[iOS] Fixes DisplayLinkManager leaks (#22194)

Author: zhongwuzw

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

@@ -635,7 +635,7 @@ - (BOOL)createShell:(NSString*)entrypoint
 }
 
 - (void)initializeDisplays {
-  double refresh_rate = [[[DisplayLinkManager alloc] init] displayRefreshRate];
+  double refresh_rate = [DisplayLinkManager displayRefreshRate];
   auto display = flutter::Display(refresh_rate);
   _shell->OnDisplayUpdates(flutter::DisplayUpdateType::kStartup, {display});
 }

shell/platform/darwin/ios/framework/Source/vsync_waiter_ios.h

@@ -12,8 +12,6 @@
 
 @interface DisplayLinkManager : NSObject
 
-- (instancetype)init;
-
 //------------------------------------------------------------------------------
 /// @brief      The display refresh rate used for reporting purposes. The engine does not care
 ///             about this for frame scheduling. It is only used by tools for instrumentation. The
@@ -23,7 +21,7 @@
 ///
 /// @return     The refresh rate in frames per second.
 ///
-- (double)displayRefreshRate;
++ (double)displayRefreshRate;
 
 @end
 

shell/platform/darwin/ios/framework/Source/vsync_waiter_ios.mm

@@ -91,26 +91,16 @@ - (void)dealloc {
 
 @end
 
-@implementation DisplayLinkManager {
-  fml::scoped_nsobject<CADisplayLink> display_link_;
-}
-
-- (instancetype)init {
-  self = [super init];
-
-  if (self) {
-    display_link_ = fml::scoped_nsobject<CADisplayLink> {
-      [[CADisplayLink displayLinkWithTarget:self selector:@selector(onDisplayLink:)] retain]
-    };
-    display_link_.get().paused = YES;
-  }
+@implementation DisplayLinkManager
 
-  return self;
-}
-
-- (double)displayRefreshRate {
++ (double)displayRefreshRate {
   if (@available(iOS 10.3, *)) {
-    auto preferredFPS = display_link_.get().preferredFramesPerSecond;  // iOS 10.0
+    fml::scoped_nsobject<CADisplayLink> display_link = fml::scoped_nsobject<CADisplayLink> {
+      [[CADisplayLink displayLinkWithTarget:[[DisplayLinkManager new] autorelease]
+                                   selector:@selector(onDisplayLink:)] retain]
+    };
+    display_link.get().paused = YES;
+    auto preferredFPS = display_link.get().preferredFramesPerSecond;  // iOS 10.0
 
     // From Docs:
     // The default value for preferredFramesPerSecond is 0. When this value is 0, the preferred
@@ -131,10 +121,4 @@ - (void)onDisplayLink:(CADisplayLink*)link {
   // no-op.
 }
 
-- (void)dealloc {
-  [display_link_.get() invalidate];
-
-  [super dealloc];
-}
-
 @end