UIPanGestureRecognizer starting point is off

Question

I have a UIView that has a UIPanGestureRecognizer attached to it the gesture is working fine except that the starting point is not where the pan first started it is usually

Answer 1

The documentation says that the pan gesture starts when the fingers have "moved enough to be considered a pan." This movement is to distinguish a press from a drag, since the user's finger could move around a bit while they are trying to press without dragging.

I think this is the difference you're seeing between the first touch point and the first point considered part of the drag.

Answer 2

You can detect initial touch position of a gesture with the gesture recognizer's delegate method

- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldReceiveTouch:(UITouch *)touch

Answer 3

The start point changing reason is as @Douglas said:

• the pan gesture starts when the fingers have moved enough to be considered a pan.

And the starting point and translation are both calculated after the pan gesture is recognized.

I use the following way to get the "real" start point:

For the view which has the pan gesture, override the -(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event method, and store the "real" starting point for later use:

-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    [super touchesBegan:touches withEvent:event];
    self.touchStartPoint = [[[touches allObjects] firstObject] locationInView:self];
}

Answer 4

Yes the difference is because the gesture recognizer waits a undetermined distance of movement before becoming active. What you can do is create your own UIPanGestureRecognizer and set the state to UIGestureRecognizerStateChanged in the touchesMoved override method.

NOTE: I used touhcesMoved instead of touchesBegan because I wanted it to start when the users touch moved and not instantly.

Here is some example code for a custom gesture recognizer that does this:

#import "RAUIPanGestureRecognizer.h"

@implementation RAUIPanGestureRecognizer 


#pragma mark - UIGestureRecognizerSubclass Methods

- (void)reset
    { [super reset ]; }

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event;
    { [super touchesBegan:touches withEvent:event ]; }

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
    { 
        [self setState:UIGestureRecognizerStateChanged ];
        [super touchesMoved:touches withEvent:event ]; 
    }

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
    { [super touchesEnded:touches withEvent:event ]; }

- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event
    { [super touchesCancelled:touches withEvent:event ]; }


@end

Answer 5

  CGPoint beg = [panRecognizer locationInView:_scrollView];
  CGPoint trans = [panRecognizer translationInView:_scrollView];
  CGPoint firstTouch = CGPointSubtract(beg, trans);

Put this code in the UIGestureRecognizerStateBegan case

Answer 6

To get around this you could try reseting the translation point when the gesture recognizer begins. For example, start your action method out like so:

- (void)panGesture:(UIPanGestureRecognizer *)recognizer;
{
    if ( recognizer.state == UIGestureRecognizerStateBegan )
    {
        CGPoint point = ...; // The view's initial origin.
        UIView *superview = [recognizer.view superview];
        [recognizer setTranslation:point inView:superview];
    }
}