UIImageView stops resizing when fast scrolling a UIScrollView - iphone

Question

So i got an imageView inside a ScrollView that should resize, which works the way i want (see a small video here: https://dl.dropbox.com/u/80699/scroll.m4v)
what i did is setting up a UIScrollViewDelegate and using the scrollViewDidScroll method to resize my image based on the scrolling offset
- (void)scrollViewDidScroll:(UIScrollView *)aScrollView
{
CGFloat scrollViewOffset = aScrollView.contentOffset.y;
if(scrollViewOffset < 0.0f) {
// postition top
CGRect imageViewRect = self.imageView.frame;
imageViewRect.origin.y = scrollViewOffset;
if(scrollViewOffset < 0.0f && scrollViewOffset >= -50.0f) {
CGFloat newBackdropHeight = kImageHeight - scrollViewOffset;
imageViewRect.size.height = newBackdropHeight;
}
self.imageView.frame = imageViewRect;
}
}
whats basically happening here is, that if the user is scrolling upwards when he is on the top (bounces enabled) the imageView expands with the scroll until a certain amount of offset(here 60px).
the problem with this is, that if i scroll very fast, the image stops resizing, but the rest of the scrollview scrolls fast down like it would normally do. then, when the scrollview snaps back, the image expands immediately and then scales down like it should (see video here: https://dl.dropbox.com/u/80699/scroll2.m4v).
with this behavior, the user-expierience is not very nice and the user sees a jumping image.
does anybody know how i could fix this?
here is a small sample project if you want to see the behavior yourself: https://dl.dropbox.com/u/80699/scroll.zip
thanks for your help!
if anything is unclear, PLEASE leave a comment

Answer 1

I had a quick look at your test project, and I believe the issue is that when you scroll quickly, by the time the callback is called the Y offset is greater than -50, so the image view is not resized.
I solved this by removing the inner if condition and giving the backdrop a maximum height:
if(scrollViewOffset < 0.0f) {
// postition top
CGRect imageViewRect = self.imageView.frame;
imageViewRect.origin.y = scrollViewOffset;
CGFloat newBackdropHeight = kImageHeight - MAX(scrollViewOffset,-50.0);
imageViewRect.size.height = newBackdropHeight;
self.imageView.frame = imageViewRect;
}
Hope that helps