How to make a UIScrollView snap to icons (like App Store: Feature)

Question

What I want to get is the same behaviour that this scroll view has:

Answer 1

You can either turn on pagingEnabled, or use decelerationRate = UIScrollViewDecelerationRateFast combined with scrollViewDidEndDragging and scrollViewDidEndDecelerating (which will minimize the effect of slowing scrolling to one location and then animating again to another location). It's probably not precisely what you want, but it's pretty close. And by using animateWithDuration it avoids the instantaneous jumping of that final snap.

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
{
    if (!decelerate)
        [self snapScrollView:scrollView];
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
    [self snapScrollView:scrollView];
}

You can then write a snapScrollView, such as:

- (void)snapScrollView:(UIScrollView *)scrollView
{
    CGPoint offset = scrollView.contentOffset;

    if ((offset.x + scrollView.frame.size.width) >= scrollView.contentSize.width)
    {
        // no snap needed ... we're at the end of the scrollview
        return;
    }

    // calculate where you want it to snap to

    offset.x = floorf(offset.x / kIconOffset + 0.5) * kIconOffset;

    // now snap it to there

    [UIView animateWithDuration:0.1
                     animations:^{
                         scrollView.contentOffset = offset;
                     }];
}