Why does my pan gesture only nudge its view and stop?

Question

Here is the UIPanGesture subclass:

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    NSLog(@"touches began");
   self ->origLoc = [[touches anyObject] locationInView:self.view.superview];
   [super touchesBegan:touches withEvent:event];
}


- (void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
if (self.state == UIGestureRecognizerStatePossible) {
    CGPoint loc = [[touches anyObject] locationInView:self.view.superview];
    CGFloat deltaX = fabsf(loc.x - origLoc.x);
    CGFloat deltaY = fabsf(loc.y - origLoc.y);
    if (deltaY >= deltaX) {
        self.state = UIGestureRecognizerStateFailed;
    } else {
    [super touchesMoved:touches withEvent:event];
    }
}
}

- (CGPoint) translationInView:(UIView *)v {

CGPoint proposedTranslation = [super translationInView:v];
proposedTranslation.y = 0;
return proposedTranslation;

}

Here is the method that gets called by my subclassed UIPanGesture:

- (void)dragging: (UIPanGestureRecognizer *)p {    

UIView *vv = p.view;

if (p.state == UIGestureRecognizerStateBegan || p.state == UIGestureRecognizerStateChanged) {
    CGPoint delta = [p translationInView:vv.superview];
    CGPoint c = vv.center;
    c.x += delta.x;
    c.y += delta.y;
    vv.center = c;
    [p setTranslation:CGPointZero inView:vv.superview];
 }    
}

When I try to drag my view around, it just moves a very small amount and then stops. I need to keep nudging it to get it to move. Any ideas?

Thanks in advance,

Michael.

Answer 1

In my prior comments, I wasn't clear why you were subclassing, but you clarified and it's because you have multiple gesture handlers going on. Very good.

So here is some sample code for a custom pan gesture, HorizontalPanGestureRecognizer, which is triggered only if the first movement is horizontal:

// HorizontalPanGestureRecognizer.h

#import <UIKit/UIKit.h>

@interface HorizontalPanGestureRecognizer : UIPanGestureRecognizer
@end

And

// HorizontalPanGestureRecognizer.m

#import "HorizontalPanGestureRecognizer.h"
#import <UIKit/UIGestureRecognizerSubclass.h>

@interface HorizontalPanGestureRecognizer ()
{
    BOOL _hasConfirmedDirection;
    BOOL _wasHorizontal;
    CGPoint _origLoc;
}
@end

@implementation HorizontalPanGestureRecognizer

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

    UITouch *touch = [touches anyObject];

    _hasConfirmedDirection = NO;

    _origLoc = [touch locationInView:self.view];
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    [super touchesMoved:touches withEvent:event];
    if (self.state == UIGestureRecognizerStateFailed) return;

    if (!_hasConfirmedDirection)
    {
        CGPoint translation = [self translationInView:self.view];

        _hasConfirmedDirection = YES;
        _wasHorizontal = (fabs(translation.x) > fabs(translation.y));
    }
    if (!_wasHorizontal)
        self.state = UIGestureRecognizerStateFailed;
}

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

    if (!_wasHorizontal)
        self.state = UIGestureRecognizerStateFailed;
}

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

    if (!_wasHorizontal)
        self.state = UIGestureRecognizerStateFailed;
}

- (CGPoint)locationInView:(UIView *)view
{
    CGPoint superLocation = [super locationInView:view];

    if (_hasConfirmedDirection)
        superLocation.y = _origLoc.y;

    return superLocation;
}

- (CGPoint)translationInView:(UIView *)view
{
    CGPoint superTranslation = [super translationInView:view];

    if (_hasConfirmedDirection)
        superTranslation.y = 0.0f;

    return superTranslation;
}

@end

And then you can have your handler in your main view controller use it appropriately (in this example, just dragging a UILabel around). In in viewDidLoad, create the gesture:

HorizontalPanGestureRecognizer *recognizer = [[HorizontalPanGestureRecognizer alloc] initWithTarget:self action:@selector(handleCustomPan:)];
[self.view addGestureRecognizer:recognizer];

And then the handler would look something like:

- (void)handleCustomPan:(UIPanGestureRecognizer *)sender 
{    
    switch (sender.state) {
        case UIGestureRecognizerStateChanged:
            if (!_panLabel)
            {
                // In my case I'm creating a UILabel to drag around, whereas you might just drag
                // whatever countrol you want to drag.
                //
                // But, regardless, I'm keeping track of the original center in _panLabelOrigCenter

                [self makePanLabel:sender]; 
            }

            CGPoint translate = [sender translationInView:self.view];
            _panLabel.center = CGPointMake(_panLabelOrigCenter.x + translate.x, _panLabelOrigCenter.y + translate.y);

            break;

        case UIGestureRecognizerStateEnded:
            [self removePanLabel];
            break;

        default:
            break;
    }
}

(Obviously, this is ARC code. If non-ARC, add the necessary additional lines of code for routine memory management.)

Answer 2

That failure condition in touches moved looks like it will be super sensitive to early motion.

Consider making it less sensitive ...

- (BOOL)movementIsHorizontalX:(CGFloat)x andY:(CGFloat) y {

    CGFloat absX = fabs(x);
    CGFloat absY = fabs(y);

    // if x is small, then accept small y values but not large ones
    if (absX < 4) return absY < 4;

    // now we can express horizontal-ness as a slope.  we've ruled out small x, so this is a stable calculation
    CGFloat slope = absY / absX;
    return slope < 0.10;

    // you could generalize by passing in the x threshold and percentage slope constants
}

Answer 3

I realized my error. I moved

    [super touchesMoved: touches withEvent:event];  

outside the if statement, and now it seems to work fine. Not really 100% sure why it works now, but I am happy it does.

Now to figure out how to prevent the UIScrollView from accepting the pan gesture until this one cancels/fails...