I have looked at http://stackoverflow.com/questions/1493950/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