Fix retain cycle in FlutterMetalLayer (flutter/engine#54119)

*Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.*

*List which issues are fixed by this PR. You must list at least one issue.*

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

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

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterMetalLayer.mm

@@ -50,6 +50,7 @@ extern CFTimeInterval display_link_target;
   BOOL _displayLinkForcedMaxRate;
 }
 
+- (void)onDisplayLink:(CADisplayLink*)link;
 - (void)presentTexture:(FlutterTexture*)texture;
 - (void)returnTexture:(FlutterTexture*)texture;
 
@@ -163,6 +164,26 @@ extern CFTimeInterval display_link_target;
 
 @end
 
+@interface FlutterMetalLayerDisplayLinkProxy : NSObject {
+  __weak FlutterMetalLayer* _layer;
+}
+
+@end
+
+@implementation FlutterMetalLayerDisplayLinkProxy
+- (instancetype)initWithLayer:(FlutterMetalLayer*)layer {
+  if (self = [super init]) {
+    _layer = layer;
+  }
+  return self;
+}
+
+- (void)onDisplayLink:(CADisplayLink*)link {
+  [_layer onDisplayLink:link];
+}
+
+@end
+
 @implementation FlutterMetalLayer
 
 @synthesize preferredDevice = _preferredDevice;
@@ -179,7 +200,9 @@ extern CFTimeInterval display_link_target;
     self.pixelFormat = MTLPixelFormatBGRA8Unorm;
     _availableTextures = [[NSMutableSet alloc] init];
 
-    _displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(onDisplayLink:)];
+    FlutterMetalLayerDisplayLinkProxy* proxy =
+        [[FlutterMetalLayerDisplayLinkProxy alloc] initWithLayer:self];
+    _displayLink = [CADisplayLink displayLinkWithTarget:proxy selector:@selector(onDisplayLink:)];
     [self setMaxRefreshRate:DisplayLinkManager.displayRefreshRate forceMax:NO];
     [_displayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];
     [[NSNotificationCenter defaultCenter] addObserver:self
@@ -190,6 +213,11 @@ extern CFTimeInterval display_link_target;
   return self;
 }
 
+- (void)dealloc {
+  [_displayLink invalidate];
+  [[NSNotificationCenter defaultCenter] removeObserver:self];
+}
+
 - (void)setMaxRefreshRate:(double)refreshRate forceMax:(BOOL)forceMax {
   // This is copied from vsync_waiter_ios.mm. The vsync waiter has display link scheduled on UI
   // thread which does not trigger actual core animation frame. As a workaround FlutterMetalLayer

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterMetalLayerTest.mm

@@ -270,4 +270,24 @@
   [self removeMetalLayer:layer];
 }
 
+- (void)testDealloc {
+  __weak FlutterMetalLayer* weakLayer;
+  @autoreleasepool {
+    FlutterMetalLayer* layer = [self addMetalLayer];
+    weakLayer = layer;
+    TestCompositor* compositor = [[TestCompositor alloc] initWithLayer:layer];
+
+    id<CAMetalDrawable> drawable = [layer nextDrawable];
+    BAIL_IF_NO_DRAWABLE(drawable);
+    [drawable present];
+    [compositor commitTransaction];
+
+    [self removeMetalLayer:layer];
+    // Deallocating the layer after removing is not synchronous.
+    CFRunLoopRunInMode(kCFRunLoopDefaultMode, 1, YES);
+  }
+
+  XCTAssertNil(weakLayer);
+}
+
 @end