Swift Gesture Recognizer That Tracks Only First Touch In Continuous Gesture

0
votes

I'm trying to use a gesture recognizer to allow the user to resize a view on the screen by dragging it with one finger... as such, I want to track all changes to the touch. I initially tried to use a PanGestureRecognizer for this, but I've run into an issue when the user touches the view with multiple fingers. I only want to track the first touch (ie the first finger the user places on the screen during a gesture), but I can't find a way to prevent additional touches from interfering with my ability to track the first. My initial solution was to simply set

recognizer.maximumNumberOfTouches = 1

However, this causes the entire gesture to be cancelled in the event of a second touch (I want to continue tracking the first touch in this case). But, of course, not setting this value to 1 causes the gesture to actually track multiple touches. How can I make a gesture recognizer that tracks all changes to only the first touch in the gesture, and isn't cancelled by additional touches on the screen?

2
iosswiftuigesturerecognizeruipangesturerecognizer
What happen if you remove recognizer.maximumNumberOfTouches = 1 this line? – TheTiger
With a standard PanGestureRecognizer, it tracks both touches. With my custom recognizer, it cancels the entire gesture (but seems to be slightly delayed in doing so). – kmell96
Just get rid of that touchesMoved implementation. – matt
@matt But after began if user touches his second finger in that case touchBegan will not call again.. isn't it? – TheTiger
Right but you don’t want it to. You want to just keep tracking the first finger, and that’s just what will happen – matt

2 Answers

-1
votes

From your comments and answer, it sounds like your use of a pan gesture recognizer was always wrong and you were doing something wrong in your action handler. No correct action handler for a pan gesture recognizer would ever use location(in:) for anything. location(in:) gives you a centroid of all touches, which is exactly what you say you don't want — and in any case there is no need to consult it.

  • If your goal is to drag a view, you use translation(in:).

  • If your goal is to track the actual location of a particular touch, you use location(ofTouch:in:).

  • If you discover that a touch is being delivered and you don't want it tracked, call ignore(_:for:).

-1
votes

Here is a UIGestureRecognizer subclass that will track all changes the first touch the user places on the screen at the start of a gesture, and will ignore all other fingers without cancelling the gesture. Note that if you're using functions/properties of the gesture recognizer (such as location(in:)), you'll probably have to override those as well.

class FirstTouchGestureRecognizer: UIGestureRecognizer {
    private var firstTouch: UITouch? = nil

    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent) {
        if firstTouch == nil {
            firstTouch = touches.first!
            self.state = .began
        }
    }

    override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent) {
        if firstTouch != nil && touches.contains(firstTouch!) {
            self.state = .changed
        }
    }

    override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent) {
        if firstTouch != nil && touches.contains(firstTouch!) {
            firstTouch = nil
            self.state = .ended
        }
    }
}