如何使用AVPlayer设置低质量或高质量视频流？我的代码如下:self.videoPlayerItem=[AVPlayerItemplayerItemWithURL:videoUrl];videoPlayer=[AVPlayerplayerWithPlayerItem:self.videoPlayerItem];videoPlayer.actionAtItemEnd=AVPlayerActionAtItemEndNone;CGRectframe=self.frame;videoPlayerLayer=[AVPlayerLayerlayer];[videoPlayerLayersetP