使用 UITapGestureRecognizer

Question

iPhone 开发新手。我有一个视图，其中包含 UIScrollView，其中包含 UIImageView。我在图像视图上添加了一个（双击）点击手势识别器，这使得警报框打开。由于某种原因，我确信我只是弱智，它打开了 3 次。

这是我的代码：

- (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];
}

几天前我刚刚开始 iPhone 开发。这个问题有点让我想起我在 javascript 中处理过的事件冒泡问题。有什么想法吗？

Answer 1

不确定确切的原因是什么，但 UIAlertView 不知何故导致手势再次触发。解决方法是使用 PerformSelector 在手势处理程序外部执行显示：

-(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];
}

编辑：
手势识别器会经历手势中的不同状态（开始、更改等），并且每次状态更改时都会调用处理程序方法。因此，更好且可能正确的解决方案是检查处理程序顶部的手势识别器的状态属性：

-(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];
}