Cancel UIScrollView bounce after dragging

问题:

I have a horizontal UIScrollView. I want to do a variation of the "pull-to-reset" animation, where I pull all the way past the right edge of the scroll view's content size, release my finger, and have the scroll view fly back to (0, 0) content offset.

My delegate method looks like this:

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {
    //check if it exceeds a certain critical value
    if (scrollView.contentOffset.x - (scrollView.contentSize.width - IMAGE_WIDTH) > 80) {
        [self doAnimatedScrollTo:CGPointMake(0, 0)];
    }
}

where doAnimatedScrollTo: is a custom animation method necessary because I want to control the duration of the animation.

While this works, it seems that the animation is queued up. The UIScrollView "bounce" animation happens first, then my animation occurs.

Is there a way to cancel the bounce animation, keep the content offset from "snapping" back, and then perform my animation?

回答1:

try this

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {
  //check if it exceeds a certain critical value
  if (scrollView.contentOffset.x - (scrollView.contentSize.width - IMAGE_WIDTH) > 80) {
    [scrollView setContentOffset:CGPointMake(0, 0) animated:YES];
  }
}

回答2:

I accomplished cancelling the bounce back animation of a UIScrollView.

I wanted to leave the default behaviour during a rapid scroll to the top when it bounces. However if scrollview is already at the top and then the user pulls it down and releases (analogous to pull to refresh) I wanted to take the control over the bounce back and do something custom.

In scrollview delegate I track the initial position:

- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
{
    if (scrollView.contentOffset.y < 0.1)
    {
        isPullingTop = YES;
    }
}

In scrollview delegate detect if the flag is set and scrollview is dragged enough

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
{
    if (isPullingTop && scrollView.contentOffset.y < -30) {

        overrideBounce = YES;
    }
    isPullingTop = NO;
}

I subclass scrollview and override the setContentOffset:

-(void)setContentOffset:(CGPoint)contentOffset
{
    if (!overrideBounce)
    {
        [super setContentOffset:contentOffset];
    }
    else
    {
        //customs stuff goes here , for example an animation
        overrideBounce = NO;
    }
}