It is a demo for different kinds of customized collectionViewLayout.
CollectionViewLayout customization is an interesting thing. If you look carefully at the layout, you might have two questions about the layout. First, what is the contentSize of the whole layout ? Second, what does the cell look like ? (The position of the cell ? Is the cell rotated ?) for each different indexPath.
In order to answer the questions, UICollectionViewLayout provides three important methods for us to customize the layout.
We describe the whole layout by creating an array of UICollectionViewLayoutAttributes's instance in prepareLayout. UICollectionViewLayoutAttributes is used to describe what the cell looks like.
The layout provides an array of UICollectionViewLayoutAttributes's instance in the specific area (Part of layoutAttributes we created in prepareLayout) to collectionView that we scrolled by layoutAttributesForElementsInRect.
Basically, the contentSize of layout is calculated in prepareLayout.
