有时候你的App需要控制从状态栏下拉或者底部栏上滑，这个会跟系统的下拉通知中心手势和上滑控制中心手势冲突。
如果你要优先自己处理手势可以将系统手势延迟。
@interfaceUIViewController(UIScreenEdgesDeferringSystemGestures)
// Override to return a child view controller or nil. If non-nil, that view controller's screen edges deferring system gestures will be used. If nil, self is used. Whenever the return value changes, -setNeedsScreenEdgesDeferringSystemGesturesUpdate should be called.
- (nullableUIViewController*)childViewControllerForScreenEdgesDeferringSystemGestures API_AVAILABLE(ios(11.0)) API_UNAVAILABLE(watchos, tvos);
// Controls the application's preferred screen edges deferring system gestures when this view controller is shown. Default is UIRectEdgeNone.
- (UIRectEdge)preferredScreenEdgesDeferringSystemGestures API_AVAILABLE(ios(11.0)) API_UNAVAILABLE(watchos, tvos);
// This should be called whenever the return values for the view controller's screen edges deferring system gestures have changed.
- (void)setNeedsUpdateOfScreenEdgesDeferringSystemGestures API_AVAILABLE(ios(11.0)) API_UNAVAILABLE(watchos, tvos);
@end
例如：
- (UIRectEdge)preferredScreenEdgesDeferringSystemGestures
{
returnUIRectEdgeTop;
}
设置后下拉状态栏只会展示指示器，继续下拉才能将通知中心拉出来。如果返回UIRectEdgeNone则会直接下拉出来。
