This is needed as otherwise each and every backend needs to handle the fact
that some "bad" clients might send {'field': 'value'} instead of
{'field': ['value']} Though the real problem isn't the clients but our
organically grown query API.
This change has us checking the return value of change_track when deciding if
the play call was a success or if the track is unplayable. Which ensures that
the following can no longer happen: 1) play stream 2) play stream that fails
change_track 3) stream 1) continues playing. Correct behavior being the next
stream playing instead.
change_track(track) simply calls translate_uri(uri) by default now so that 90%
of clients with custom URIs should be able to just implement this one method on
the backend any ignore everything else.
All uris given to lookup should be in the result even if there is no
backend to handle the uri, and the lookup result thus is an empty list.
As a side effect, future.get() is now called in the order of the URIs
in the `uris` list, making it easier to mock out
backend.library.lookup() in core layer tests.
While trying to remove traces of stop calls in core to get gapless working I
found we had no way to switch to switch tracks without triggering a play. This
change fixes this by changing the backends playback provider API.
- play() now _only_ starts playback and does not take any arguments.
- prepare_change() has been added, this could have been avoided with a kwarg to
change_track(track), but that would break more backends.
- core has been updated to call prepare_change+change_track+play as needed.
- tests have been updated to handle this change.
Longer term I hope to completely rework the playback API in backends, as 99% of
our backends only use change_track(track) to translate URIs. So we should make
simple case simple, and handle mopidy-spotify / appsrc in some other way.
Cherry picked from the WIP gapless branch.
Instead of changing the signature to add(uri, name) I opted for
renaming it to _add_track(track).
Since it's internal we may change it whenever we like to. Since you need
different logic for extracting an interesting name from a track and from
a ref or a stream title, it makes sense to add another method for adding
refs/stream titles to the history when that time comes.
Fixes#1056
