Ios – How to capture the point initially tapped in a UIPanGestureRecognizer

iosiphoneobjective cpanuigesturerecognizer

I have an app that lets the user trace lines on the screen. I am doing so by recording the points within a UIPanGestureRecognizer:

-(void)handlePanFrom:(UIPanGestureRecognizer *)recognizer
{
    CGPoint pixelPos = [recognizer locationInView:rootViewController.glView];
    NSLog(@"recorded point %f,%f",pixelPos.x,pixelPos.y);
}

That works fine. However, I'm very interested in the first point the user tapped before they began panning. But the code above only gives me the points that occurred after the gesture was recognized as a pan (vs. a tap.)

From the documentation, it appears there may be no easy way to determine the initially-tapped location within the UIPanGestureRecognizer API. Although within UIPanGestureRecognizer.h, I found this declaration: 

CGPoint _firstScreenLocation;

…which appears to be private, so no luck. I'm considering going outside the UIGestureRecognizer system completely just to capture that initailly-tapped point, and later refer back to it once I know that the user has indeed begun a UIPanGesture. I Thought I would ask here, though, before going down that road.

Best Answer

Late to the party, but I notice that nothing above actually answers the question, and there is in fact a way to do this. You must subclass UIPanGestureRecognizer and include:

#import <UIKit/UIGestureRecognizerSubclass.h>

either in the Objective-C file in which you write the class or in your Swift bridging header. This will allow you to override the touchesBegan:withEvent method as follows:

class SomeCoolPanGestureRecognizer: UIPanGestureRecognizer {
    private var initialTouchLocation: CGPoint!

    override func touchesBegan(touches: Set<UITouch>, withEvent event: UIEvent) {
        super.touchesBegan(touches, withEvent: event)
        initialTouchLocation = touches.first!.locationInView(view)
    }
}

Then your property initialTouchLocation will contain the information you seek. Of course in my example I make the assumption that the first touch in the set of touches is the one of interest, which makes sense if you have a maximumNumberOfTouches of 1. You may want to use more sophistication in finding the touch of interest.

Edit: Swift 5


import UIKit.UIGestureRecognizerSubclass

class InitialPanGestureRecognizer: UIPanGestureRecognizer {
  private var initialTouchLocation: CGPoint!

  override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent) {
    super.touchesBegan(touches, with: event)
    initialTouchLocation = touches.first!.location(in: view)
  }
}
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