How to use ffmpeg for muxing steam into mp4 file and saving to iOS photo galllery

Question

I am trying to mux a live stream into a mp4 file by using FFmpeg on iOS. After the muxing, how should I save this mp4 file into my iOS photo gallery?

My mp4 file path is something like this: file:///var/mobile/Containers/Data/Application/560F1377-D16D-4322-8CBC-C6F521BA326D/Documents/temp.mp4

I use: videoAtPathIsCompatibleWithSavedPhotosAlbum to make sure this is the right format file that can be saved into iOS photo gallery, but it's always return No.

Any comment will be appreciated!