Resize UICollectionView while scrolling - iphone

Question

My collection view begins life short, at the bottom of it's view controller's view. When the user scrolls up a little bit, I'd like the collection to fill the view. I'd like it to shrink again when the user pulls down into the bounce area.
Here's the code that I'd like to make work (and I think should work):
- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
if (scrollView == self.collectionView) {
CGFloat offsetY = self.collectionView.contentOffset.y;
if (offsetY > 20.0) {
[self setCollectionViewExpanded:YES];
} else if (offsetY < -10.0) {
[self setCollectionViewExpanded:NO];
}
}
}
- (void)setCollectionViewExpanded:(BOOL)expanded {
// avoid starting a do-nothing animation
BOOL currentlyExpanded = self.collectionView.frame.origin.y == 0.0;
if (expanded == currentlyExpanded) return;
// note: the collection view is the uppermost subview of the vc's view
// expanding to view bounds makes it cover every other view
CGRect newFrame = (expanded)? self.view.bounds : CGRectMake(0.0, 240.0, 320.0, self.view.bounds.size.height-240.0);
[UIView animateWithDuration:0.3 animations:^{
self.collectionView.frame = newFrame;
}];
}
But after the expand happens, the cells remain in the same position relative to the parent view (still near the bottom) and the collection view stops responding to panning touches and sends no further didScroll notification.
I can get the content repositioned properly by doing a reloadData in an animation completion block, but the scrolling touches stop working either way.
A while back, I tried something like this with a table view (or another kind of scroll view) and ran into a similar hangup. Much obliged if anyone has this solved...

Answer 1

Can you describe your view hierarchy a bit better? Is the UICollectionView a subview of the UIScrollView?
Is self.view a UIScrollView?
In that case, the bounds origin is the scrollview's contentOffset (not 0,0). That could give your UICollectionView an unexpected frame.
If the UICollectionView is a subview of the scrollview, the frame origin is the origin of the view in the scrollview-space. So don't change the frame's origin (just its size) and set the contentOffset of the scrollView to make it visible.