AVCompositionTrack object provides the low-level representation of tracks a track in an
AVComposition object, comprising a media type, a track identifier, and an array of
AVCompositionTrackSegment objects, each comprising a URL, and track identifier, and a time mapping.
timeMapping.target.start of the first track segment in a composition track is
kCMTimeZero, and the
timeMapping.target.start of each subsequent track segment equals
The AVFoundation framework also provides a mutable subclass,