iOS 关于mutableComposition的旋转属性

当使用AVMutableComposition进行谱曲，那么直接对视频轨道使用preferredTransform属性即可。如果是对整个谱曲进行设置，则通过AVMutableVideoComposition给exportSession进行全局设置。

AVMutableComposition

If you are using an AVMutableComposition to edit your visual assets, you can set the preferredTransform property on any of the video tracks (AVMutableCompositionTrack).

AVMutableVideoComposition

If you are using an AVMutableVideoComposition to edit the video tracks in a video composition, use an AVMutableVideoCompositionLayerInstructionobject to modify the transform to apply to a given track in the video composition. Use the setTransform:atTime: method to set a value of the transform at a time within the time range of the instruction, or use the setTransformRampFromStartTransform:toEndTransform:timeRange: method to set a transform ramp to apply during a given time range.

https://developer.apple.com/library/archive/qa/qa1744/_index.html