pub unsafe static AVTrackAssociationTypeForcedSubtitlesOnly: &'static AVTrackAssociationTypeAVAssetTrack only.Expand description
Indicates an association between a subtitle track typically containing both forced and non-forced subtitles with another subtitle track that contains only forced subtitles, for use when the user indicates that only essential subtitles should be displayed. When such an association is established, the forced subtitles in both tracks are expected to present the same content in the same language but may have different timing.
Associations of type AVTrackAssociationTypeForcedSubtitlesOnly are supported only between subtitle tracks. This association is not symmetric; when used with -[AVAssetWriterInput addTrackAssociationWithTrackOfInput:type:], the receiver should be an instance of AVAssetWriterInput with a corresponding subtitle track that contains non-forced subtitles, and the input parameter should be an instance of AVAssetWriterInput with a corresponding subtitle track that contains forced subtitles only.
See also Apple’s documentation