翻译：AVPlayerItemOutput

@class AVPlayerItemOutput

@abstract

AVPlayerItemOutput是一个封装了它的子类的通用API的抽象类。

@discussion

1.AVPlayerItemOutput实例允许你获得一个在AVPlayer播放过程中的AVAsset的一个独立的采样值。为了在当多个AVPlayerItemOutput应用在单一source时能优雅降档（graceful degradation），所有AVPlayerItemOutput子类只提供当前sample或者容易取得的future samples。所有在current sample之前的samples将被AVPlayerItemOutput自动丢弃。

2.你可以通过AVPlayerItem的下面两个方法来管理让一个AVPlayerItem作为source input与AVPlayerItemOutput之间的联系：（太长了好难捋顺语句…）
• addOutput:
• removeOutput:

3.当一个AVPlayerItemOutput与一个AVPlayerItem产生关联，在多个使能了的tracks中，samples会根据AVPlayer遵守的混合，合并或丢弃的规则来提供当做他们自己的渲染目的。（不会译直接上原文吧，samples are provided for a media type in accordance with the rules for mixing, composition, or exclusion that the AVPlayer honors among multiple enabled tracks of that media type for its own rendering purposes）。例如，video media会根据AVPlayerItem.videoComposition提供的指令来合并。

@class AVPlayerItemVideoOutput

@abstract

一个AVPlayerItemOutput的子类，把video images用CVPixelBuffers来“出售”？（vends）。

@discussion

最好是用一个AVPlayerItemVideoOutput连接一个CVDisplayLink or CADisplayLink的services，来确保与屏幕硬件刷新的同步。为了优化效率，这里可以有机会让services“静默”下来。例如当playback暂停或者在playback of empty edits的过程中。下面这个采样代码展示了在你使用AVPlayerItemVideoOutput时如何“静默”CVDisplayLink。

    (void)CVDisplayLinkCreateWithActiveCGDisplays( &myDisplayLink );    CVDisplayLinkSetOutputCallback( myDisplayLink, myDisplayCallback, self );    [myPlayerItem addOutput:myVideoOutput];    [myVideoOutput setDelegate:self queue:myDispatchQueue];    ...    static CVReturn myDisplayCallback ( CVDisplayLinkRef displayLink,                                         const CVTimeStamp *inNow,                                         const CVTimeStamp *inOutputTime,                                         CVOptionFlags flagsIn,                                         CVOptionFlags *flagsOut,                                         void *displayLinkContext )    {        MYObject *self = displayLinkContext;        CMTime outputItemTime = [[self myVideoOutput] itemTimeForCVTimeStamp:*inOutputTime];        if ( [[self myVideoOutput] hasNewPixelBufferForItemTime:outputItemTime] ) {            CVPixelBufferRef pixBuff = NULL;            CMTime presentationItemTime = kCMTimeZero;            self->myLastHostTime = inOutputTime->hostTime;            pixBuff = [[self myVideoOutput] copyPixelBufferForItemTime:outputItemTime itemTimeForDisplay:&presentationItemTime];            // Use pixBuff here            // presentationItemTime is the item time appropriate for            // the next screen refresh            CVBufferRelease( pixBuff );        }        else {            CMTime elapsedTime = CMClockMakeHostTimeFromSystemUnits( inNow->hostTime - self->myLastHostTime );            if ( CMTimeGetSeconds( elapsedTime ) > NON_QUIESCENT_PERIOD_IN_SECONDS ) {                [[self myVideoOutput] requestNotificationOfMediaDataChangeWithAdvanceInterval:MY_STARTUP_TIME_IN_SECONDS];                CVDisplayLinkStop( displayLink );            }        }        return kCVReturnSuccess;    }    - (void)outputMediaDataWillChange:(AVPlayerItemOutput *)output    {           // Start running again        myLastHostTime = CVGetCurrentHostTime();        CVDisplayLinkStart( myDisplayLink );    }