UIGestureRecognizer有用文档摘抄

已提供的手势：

The concrete subclasses of UIGestureRecognizer are the following:

• UITapGestureRecognizer

• UIPinchGestureRecognizer

• UIRotationGestureRecognizer

• UISwipeGestureRecognizer

• UIPanGestureRecognizer

• UILongPressGestureRecognizer

手势对UITouch的影响：

A window delivers touch events to a gesture recognizer before it delivers them to the hit-tested view attached to the gesture recognizer. Generally, if a gesture recognizer analyzes the stream of touches in a multi-touch sequence and does not recognize its gesture, the view receives the full complement of touches. If a gesture recognizer recognizes its gesture, the remaining touches for the view are cancelled. The usual sequence of actions in gesture recognition follows a path determined by default values of the cancelsTouchesInView, delaysTouchesBegan, delaysTouchesEnded properties:

• cancelsTouchesInView—If a gesture recognizer recognizes its gesture, it unbinds the remaining touches of that gesture from their view (so the window won’t deliver them). The window cancels the previously delivered touches with a (touchesCancelled:withEvent:) message. If a gesture recognizer doesn’t recognize its gesture, the view receives all touches in the multi-touch sequence.

• delaysTouchesBegan—As long as a gesture recognizer, when analyzing touch events, has not failed recognition of its gesture, the window withholds delivery of touch objects in the UITouchPhaseBegan phase to the attached view. If the gesture recognizer subsequently recognizes its gesture, the view does not receive these touch objects. If the gesture recognizer does not recognize its gesture, the window delivers these objects in an invocation of the view’s touchesBegan:withEvent: method (and possibly a follow-up touchesMoved:withEvent: invocation to inform it of the touches current location).

• delaysTouchesEnded—As long as a gesture recognizer, when analyzing touch events, has not failed recognition of its gesture, the window withholds delivery of touch objects in the UITouchPhaseEnded phase to the attached view. If the gesture recognizer subsequently recognizes its gesture, the touches are cancelled (in a touchesCancelled:withEvent: message). If the gesture recognizer does not recognize its gesture, the window delivers these objects in an invocation of the view’s touchesEnded:withEvent: method.

如何确定UIGestureRecognizer是否可用？

To determine whether a class is available at runtime in a given iOS release, you typically check whether the class is nil. Unfortunately, this test is not cleanly accurate for UIGestureRecognizer. Although this class was publicly available starting with iOS 3.2, it was in development a short period prior to that. Although the class exists in an earlier release, use of it and other gesture-recognizer classes are not supported in that earlier release. You should not attempt to use instances of those classes.

To determine at runtime whether you can use gesture recognizers in your application, test whether the class exists and, if it does, allocate an instance and see check if it responds to the selector locationInView:. This method was not added to the class until iOS 3.2. The code might look like the following:

UIGestureRecognizer *gestureRecognizer = [[UIGestureRecognizer alloc] initWithTarget:self action:@selector(myAction:)];

if (![gestureRecognizer respondsToSelector:@selector(locationInView:)]) {

    [gestureRecognizer release];

    gestureRecognizer = nil;

}

// do something else if gestureRecognizer is nil

UIGestureRecognizer的state（重要）：

Recognizers for discrete gestures transition from UIGestureRecognizerStatePossible to UIGestureRecognizerStateFailed or UIGestureRecognizerStateRecognized. Recognizers for continuous gesture transition from UIGestureRecognizerStatePossible to these phases in the given order: UIGestureRecognizerStateBegan, UIGestureRecognizerStateChanged, and UIGestureRecognizerStateEnded. If, however, they receive a cancellation touch, they should transition to UIGestureRecognizerStateCancelled. If recognizers for continuous gestures cannot interpret a multi-touch sequence as their gesture, they transition to UIGestureRecognizerStateFailed.