rankAmateur rankAmateur - 5 days ago 5
iOS Question

Why are UIViewController touchesBegan, touchesMoved & touchesEnded only being called when the first of two touches begins, moves or ends?

I'm having an issue with handling more than one touch through the touchesBegan/Moved/Ended methods of UIViewController. I'm also seeing the same behaviour in a cocos2d app (using ccTouchesBegan/Moved/Ended) so I think this question can be applied to all touch handling in iOS. I've put the code that I'm using below, followed by the results that I'm seeing.

All methods are implemented on a UIViewController subclass.

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
NSLog(@"Touches Began");
[self logTouchesFor: event];

[super touchesEnded: touches withEvent: event];
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
NSLog(@"Touches Moved");
[self logTouchesFor: event];

[super touchesEnded: touches withEvent: event];
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
NSLog(@"Touches Ended");
[self logTouchesFor: event];

[super touchesEnded: touches withEvent: event];
}

-(void)logTouchesFor:(UIEvent *)event
{
int count = 1;

for (UITouch *touch in event.allTouches)
{
CGPoint location = [touch locationInView: self.view];

NSLog(@"%d: (%.0f, %.0f)", count, location.x, location.y);
count++;
}
}


Now for the interesting results...

Single Touches Work as Expected

Let's say I touch the screen with my thumb. I see in the output window that touchesBegan has been called as expected. I move my thumb around and touchesMoved gets called. Then I lift my thumb off the screen and touchesEnded gets called. All this is as expected, I'm including it in the question as a control case - just to be clear that my view controller is receiving touch events and I haven't missed a
vc.view.userInteractionEnabled = YES
anywhere.

The Second Touch Doesn't Cause touchesBegan, touchesMoved or touchesEnded to be called

This is the most interesting one. Let's say I touch the screen with my thumb (touchesBegan is called) and hold it still on the screen. Then I touch somewhere else on the screen with my index finger, whilst keeping my thumb in the same place. TouchesBegan is not called. Then let's say I move my index finger whilst keeping my thumb absolutely still (this can be tricky but it is possible). TouchesMoved is not called. Then, I lift my index finger off the screen. TouchesEnded is not called. Finally, I move my thumb and touchesMoved is called. Then I lift my thumb from the screen and touchesEnded is called.

Just to be clear: I've set
self.view.multipleTouchEnabled = YES
in my
viewDidLoad
method.

Information About the Second Touch is Available, Providing the First Touch Moves

This time I do something very similar to the example immediately above. I touch the screen with my thumb, then index finger whilst keeping my thumb still. TouchesBegan is called when my thumb hits the screen, but not my index finger. Now I move my thumb, and touchesMoved is called. Not only that, but there are two touches in the event.allTouches array (and yes, the second touch is where I would expect it to be). This means that the system is aware that I have touched the screen a second time, but I am not being notified through the touch handling methods on my view controller.

How Can I be Notified About Changes to the Second Touch?

I'd really like to be able to respond to changes in the location or state of the second touch as they happen, rather than when the first touch also changes. A lot of the time this won't be an issue because it's very hard to change one touch without impacting on the other, but I have at least one situation where it can be an issue. Am I missing something obvious? Has anyone else noticed this behaviour or had issues with it?

In case it's relevant, I'm using an iPhone 3GS running iOS 5.1.

Answer

Ok, I started painstakingly rebuilding my project, adding in the files one at a time, and I think I've got it...

There was a subview of the view in question which had userInteractionEnabled == YES. Once I set this to NO, my view controller started getting touchesBegan/Moved/Ended calls for each touch. Presumably, the second touch was being claimed by the subview and not making it to my view controller, but I have no idea why this would only happen with the second touch and not the first.

I haven't figured out what's going on with the cocos2d project yet, but presumably it's a different issue as there are no subviews or nodes that could be doing the same thing.

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