Unable to setContentOffset in scrollViewDidEndDragging

Question

I am attempting to modify the contentOffset of my scroll within the following delegate method:

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate

I have tried both of the following:

[UIView animateWithDuration:.2 animations:^ {
   [scrollView setContentOffset:CGPointZero animated:NO];
}];

and

[UIView animateWithDuration:.2 animations:^ {
   CGRect svBounds = self.bounds;
   svBounds.origin.y = 0;
   self.bounds = svBounds; 
}];

The problem is that although this changes the offset right away (proven by logs after the animation is complete), it doesn't change the visible scroll location. This is further proven by subsequent delegate methods of my scroll view which indicate that the bounds have indeed not changed at all. Meaning that the y location is not 0.

Is it forbidden to change the content offset in this particular delegate method? If so, when is it possible for me to change the offset? I'm trying to perform an animation of returning the visible scroll area to the top once the user finishes dragging (after a certain amount).

Thanks!

I was able to get this to work in scrollViewWillBeginDecelerating. This wasn't working in both scrollViewDidEndDragging and scrollViewWillEndDragging. Perhaps because they are still accelerating at the time those methods are called? I am not sure...I don't know why this is though, perhaps someone else can shed light. — AbuZubair

tomeron11 tomeron11 · Accepted Answer · 2014-11-23T12:19:42

what i did eventually is dispatching again to the main queue. meaning:

-(void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {
NSLog(@"END DRAG");
CGFloat yVelocity = [scrollView.panGestureRecognizer velocityInView:scrollView].y;
if (yVelocity < 0) {
    NSLog(@"Up");
} else {
    NSLog(@"Down");
}

if (yVelocity < 0 && scrollView.contentOffset.y < -100 && scrollView.contentOffset.y > -200) //UP - Hide
{
    NSLog(@"hiding");
    dispatch_async(dispatch_get_main_queue(), ^{
        [scrollView setContentOffset:CGPointMake(0, 0) animated:YES];
    });
}
else if (yVelocity > 0 && scrollView.contentOffset.y < -100) //DOWN - Show
{
    NSLog(@"showing");
    dispatch_async(dispatch_get_main_queue(), ^{
        [scrollView setContentOffset:CGPointMake(0, -300) animated:YES];
    });
}

}

and it actually works :-)

Unable to setContentOffset in scrollViewDidEndDragging

3 Answers