Is there a way to switch between multiple video tracks in an AVMutableComposition? I want to assign a different preferredTransform to each, and switch between them.
I tried it by assigning alternating assets to an A track and a B track, but no video showed for the B track time.
It looks like to perform the switching between multiple video tracks ("video downmixing") you need to use AVAssetWriter. According to the docs the class has "automatic interleaving of media data for multiple concurrent tracks".
So, if you want to preview a composition using multiple video tracks, you either need to write it out using AVAssetWriter or compose it in an AVMutableComposition using a single video track only.