[macOS] Multi-view thread synchronizer

ci/licenses_golden/licenses_flutter

@@ -2744,6 +2744,9 @@ ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterTex
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterTextInputSemanticsObjectTest.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterTextureRegistrar.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterTextureRegistrar.mm + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterThreadSynchronizer.h + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterThreadSynchronizer.mm + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterThreadSynchronizerTest.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterUmbrellaImportTests.m + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterView.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterView.mm + ../../../flutter/LICENSE
@@ -5410,6 +5413,7 @@ FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterTextu
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterTextureRegistrar.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterThreadSynchronizer.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterThreadSynchronizer.mm
+FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterThreadSynchronizerTest.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterUmbrellaImportTests.m
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterView.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterView.mm

shell/platform/darwin/macos/BUILD.gn

@@ -182,6 +182,7 @@ executable("flutter_desktop_darwin_unittests") {
     "framework/Source/FlutterSurfaceManagerTest.mm",
     "framework/Source/FlutterTextInputPluginTest.mm",
     "framework/Source/FlutterTextInputSemanticsObjectTest.mm",
+    "framework/Source/FlutterThreadSynchronizerTest.mm",
     "framework/Source/FlutterViewControllerTest.mm",
     "framework/Source/FlutterViewControllerTestUtils.h",
     "framework/Source/FlutterViewControllerTestUtils.mm",

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

@@ -388,6 +388,8 @@ @implementation FlutterEngine {
   // A method channel for miscellaneous platform functionality.
   FlutterMethodChannel* _platformChannel;
 
+  FlutterThreadSynchronizer* _threadSynchronizer;
+
   int _nextViewId;
 }
 
@@ -427,6 +429,7 @@ - (instancetype)initWithName:(NSString*)labelPrefix
                            object:nil];
 
   _platformViewController = [[FlutterPlatformViewController alloc] init];
+  _threadSynchronizer = [[FlutterThreadSynchronizer alloc] init];
   [self setUpPlatformViewChannel];
   [self setUpAccessibilityChannel];
   [self setUpNotificationCenterListeners];
@@ -589,7 +592,7 @@ - (void)registerViewController:(FlutterViewController*)controller forId:(Flutter
   NSAssert(![controller attached],
            @"The incoming view controller is already attached to an engine.");
   NSAssert([_viewControllers objectForKey:@(viewId)] == nil, @"The requested view ID is occupied.");
-  [controller attachToEngine:self withId:viewId];
+  [controller setUpWithEngine:self viewId:viewId threadSynchronizer:_threadSynchronizer];
   NSAssert(controller.viewId == viewId, @"Failed to assign view ID.");
   [_viewControllers setObject:controller forKey:@(viewId)];
 }
@@ -928,11 +931,8 @@ - (void)shutDownEngine {
     return;
   }
 
-  NSEnumerator* viewControllerEnumerator = [_viewControllers objectEnumerator];
-  FlutterViewController* nextViewController;
-  while ((nextViewController = [viewControllerEnumerator nextObject])) {
-    [nextViewController.flutterView shutdown];
-  }
+  [_threadSynchronizer shutdown];
+  _threadSynchronizer = nil;
 
   FlutterEngineResult result = _embedderAPI.Deinitialize(_engine);
   if (result != kSuccess) {
@@ -1117,6 +1117,10 @@ - (NSPasteboard*)pasteboard {
   return flutter::GetSwitchesFromEnvironment();
 }
 
+- (FlutterThreadSynchronizer*)testThreadSynchronizer {
+  return _threadSynchronizer;
+}
+
 #pragma mark - FlutterBinaryMessenger
 
 - (void)sendOnChannel:(nonnull NSString*)channel message:(nullable NSData*)message {

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

@@ -25,13 +25,16 @@
 // CREATE_NATIVE_ENTRY and MOCK_ENGINE_PROC are leaky by design
 // NOLINTBEGIN(clang-analyzer-core.StackAddressEscape)
 
+constexpr int64_t kDefaultViewId = 0ll;
+
 @interface FlutterEngine (Test)
 /**
  * The FlutterCompositor object currently in use by the FlutterEngine.
  *
  * May be nil if the compositor has not been initialized yet.
  */
 @property(nonatomic, readonly, nullable) flutter::FlutterCompositor* macOSCompositor;
+
 @end
 
 @interface TestPlatformViewFactory : NSObject <FlutterPlatformViewFactory>
@@ -438,7 +441,7 @@ - (nonnull NSView*)createWithViewIdentifier:(int64_t)viewId arguments:(nullable
                 result:^(id result){
                 }];
 
