Downloaded is when the song is downloaded to the disk.

Downloading is when the song is downloading.

Failed is when the song failed to download, will be retried.

Initial is the initial state of every song.

NotAvailable means the song cannot be cached by now.

Pending is when the song is waiting for the download to start, either because previous songs are still downloading or it's queue-jumped by a higher priority song.

Removed is when the song is removed from the playlist.

Requesting is when the song is requesting the download.

StateMachine is the state machine for a song.