UIRefreshControl on UICollectionView only works if the collection fills the height of the container

http://stackoverflow.com/questions/14678727/uirefreshcontrol-on-uicollectionview-only-works-if-the-collection-fills-the-heig

UICollectionView 的下拉刷新

默认情况下 UICollectionView 在内容不满是，是无法下拉刷新的

所以要设置

self.collectionView.alwaysBounceVertical = YES;