-  [viewController.flutterView.threadSynchronizer blockUntilFrameAvailable];
+  [engine.testThreadSynchronizer blockUntilFrameAvailable];
 
   CALayer* rootLayer = viewController.flutterView.layer;
 
@@ -629,9 +632,10 @@ - (nonnull NSView*)createWithViewIdentifier:(int64_t)viewId arguments:(nullable
   [threadSynchronizer shutdown];
 
   std::thread rasterThread([&threadSynchronizer] {
-    [threadSynchronizer performCommit:CGSizeMake(100, 100)
-                               notify:^{
-                               }];
+    [threadSynchronizer performCommitForView:kDefaultViewId
+                                        size:CGSizeMake(100, 100)
+                                      notify:^{
+                                      }];
   });
 
   rasterThread.join();

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

@@ -195,4 +195,8 @@ typedef NS_ENUM(NSInteger, FlutterAppExitResponse) {
 
 @end
 
+@interface FlutterEngine (Tests)
+- (nonnull FlutterThreadSynchronizer*)testThreadSynchronizer;
+@end
+
 NS_ASSUME_NONNULL_END

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

@@ -1,21 +1,29 @@
+// 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.
+
 #import <Cocoa/Cocoa.h>
 
 /**
  * Takes care of synchronization between raster and platform thread.
+ *
+ * All methods of this class must be called from the platform thread,
+ * except for performCommitForView:size:notify:.
  */
 @interface FlutterThreadSynchronizer : NSObject
 
 /**
- * Blocks current thread until there is frame available.
- * Used in FlutterEngineTest.
+ * Creates a FlutterThreadSynchronizer that uses the OS main thread as the
+ * platform thread.
  */
-- (void)blockUntilFrameAvailable;
+- (nullable instancetype)init;
 
 /**
- * Called from platform thread. Blocks until commit with given size (or empty)
- * is requested.
+ * Blocks until all views have a commit with their given sizes (or empty) is requested.
  */
-- (void)beginResize:(CGSize)size notify:(nonnull dispatch_block_t)notify;
+- (void)beginResizeForView:(int64_t)viewId
+                      size:(CGSize)size
+                    notify:(nonnull dispatch_block_t)notify;
 
 /**
  * Called from raster thread. Schedules the given block on platform thread
@@ -26,11 +34,65 @@
  *
  * The notify block is guaranteed to be called within a core animation transaction.
  */
-- (void)performCommit:(CGSize)size notify:(nonnull dispatch_block_t)notify;
+- (void)performCommitForView:(int64_t)viewId
+                        size:(CGSize)size
+                      notify:(nonnull dispatch_block_t)notify;
+
+/**
+ * Requests the synchronizer to track another view.
+ *
+ * A view must be registered before calling begineResizeForView: or
+ * performCommitForView:. It is typically done when the view controller is
+ * created.
+ */
+- (void)registerView:(int64_t)viewId;
+
+/**
+ * Requests the synchronizer to no longer track a view.
+ *
+ * It is typically done when the view controller is destroyed.
+ */
+- (void)deregisterView:(int64_t)viewId;
 
 /**
- * Called when shutting down. Unblocks everything and prevents any further synchronization.
+ * Called when the engine shuts down.
+ *
+ * Prevents any further synchronization and no longer blocks any threads.
  */
 - (void)shutdown;
 
 @end
+
+@interface FlutterThreadSynchronizer (TestUtils)
+
+/**
+ * Creates a FlutterThreadSynchronizer that uses the specified queue as the
+ * platform thread.
+ */
+- (nullable instancetype)initWithMainQueue:(nonnull dispatch_queue_t)queue;
+
+/**
+ * Blocks current thread until the mutex is available, then return whether the
+ * synchronizer is waiting for a correct commit during resizing.
+ *
+ * After calling an operation of the thread synchronizer, call this method,
+ * and when it returns, the thread synchronizer can be at one of the following 3
+ * states:
+ *
+ *  1. The operation has not started at all (with a return value FALSE.)
+ *  2. The operation has ended (with a return value FALSE.)
+ *  3. beginResizeForView: is in progress, waiting (with a return value TRUE.)
+ *
+ * By eliminating the 1st case (such as using the notify callback), we can use
+ * this return value to decide whether the synchronizer is in case 2 or case 3,
+ * that is whether the resizing is blocked by a mismatching commit.
+ */
+- (BOOL)isWaitingWhenMutexIsAvailable;
+
+/**
+ * Blocks current thread until there is frame available.
+ * Used in FlutterEngineTest.
+ */
+- (void)blockUntilFrameAvailable;
+
+@end