Using UITapGestureRecognizer

Question

New to iPhone dev. I have a view which contains a UIScrollView which contains a UIImageView. I added a (double) tap gesture recognizer on the image view which makes an alert box open. For some reason, and I'm sure I'm just retarded, it opens 3 times.

Here's my code:

- (void)viewDidLoad {

    scrollView.delegate = self;

    UIImage* image = imageView.image;
    imageView.bounds = CGRectMake(0, 0, image.size.width, image.size.height);
    scrollView.contentSize = image.size;

    UITapGestureRecognizer *tapGesture = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(handleTapGesture:)];
    tapGesture.numberOfTapsRequired = 2;
    [imageView addGestureRecognizer:tapGesture];
    [tapGesture release];

    NSLog(@"LOADED");

    [super viewDidLoad];
}

-(IBAction) handleTapGesture:(UIGestureRecognizer *) sender {
    CGPoint tapPoint = [sender locationInView:imageView];
    int tapX = (int) tapPoint.x;
    int tapY = (int) tapPoint.y;
    NSLog(@"TAPPED X:%d Y:%d", tapX, tapY);
    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Hello" message:@"How are you?" delegate:nil cancelButtonTitle:@"I'm awesome." otherButtonTitles:nil];
    [alert show];
    [alert release];
}

I just started iPhone dev a few days ago. This problem kind of reminds me of event bubbling issues I've dealt with in javascript. Any ideas?

Answer 1

Not sure what the exact reason is but the UIAlertView is somehow causing the gesture to fire again. A workaround is to execute the showing outside the gesture handler using performSelector:

-(void) handleTapGesture:(UIGestureRecognizer *) sender {
    CGPoint tapPoint = [sender locationInView:imageView];
    int tapX = (int) tapPoint.x;
    int tapY = (int) tapPoint.y;
    NSLog(@"TAPPED X:%d Y:%d", tapX, tapY);
    [self performSelector:@selector(showMessage) withObject:nil afterDelay:0.0];
}

- (void)showMessage
{
    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Hello" message:@"How are you?" delegate:nil cancelButtonTitle:@"I'm awesome." otherButtonTitles:nil];
    [alert show];
    [alert release];
}

Edit:
The gesture recognizer goes through different states in the gesture (Began, Changed, etc) and it calls the handler method each time the state changes. So a better and probably correct solution is to check the state property of the gesture recognizer at the top of the handler:

-(void) handleTapGesture:(UIGestureRecognizer *) sender {
    if (sender.state != UIGestureRecognizerStateEnded)  // <---
        return;                                         // <---

    CGPoint tapPoint = [sender locationInView:imageView];
    int tapX = (int) tapPoint.x;
    int tapY = (int) tapPoint.y;
    NSLog(@"TAPPED X:%d Y:%d", tapX, tapY);

    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Hello" message:@"How are you?" delegate:nil cancelButtonTitle:@"I'm awesome." otherButtonTitles:nil];
    [alert show];
    [alert release];
}