Changes to UIScrollView contentOffset don't stick when decelerating

Question

I have looked at UIScrollView. Any thoughts on implementing "infinite" scroll/zoom?, but it didn't exactly address my issue. I'm puzzled, though, because I would guess that other folks haven't seen my problem, so maybe it has to do with the details of my code. The sample code provided by Allisone is similar to mine, although I use scrollViewDidScroll to monitor the contentOffset instead of custom KVO approach.

When my UIScrollView is decelerating, if I insert a subview to the left of the UIScrollView's bounds and increase contentOffset by the width of my inserted subview, my change to contentOffset is ignored; the content "jumps" by the width of the inserted subview, and the contentOffset value stream continues on its current trajectory in subsequent invocations of scrollViewDidScroll. While simply tracking, there is no problem. Is there something I'm likely to be doing wrong? It's almost as if the bounds or contentOffset is sticky with regard to the deceleration events.

Use case: the UIScrollView has very large virtual content that is dynamically paged in and out, and when the user is smoothly scrolling through content in the UIScrollView (toward the left, say), additional content should be inserted on the far left of the scrollview without disrupting the smooth scrolling that is currently going on. As I said, this works fine if deceleration is disabled or I rely on dragging rather than flicking.

At first I hoped that the problem was caused by changing the contents of the UIScrollView from within the callout to scrollViewDidScroll, so I double-checked by doing a delayed performSelector, but the problem remained.

Thanks, Kevin

Answer 1

Even though not exactly what you had in mind, but using setContentOffset:animated will have the desired outcome ( alter the content offset to the right coordinates ) although it will be animated.

Answer 2

Faced up with the same issue. Seems like strange UIScrollView bug. I fixed it on the base of StreetScroller example from Apple. InfiniteScrollView inherits from UIScrollView and works good in there. But if you want to create custom class which uses UIScrollView inside itself, you can subclass this UIScrollView and call delegate when it needs to recenter content like this:

tScrollView : UIScrollView

In tScrollView.m:

- (void)recenterIfNecessary {
    if (_tDelegate && [_tDelegate respondsToSelector:@selector(offsetForRecenterScrollView:)])
        self.contentOffset = [_tDelegate offsetForRecenterScrollView:self];
}
- (void)layoutSubviews {
    [super layoutSubviews];
    [self recenterIfNecessary];
}

Implement offsetForRecenterScrollView: in delegate and return new CGPoint to set in scrollView.contentOffset.