The amount of content that has been played for the media item.
Mac Catalyst 13.0+
This property uses the values from 0.0 to 1.0. The value 0.0 indicates that the media item has not been viewed, while 1.0 indicates that the media item has been completely played. This property’s default, –1.0 indicates that no progress indicator is shown. The progress indicator is shown automatically if the playbackProgress property has a valid value and the app responds affirmatively to the childItemsDisplayPlaybackProgressAtIndexPath: method.