[iOS] Eliminate strong retain cycle from VSyncClient

cbracken · cbracken · commit dca7b2c7bf39 · 2026-05-23T21:07:48.000+09:00
diff --git a/engine/src/flutter/shell/platform/darwin/ios/framework/Source/VSyncClient.swift b/engine/src/flutter/shell/platform/darwin/ios/framework/Source/VSyncClient.swift
@@ -58,7 +58,8 @@ public class VSyncClient: NSObject {
 
     super.init()
 
-    let link = CADisplayLink(target: self, selector: #selector(onDisplayLink(_:)))
+    let relay = DisplayLinkRelay(target: self)
+    let link = CADisplayLink(target: relay, selector: #selector(DisplayLinkRelay.onDisplayLink(_:)))
     link.isPaused = true
     self.displayLink = link
 
@@ -186,3 +187,21 @@ public class VSyncClient: NSObject {
     callback(timestamp, targetTimestamp)
   }
 }
+
+/// A weak proxy target for `CADisplayLink` callbacks to prevent retain cycles.
+///
+/// `CADisplayLink` strongly retains its target. If the display link directly targeted `VSyncClient`,
+/// it would form a strong retain cycle (since `VSyncClient` also strongly retains the `CADisplayLink`
+/// instance). Instead, we route display link callbacks through this intermediate relay holding `VSyncClient` weakly.
+private final class DisplayLinkRelay: NSObject {
+  private weak var target: VSyncClient?
+
+  init(target: VSyncClient) {
+    self.target = target
+  }
+
+  @objc
+  func onDisplayLink(_ link: CADisplayLink) {
+    target?.onDisplayLink(link)
+  }
+}
diff --git a/engine/src/flutter/shell/platform/darwin/ios/framework/Source/VSyncClientTest.swift b/engine/src/flutter/shell/platform/darwin/ios/framework/Source/VSyncClientTest.swift
@@ -197,4 +197,20 @@ class VSyncClientTest: XCTestCase {
     waitForExpectations(timeout: 1.0, handler: nil)
     XCTAssertNil(weakClient)
   }
+
+  func testDeallocatesWithoutExplicitInvalidation() {
+    let threadTaskRunner = TaskRunnerTestHelper.makeTaskRunner(withLabel: "VSyncClientTest")
+    weak var weakClient: VSyncClient?
+
+    autoreleasepool {
+      let client = VSyncClient(
+        taskRunner: threadTaskRunner,
+        isVariableRefreshRateEnabled: false,
+        maxRefreshRate: 60.0
+      ) { _, _ in }
+      weakClient = client
+    }
+
+    XCTAssertNil(weakClient)
+  }
 }
