__bridge转换Objective-C 和 Core Foundation 指针,不移交持有权.
__bridge_retained或CFBridgingRetain转换 Objective-C 指针到Core Foundation 指针并移交持有权.
你要负责调用CFRelease或一个相关的函数来释放对象.
__bridge_transfer或CFBridgingRelease传递一个非Objective-C 指针到 Objective-C 指针并移交持有权给ARC.
ARC负责释放对象.
http://blog.csdn.net/junjun150013652/article/details/53506504