Avoid accessing the Cocoa view on the GPU or IO task runners. (#13295)

The view was being accessed from a background thread so its OpenGL context could be accessed. This tripped thread safety assertions in Cocoa. Now the OpenGL context is stashed in the FlutterEngine instance itself.

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

@@ -150,6 +150,10 @@ @implementation FlutterEngine {
   // The context provided to the Flutter engine for resource loading.
   NSOpenGLContext* _resourceContext;
 
+  // The context that is owned by the currently displayed FlutterView. This is stashed in the engine
+  // so that the view doesn't need to be accessed from a background thread.
+  NSOpenGLContext* _mainOpenGLContext;
+
   // A mapping of channel names to the registered handlers for those channels.
   NSMutableDictionary<NSString*, FlutterBinaryMessageHandler>* _messageHandlers;
 
@@ -215,18 +219,26 @@ - (BOOL)runWithEntrypoint:(NSString*)entrypoint {
   flutterArguments.platform_message_callback = (FlutterPlatformMessageCallback)OnPlatformMessage;
   flutterArguments.custom_dart_entrypoint = entrypoint.UTF8String;
 
-  FlutterEngineResult result = FlutterEngineRun(
+  FlutterEngineResult result = FlutterEngineInitialize(
       FLUTTER_ENGINE_VERSION, &rendererConfig, &flutterArguments, (__bridge void*)(self), &_engine);
   if (result != kSuccess) {
-    NSLog(@"Failed to start Flutter engine: error %d", result);
+    NSLog(@"Failed to initialize Flutter engine: error %d", result);
+    return NO;
+  }
+
+  result = FlutterEngineRunInitialized(_engine);
+  if (result != kSuccess) {
+    NSLog(@"Failed to run an initialized engine: error %d", result);
     return NO;
   }
+
   [self updateWindowMetrics];
   return YES;
 }
 
 - (void)setViewController:(FlutterViewController*)controller {
   _viewController = controller;
+  _mainOpenGLContext = controller.flutterView.openGLContext;
   if (!controller && !_allowHeadlessExecution) {
     [self shutDownEngine];
     _resourceContext = nil;
@@ -281,33 +293,26 @@ - (void)sendPointerEvent:(const FlutterPointerEvent&)event {
 #pragma mark - Private methods
 
 - (bool)engineCallbackOnMakeCurrent {
-  if (!_viewController.flutterView) {
+  if (!_mainOpenGLContext) {
     return false;
   }
-  [_viewController.flutterView makeCurrentContext];
+  [_mainOpenGLContext makeCurrentContext];
   return true;
 }
 
 - (bool)engineCallbackOnClearCurrent {
-  if (!_viewController.flutterView) {
-    return false;
-  }
   [NSOpenGLContext clearCurrentContext];
   return true;
 }
 
 - (bool)engineCallbackOnPresent {
-  if (!_viewController.flutterView) {
-    return false;
+  if (!_mainOpenGLContext) {
   }
-  [_viewController.flutterView onPresent];
+  [_mainOpenGLContext flushBuffer];
   return true;
 }
 
 - (bool)engineCallbackOnMakeResourceCurrent {
-  if (!_viewController.flutterView) {
-    return false;
-  }
   [self.resourceContext makeCurrentContext];
   return true;
 }
@@ -344,11 +349,18 @@ - (void)engineCallbackOnPlatformMessage:(const FlutterPlatformMessage*)message {
  * Note: Called from dealloc. Should not use accessors or other methods.
  */
 - (void)shutDownEngine {
-  if (_engine) {
-    FlutterEngineResult result = FlutterEngineShutdown(_engine);
-    if (result != kSuccess) {
-      NSLog(@"Failed to shut down Flutter engine: error %d", result);
-    }
+  if (_engine == nullptr) {
+    return;
+  }
+
+  FlutterEngineResult result = FlutterEngineDeinitialize(_engine);
+  if (result != kSuccess) {
+    NSLog(@"Could not de-initialize the Flutter engine: error %d", result);
+  }
+
+  result = FlutterEngineShutdown(_engine);
+  if (result != kSuccess) {
+    NSLog(@"Failed to shut down Flutter engine: error %d", result);
   }
   _engine = nullptr;
 }

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

@@ -35,14 +35,4 @@
 - (nullable instancetype)initWithCoder:(nonnull NSCoder*)coder NS_UNAVAILABLE;
 - (nonnull instancetype)init NS_UNAVAILABLE;
 
-/**
- * Sets this view as the current context object for OpenGL drawing.
- */
-- (void)makeCurrentContext;
-
-/**
- * Called when the OpenGL display should be updated.
- */
-- (void)onPresent;
-
 @end

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

@@ -26,14 +26,6 @@ - (instancetype)initWithFrame:(NSRect)frame
   return self;
 }
 
-- (void)makeCurrentContext {
-  [self.openGLContext makeCurrentContext];
-}
-
-- (void)onPresent {
-  [self.openGLContext flushBuffer];
-}
-
 #pragma mark - NSView overrides
 
 /**