AirPlay：连接到Apple TV时，音量控制被禁用

After discussion with a DTS Engineer, he found a workaround (rdar://42881405 Volume control is disabled when connected to Apple TV using AirPlay).

"According to engineering, the disabling of the volume control is correct behavior for certain Apple TV configurations, where the audio is being sent to the actual TV via HDMI. In that case, volume is controlled by the TV itself.

An alteration to this standard behavior is made for audio-only apps (such as Podcasts and Overcast). In those cases, the volume control is enabled anyway, and it provides a software volume adjustment of the audio in addition to the hardware volume control. The reason you weren’t getting this is that you used AVQueuePlayer, which is regarded as a video player, not a pure audio player. I modified your sample project to use AVAudioPlayer instead, and the volume control was enabled for AirPlay output as expected.

但是，AVAudioPlayer无法播放流式资产，因此在您的用例中可能不是可行的解决方案。我仍在研究是否可以通过其他播放技术获得纯音频行为。”

解：

基本上，将allowsExternalPlaybackAVPlayer / AVQueuePlayer的属性设置为false会禁止将视频播放路由到AirPlay，并且（作为副作用）允许纯音频播放。

最后说明：

即使这样，我认为使用newAVSampleBufferAudioRenderer和AVSampleBufferRenderSynchronizerclass也可以，但是设置起来要复杂得多。

已创建的rdar：// 42966681：提供用于使用AVPlayer / AVQueuePlayer将应用程序指定为“仅音频”的API。