@class AVPlayer
@abstract
AVPlayer爲single-item playback提供了一個playback接口。
@discussion
1.AVPlayer對本地和遠端的媒體文件都運行的非常好，爲多個客戶提供合關於快速播放或者需要等待額外的數據才能繼續進行的合適信息。
2.與視覺相關的items將給一個AVPlayer實例展現在一個CoreAnimation layer of class AVPlayerLayer.
3.To allow clients to add and remove their objects as key-value observers safely, AVPlayer serializes notifications of
changes that occur dynamically during playback on a dispatch queue. By default, this queue is the main queue. See dispatch_get_main_queue().
To ensure safe access to AVPlayer's nonatomic properties while dynamic changes in playback state may be reported, clients must
serialize their access with the receiver's notification queue. In the common case, such serialization is naturally achieved
by invoking AVPlayer's various methods on the main thread or queue.