Question

I can’t find anywhere in the documentation. When this message is called on my subclass of UIView:

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event

How can I get the touch for which this message was called ?

On both the NSSet and UIEvent I can get only sets of touches, but no unique identifier so I can determine which touch triggered the message.

PS: why on hell would they send a NSSet of all touches, and also the possibility to get the same set from [[event allTouches] anyObject]

Answer 1

You can’t get the touch, because sometimes multiple touches triggered the message. If the user has two fingers on the screen, and moves both, you can get a single touchesMoved:withEvent: in which both touches are updated.

You need to process every touch in the touches set. If you’ve disabled multitouch for the view, so that you know there will only ever be one touch in the set, you can use touches.anyObject to get the touch. But if you haven’t disabled multitouch, you need to loop over all of the touches in the set.

The message includes a set of touches separate from event.allTouches because the user might have three fingers down but only move one or two of them. The touches set only contains the moved touches, but event.allTouches contains all of the user’s touches, including the touches that have not moved since the last message.

The unique identifier for the touch is the UITouch object itself. When the user puts a finger on the screen, iOS creates a UITouch object. It updates that object as the user moves his finger. So you can use the UITouch object as the key in an NSDictionary, or you can attach your own objects to it using objc_setAssociatedObject.