Question

Are there known issues with gesture recognizers and the UIView class methods for animation?

I am having problems with a sequence of animations on a UIImageView from UIGestureRecognizer callback. If the sequence of animations is started from a standard callback like TouchUpInside, the animation works fine. If it is started via the UILongPressGestureRecognizer, then the first animation jumps to the end and the second animation immediately begins.

Here’s a sample that illustrates my problem. In the .xib for the project, I have a UIImageView that is connected to the viewToMove IBOutlet. I also have a UIButton connected to the startButton IBOutlet, and I have connected its TouchUpInside action to the startButtonClicked IBAction. The TouchUpInside action works as I want it to, but the longPressGestureRecognizer skips to the end of the first animation after about half a second. When I NSLog the second animation (animateTo200) I can see that it is called twice when a long press starts the animation but only once when the button’s TouchUpInside action starts the animation.

- (void)viewDidLoad {
[super viewDidLoad];

UILongPressGestureRecognizer *longPressRecognizer = [[UILongPressGestureRecognizer alloc] initWithTarget:self action:@selector(startButtonClicked)];
NSArray *recognizerArray = [[NSArray alloc] initWithObjects:longPressRecognizer, nil];
[startButton setGestureRecognizers:recognizerArray];

[longPressRecognizer release];
[recognizerArray release];
}

-(IBAction)startButtonClicked {

if (viewToMove.center.x < 150) {
    [self animateTo200:@"Right to left" finished:nil context:nil];
} else {
    [self animateTo100:@"Right to left" finished:nil context:nil];
}
}

-(void)animateTo100:(NSString *)animationID finished:(NSNumber *)finished context:(void *)context {
[UIView beginAnimations:@"Right to left" context:nil];
[UIView setAnimationDuration:4];
[UIView setAnimationDelegate:self];
[UIView setAnimationDidStopSelector:@selector(animateTo200:finished:context:)];
viewToMove.center = CGPointMake(100.0, 100.0);
[UIView commitAnimations];          
}

-(void)animateTo200:(NSString *)animationID finished:(NSNumber *)finished context:(void *)context {
[UIView beginAnimations:@"Left to right" context:nil];
[UIView setAnimationDuration:4];
viewToMove.center = CGPointMake(200.0, 200.0);
[UIView commitAnimations];          
}

Answer 1

You should change the signature of startButtonClicked to - (void)startButtonClicked:(UIGestureRegognizer *)gestureRecognizer and then query the gesture recognizer’s state property in the method. The gesture recognizer will call its action method multiple times with different states (e.g. UIGestureRecognizerStateBegan and UIGestureRecognizerStateEnded).