So here's how I think Music/iTunes works, and how I think iTunes handles the crazy-assed "Up Next" system and why it doesn't make sense to anyone.
When you press Play on any single track, that begins playing (duh). But also, Music/iTunes takes a look at the context in which you pressed Play, e.g. the alphabetical list of songs, whether you have Shuffle on, etc. and figures out what it might play next:
- Now Playing:
- Ain't Goin' to Goa
- Playing Next:
- Ain't No Love
- Ain't No Mountain High Enough
- Alejando
(from Songs, no shuffle)
If you pick another track, and hit play, it automatically clears the old Playing Next list and comes up with a new one. It would be annoying if it didn't do this.
Context includes Albums and Playlists, so if I pick a song within its album, the Playing Next reflects that:
- Now Playing:
- 22
- Playing Next:
- I Almost Do
- We Are Never Ever Getting Back Together
- Stay Stay
(from Album, no shuffle)
Here's the weird thing.
When you hit Play on an Album or Playlist, Music/iTunes doesn't queue that entire album, or entire playlist, it just Plays the first track! It relies on this Playing Next functionality to do the right thing:
- Now Playing:
- I Like It
- Playing Next:
- Fireball
- Heart Attack
- International Love
(from Playlist, no shuffle)
So where does "Add to Up Next" come in? This is another, separate, list. You can think of it as a manually created/curated queue. The weird thing is that unlike Play, it works not only per song, but also per album, or per playlist.
If playing the above, I then hit "Add to Up Next" for "22", I get this:
- Now Playing:
- I Like It
- Up Next:
- 22
- Playing Next:
- Fireball
- Heart Attack
- International Love
Once whatever you've added to Up Next finishes, it resumes whatever it had in mind to play next after the last thing you hit Play on.
Here's what's really weird...
If I hit "Play" (not "Add to Up Next") on another album or playlist, while there is something in Up Next, it has to actually ask me my intentions. If I tell it to "Keep Up Next" it actually pre-inserts the entire new album into Up Next, replaces the playing song, but also doesn't clear Playing Next - which is different to just "Add to Up Next" which would have post-appended it to it.
All this only really makes sense if you remember that this feature used to be called "iTunes DJ" and was a way for your friends to insert Rick Astley as the next song at any arbitrary point in your party's playlist.