iOS开发之-Gesture Recognizer

Gesture Recognizers

Gesture Recognizers Simplify Event Handling

Gestures Recognizers Are Attached to a View

1. To detect its gestures, a gesture recognizer must be attached to the view that a user is touching. This view is known as the hit-tested view. Delivery of events initially follows the usual path: from operating system to the application object to the window object representing the window in which the touches are occurring. 

2. Thus gesture recognizers act as observers of touch objects sent to their attached view or view hierarchy. However, they are not part of that view hierarchy and do not participate in the responder chain. 

3. For some gestures, the locationInView: and the locationOfTouch:inView: methods of UIGestureRecognizer enable clients to find the location of gestures or specific touches in the attached view or its subviews.

Gestures Trigger Action Messages

When a gesture recognizer recognizes its gesture, it sends one or more action messages to one or more targets. 

Discrete Gestures and Continuous Gestures

Implementing Gesture Recognition

Preparing a Gesture Recognizer

UITapGestureRecognizer *doubleFingerTap = [[UITapGestureRecognizer alloc]
   initWithTarget:self action:@selector(handleDoubleDoubleTap:)];

1. The action methods for handling gestures—and the selector for identifying them—are expected to conform to one of two signatures:

- (void)handleGesture
- (void)handleGesture:(UIGestureRecognizer *)sender

2. UIView methods:

addGestureRecognizer:

gestureRecognizers property

removeGestureRecognizer:

- (void)createGestureRecognizers {
    UITapGestureRecognizer *singleFingerDTap = [[UITapGestureRecognizer alloc]
        initWithTarget:self action:@selector(handleSingleDoubleTap:)];
    singleFingerDTap.numberOfTapsRequired = 2;
    [self.theView addGestureRecognizer:singleFingerDTap];
    [singleFingerDTap release];
 
    UIPanGestureRecognizer *panGesture = [[UIPanGestureRecognizer alloc]
        initWithTarget:self action:@selector(handlePanGesture:)];
    [self.theView addGestureRecognizer:panGesture];
    [panGesture release];
 
    UIPinchGestureRecognizer *pinchGesture = [[UIPinchGestureRecognizer alloc]
        initWithTarget:self action:@selector(handlePinchGesture:)];
    [self.theView addGestureRecognizer:pinchGesture];
    [pinchGesture release];
}

3. You may also add additional targets and actions to a gesture recognizer using the addTarget:action: method of UIGestureRecognizer. 

Responding to Gestures

- (IBAction)handlePinchGesture:(UIGestureRecognizer *)sender {
    CGFloat factor = [(UIPinchGestureRecognizer *)sender scale];
    self.view.transform = CGAffineTransformMakeScale(factor, factor);
}
 
- (IBAction)handlePanGesture:(UIPanGestureRecognizer *)sender {
    CGPoint translate = [sender translationInView:self.view];
 
    CGRect newFrame = currentImageFrame;
    newFrame.origin.x += translate.x;
    newFrame.origin.y += translate.y;
    sender.view.frame = newFrame;
 
    if (sender.state == UIGestureRecognizerStateEnded)
        currentImageFrame = newFrame;
}
 
- (IBAction)handleSingleDoubleTap:(UIGestureRecognizer *)sender {
    CGPoint tapPoint = [sender locationInView:sender.view.superview];
    [UIView beginAnimations:nil context:NULL];
    sender.view.center = tapPoint;
    [UIView commitAnimations];
}

Interacting with Other Gesture Recognizers

Requiring a Gesture Recognizer to Fail

1. You might want a relationship between two gesture recognizers so that one can operate only if the other one fails. For example, recognizer A doesn’t begin analyzing a multitouch sequence until recognizer B fails and, conversely, if recognizer B does recognize its gesture, recognizer A never looks at the multitouch sequence. An example where you might specify this relationship is when you have a gesture recognizer for a single tap and another gesture recognizer for a double tap; the single-tap recognizer requires the double-tap recognizer to fail before it begins operating on a multitouch sequence.