:(const FlutterSemanticsCustomAction*)action
 
 #pragma mark - Task runner integration
 
-- (void)postMainThreadTask:(FlutterTask)task targetTimeInNanoseconds:(uint64_t)targetTime {
-  const auto engine_time = _embedderAPI.GetCurrentTime();
+- (void)runTaskOnEmbedder:(FlutterTask)task {
+  if (_engine) {
+    auto result = _embedderAPI.RunTask(_engine, &task);
+    if (result != kSuccess) {
+      NSLog(@"Could not post a task to the Flutter engine.");
+    }
+  }
+}
 
-  __weak FlutterEngine* weak_self = self;
+- (void)postMainThreadTask:(FlutterTask)task targetTimeInNanoseconds:(uint64_t)targetTime {
+  __weak FlutterEngine* weakSelf = self;
   auto worker = ^{
-    FlutterEngine* strong_self = weak_self;
-    if (strong_self && strong_self->_engine) {
-      auto result = _embedderAPI.RunTask(strong_self->_engine, &task);
-      if (result != kSuccess) {
-        NSLog(@"Could not post a task to the Flutter engine.");
-      }
-    }
+    [weakSelf runTaskOnEmbedder:task];
   };
 
+  const auto engine_time = _embedderAPI.GetCurrentTime();
   if (targetTime <= engine_time) {
     dispatch_async(dispatch_get_main_queue(), worker);
 

shell/platform/darwin/macos/framework/Source/FlutterEngine_Internal.h

@@ -10,6 +10,7 @@
 
 #include "flutter/shell/platform/common/accessibility_bridge.h"
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterCompositor.h"
+#import "flutter/shell/platform/darwin/macos/framework/Source/FlutterPlatformViewController.h"
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterRenderer.h"
 
 @interface FlutterEngine ()
@@ -70,6 +71,8 @@
  */
 - (BOOL)unregisterTextureWithID:(int64_t)textureID;
 
+- (nonnull FlutterPlatformViewController*)platformViewController;
+
 // Accessibility API.
 
 /**

shell/platform/darwin/macos/framework/Source/FlutterMetalCompositor.h

@@ -7,13 +7,16 @@
 
 #include "flutter/fml/macros.h"
 #include "flutter/shell/platform/darwin/macos/framework/Source/FlutterCompositor.h"
+#import "flutter/shell/platform/darwin/macos/framework/Source/FlutterPlatformViewController.h"
 
 namespace flutter {
 
 class FlutterMetalCompositor : public FlutterCompositor {
  public:
-  explicit FlutterMetalCompositor(FlutterViewController* view_controller,
-                                  id<MTLDevice> mtl_device);
+  explicit FlutterMetalCompositor(
+      FlutterViewController* view_controller,
+      FlutterPlatformViewController* platform_views_controller,
+      id<MTLDevice> mtl_device);
 
   virtual ~FlutterMetalCompositor() = default;
 
@@ -42,7 +45,12 @@ class FlutterMetalCompositor : public FlutterCompositor {
   bool Present(const FlutterLayer** layers, size_t layers_count) override;
 
  private:
+  // Presents the platform view layer represented by `layer`. `layer_index` is
+  // used to position the layer in the z-axis.
+  void PresentPlatformView(const FlutterLayer* layer, size_t layer_index);
+
   const id<MTLDevice> mtl_device_;
+  const FlutterPlatformViewController* platform_views_controller_;
 
   FML_DISALLOW_COPY_AND_ASSIGN(FlutterMetalCompositor);
 };

shell/platform/darwin/macos/framework/Source/FlutterMetalCompositor.mm

@@ -10,9 +10,13 @@
 
 namespace flutter {
 
-FlutterMetalCompositor::FlutterMetalCompositor(FlutterViewController* view_controller,
-                                               id<MTLDevice> mtl_device)
-    : FlutterCompositor(view_controller), mtl_device_(mtl_device) {}
+FlutterMetalCompositor::FlutterMetalCompositor(
+    FlutterViewController* view_controller,
+    FlutterPlatformViewController* platform_views_controller,
+    id<MTLDevice> mtl_device)
+    : FlutterCompositor(view_controller),
+      mtl_device_(mtl_device),
+      platform_views_controller_(platform_views_controller) {}
 
 bool FlutterMetalCompositor::CreateBackingStore(const FlutterBackingStoreConfig* config,
                                                 FlutterBackingStore* backing_store_out) {
@@ -77,7 +81,6 @@
   SetFrameStatus(FrameStatus::kPresenting);
 
   bool has_flutter_content = false;
-
   for (size_t i = 0; i < layers_count; ++i) {
     const auto* layer = layers[i];
     FlutterBackingStore* backing_store = const_cast<FlutterBackingStore*>(layer->backing_store);
@@ -94,13 +97,32 @@
         break;
       }
       case kFlutterLayerContentTypePlatformView:
-        // Add functionality in follow up PR.
-        FML_LOG(WARNING) << "Presenting PlatformViews not yet supported";
+        PresentPlatformView(layer, i);
         break;
     };
   }
 
   return EndFrame(has_flutter_content);
 }
 
+void FlutterMetalCompositor::PresentPlatformView(const FlutterLayer* layer, size_t layer_position) {
+  // TODO (kaushikiska): fix this error.
+  // FML_DCHECK([[NSThread currentThread] isMainThread])
+  //     << "Must be on the main thread to handle presenting platform views";
+
+  int64_t platform_view_id = layer->platform_view->identifier;
+  NSView* platform_view = [platform_views_controller_ platformViewWithID:platform_view_id];
+
+  FML_DCHECK(platform_view) << "Platform view not found for id: " << platform_view_id;
+
+  CGFloat scale = [[NSScreen mainScreen] backingScaleFactor];
+  platform_view.frame = CGRectMake(layer->offset.x / scale, layer->offset.y / scale,
+                                   layer->size.width / scale, layer->size.height / scale);
+  if (platform_view.superview == nil) {
+    [view_controller_.flutterView addSubview:platform_view];
+  } else {
+    platform_view.layer.zPosition = layer_position;
+  }
+}
+
 }  // namespace flutter