OptionalerrorWhy this row would not play, once it has failed. Kept on the row rather than only in
musicError so the reason survives moving on to the next track — a list where the bad
row still says what is wrong with it is a list a person can act on.
The playlist to play through, when the pasted link named one.
What the player reported for this row, once it has played it. null until then, and
for a playlist row always — see MusicNowPlaying.title for what a playlist is on.
The video to play, or null for a playlist opened at its first entry.
One row of the queue.
keyrather than the video id, because the same track queued twice is two rows a person can reorder past or remove independently, and because a playlist entry has no video id at all. Every reference the queue holds internally — the history stack, the shuffle cycle — is a key for that reason.