Question

Something strange I encountered today while attaching same gesture recogniser to multiple image views. It gets attached to only the last one, in other words, it can be attached to only one view!

I had to create multiple gesture recognisers to meet my requirements.

Following is what I have done. Am I doing correct? Is that’s the only way to attach recognisers to the multiple imageviews?

Please note that I don’t want to use UITableView or UIVIew and put all imageviews in it and attach gesture recogniser to only UITableView or UIVIew. I have all image scattered and I have to detect which image is being dragged. Thanks.

[imgView1 setUserInteractionEnabled:YES];
[imgView1 setMultipleTouchEnabled:YES];

[imgView2 setUserInteractionEnabled:YES];
[imgView2 setMultipleTouchEnabled:YES];

[imgView3 setUserInteractionEnabled:YES];
[imgView3 setMultipleTouchEnabled:YES];

[imgView4 setUserInteractionEnabled:YES];
[imgView4 setMultipleTouchEnabled:YES];

[imgView5 setUserInteractionEnabled:YES];
[imgView5 setMultipleTouchEnabled:YES];

[imgView6 setUserInteractionEnabled:YES];
[imgView6 setMultipleTouchEnabled:YES];


//Attach gesture recognizer to each imagviews
gestureRecognizer1 = [[UILongPressGestureRecognizer alloc] initWithTarget:self action:@selector(gestureHandler:)];
gestureRecognizer1.delegate = self;

gestureRecognizer2 = [[UILongPressGestureRecognizer alloc] initWithTarget:self action:@selector(gestureHandler:)];
gestureRecognizer2.delegate = self;

gestureRecognizer3 = [[UILongPressGestureRecognizer alloc] initWithTarget:self action:@selector(gestureHandler:)];
gestureRecognizer3.delegate = self;

gestureRecognizer4 = [[UILongPressGestureRecognizer alloc] initWithTarget:self action:@selector(gestureHandler:)];
gestureRecognizer4.delegate = self;

gestureRecognizer5 = [[UILongPressGestureRecognizer alloc] initWithTarget:self action:@selector(gestureHandler:)];
gestureRecognizer5.delegate = self;

gestureRecognizer6 = [[UILongPressGestureRecognizer alloc] initWithTarget:self action:@selector(gestureHandler:)];
gestureRecognizer6.delegate = self;

[imgView1 addGestureRecognizer:gestureRecognizer1];
[imgView2 addGestureRecognizer:gestureRecognizer2];
[imgView3 addGestureRecognizer:gestureRecognizer3];
[imgView4 addGestureRecognizer:gestureRecognizer4];
[imgView5 addGestureRecognizer:gestureRecognizer5];
[imgView6 addGestureRecognizer:gestureRecognizer6];

Answer 1

Yes, one view per gesture recognizer. So if you want only one recognizer, put it on the superview, e.g.:

UILongPressGestureRecognizer *gestureRecognizer = [[UILongPressGestureRecognizer alloc] initWithTarget:self action:@selector(gestureHandler:)];
[self.view addGestureRecognizer:gestureRecognizer];

And then, in your handler, you can:

- (void)handleLongPress:(UILongPressGestureRecognizer *)sender
{
    CGPoint location = [sender locationInView:self.view];

    if (sender.state == UIGestureRecognizerStateBegan)
    {
        for (UIView *view in self.view.subviews)
        {
            if ([view isKindOfClass:[UIImageView class]] && CGRectContainsPoint(view.frame, location))
            {
                UIImageView *image = (UIImageView *) view;

                // ok, now you know which image you received your long press for
                // do whatever you wanted on it at this point

                return;
            }
        }
    }
}

By the way, if you do that, you don’t need to worry about enabling user interaction on the images, either.

Finally, you don’t need to worry about specifying your gesture recognizer’s delegate unless you’re going to conform to UIGestureRecognizerDelegate, which this isn’t. Also note that I’m using a local var for my recognizer because there’s no reason to hang onto it.

Update:

While the above code works fine, perhaps even better would be a custom long press gesture recognizer that would fail if the long press didn’t take place over an image (this way it’s more likely to play well in case you have other gesture recognizers taking place in your view). So:

#import <UIKit/UIGestureRecognizerSubclass.h>

@interface ImageLongPressGestureRecognizer : UILongPressGestureRecognizer
@property (nonatomic, weak) UIImageView *imageview;
@end

@implementation ImageLongPressGestureRecognizer

@synthesize imageview = _imageview;

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    self.imageview = nil;

    [super touchesBegan:touches withEvent:event];

    CGPoint location = [self locationInView:self.view];

    for (UIView *view in self.view.subviews)
    {
        if ([view isKindOfClass:[UIImageView class]] && CGRectContainsPoint(view.frame, location))
        {
            self.imageview = (UIImageView *)view;
            return;
        }
    }

    self.state = UIGestureRecognizerStateFailed;
}

@end

then create your gesture recognizer accordingly, using this new subclass:

ImageLongPressGestureRecognizer *gestureRecognizer = [[ImageLongPressGestureRecognizer alloc] initWithTarget:self action:@selector(handleLongPress:)];
[self.view addGestureRecognizer:gestureRecognizer];

and then, as a nice little benefit of this subclassing, your main gesture recognizer is simplified, namely:

- (void)handleLongPress:(ImageLongPressGestureRecognizer *)sender
{
    if (sender.state == UIGestureRecognizerStateBegan)
    {
        // you can now do whatever you want with sender.imageview, e.g. this makes it blink for you:

        [UIView animateWithDuration:0.5 
                         animations:^{
                             sender.imageview.alpha = 0.0;
                         } completion:^(BOOL finished){
                             [UIView animateWithDuration:0.5 
                                              animations:^{
                                                  sender.imageview.alpha = 1.0;
                                              } 
                                              completion:nil];
                         }];
    }
}