媒体
- 获取视频duration为NaN
Wait until the status of the AVPlayerItem is AVPlayerItemStatusReadyToPlay
https://stackoverflow.com/questions/23874574/avplayer-item-get-a-nan-duration/31408035#31408035
UI
- CollectionView
问题：当ios collectionView 调用 [ cell setSelected ] 后 不能手动选中or取消选中了
https://stackoverflow.com/questions/15330844/uicollectionview-select-and-deselect-issue
solution：在 cellForItemAtIndexPath 方法中 同时调用如下两个方法，就ok了
cell.selected = YES;
[collectionView selectItemAtIndexPath:indexPath animated:YES scrollPosition:UICollectionViewScrollPositionNone];
