WWDC 2013 Session 217 Exploring Scroll Views in iOS 7

Nested Scroll Views

At first, the session presents how to make nested scroll views working together by using a UICollectionView in UIScrollView, while each UICollectionViewCell has a UIScrollView as the subview. Meanwhile, the UICollectionView is the subclass of UIScrollView.

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
     determine delta beyond catch point
     adjust parent contentOffset by delta
     translate child by delta
}

Key points in this part:

• When subview scroll, using a delegate call to scroll parent scrollview(set contentOffset).
• When subview scroll, avoiding double scroll by transforming itself, transform itself back when scroll ends.
• When scrollViewWillEndDragging, adjust scrollview contentOffset change ratio between sub-scrollview and parent-scrollview.

The demo code has been published in the 1st release

Scrolling with UIKit Dynamics

In the second part, the session presents how to add special animation effect in the UICollectionView by customizing UICollectionFlowLayout with adding UIDynamicBehavior to UICollectionViewLayoutAttributes.

• Subclass UICollectionViewFlowLayout
• Create UIDynamicAnimator
• Create UIAttachmentBehavior for each item
• Stretch the attachments when scrolling

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds {
     CGFloat delta = newBounds.origin.y - self.collectionView.bounds.origin.y
     shift layout attribute positions by delta
     notify UIDynamicAnimator
}

The demo code has been published in the 2nd release

Hope someone could help me make a gif file and add it here for the better demo purpose.

Version 3: Memory Efficient Code

In the previous version, when prepareLayout is called, all the behaviors of every cells will be created, initialized and attached to the cell. In this version, the behaviors will be added to the cell when the cell appear on the screen, be removed when the cell disappear from the screen, very effiecient. The release code could be found here.