RT，我正在 subclassing UICollectionViewLayout，请问要重写哪些方法或者如何做才能设置 item 之间的间隔样式，比如我想在 item 之间加一条线？
重写layoutAttributesForDecorationViewOfKind:atIndexPath:
StackOverflow 上有一个相关的问题
相关资料：
- https://developer.apple.com/library/ios/documentation/WindowsViews/Conceptual/CollectionViewPGforIOS/CreatingCustomLayouts/CreatingCustomLayouts.html
- http://objccn.io/issue-3-3/
- https://developer.apple.com/library/ios/documentation/WindowsViews/Conceptual/CollectionViewPGforIOS/AWorkedExample/AWorkedExample.html#//apple_ref/doc/uid/TP40012334-CH8-SW6