UiKitViewGestureBlockingPolicy enum

How touch event callbacks and gesture recognizers of a platform view are blocked.

This replaces the engine's FlutterPlatformViewGestureRecognizersBlockingPolicy enum in FlutterPlugin.h.

In iOS, a gesture recognizer (UIGestureRecognizer) is an object that decouples the logic for recognizing a sequence of touches (like a tap, pinch, or swipe) and acting on that recognition.

When a Flutter app embeds an iOS platform view (like a WKWebView), both the Flutter framework and the native iOS view receive touch events. To prevent both systems from simultaneously reacting to the same touch (e.g., a scroll gesture scrolling both a Flutter ListView and a native UIScrollView), Flutter needs a mechanism to "block" the native view's gesture recognizers when it determines that the Flutter framework should handle the gesture.

Flutter uses two mechanisms to achieve this:

1. Synchronous Blocking (Hit Testing): During the initial touch (UIResponder.touchesBegan), Flutter performs a synchronous hit test. If the touch lands on a Flutter widget that is visually on top of the platform view, Flutter immediately blocks the native view from receiving the touch.
2. Asynchronous Blocking (Gesture Arena): If the touch lands directly on the platform view, both Flutter and the native view begin tracking the gesture. Flutter's gesture arena resolves which system wins. If Flutter wins (e.g., the user is scrolling a Flutter ListView that contains the platform view), Flutter asynchronously cancels the native view's gesture recognizers.

The default policy (fallbackToPluginDefault, which typically resolves to eager) works for most use cases. However, some native views (either from Apple or 3rd party) may have bugs where their internal gesture recognizers get stuck in a stale state if they are aggressively canceled by Flutter's asynchronous blocking. In these specific cases, you might need to change the policy to doNotBlockGesture or waitUntilTouchesEnded to work around the native view's bugs.

For more details, see: https://flutter.dev/go/ios-platform-view-touch-gesture-blocking.