Add change track to playback providers.

This method is intended to make implementing EOT handling much more streamlined
by adding a way to easily just change the URI and any other state without
messing with gstreamer pipe states

Naming of this is a bit unfortunate as it is not the same as the
core.playback's concept of change_track. At least not yet

mopidy/backends/base.py

@@ -88,8 +88,8 @@ class BaseLibraryProvider(object):
 
 class BasePlaybackProvider(object):
     """
-    :param audio: the audio actor
-    :type audio: actor proxy to an instance of :class:`mopidy.audio.Audio`
+    :param audio: audio sub-system
+    :type audio: actor proxy to a :class:`mopidy.audio.Audio` actor.
     :param backend: the backend
     :type backend: :class:`mopidy.backends.base.Backend`
     """
@@ -121,9 +121,24 @@ class BasePlaybackProvider(object):
         :rtype: :class:`True` if successful, else :class:`False`
         """
         self.audio.prepare_change()
-        self.audio.set_uri(track.uri).get()
+        self.change_track(track)
         return self.audio.start_playback().get()
 
+    def change_track(self, track):
+        """
+        Swith to provided track.
+
+        Used for handling of EOT and and in :meth:`play`.
+
+        *MAY be reimplemented by subclass.*
+
+        :param track: the track to play
+        :type track: :class:`mopidy.models.Track`
+        :rtype: :class:`True` if successful, else :class:`False`
+        """
+        self.audio.set_uri(track.uri).get()
+        return True
+
     def resume(self):
         """
         Resume playback at the same time position playback was paused.

mopidy/backends/spotify/playback.py

@@ -23,26 +23,18 @@ class SpotifyPlaybackProvider(base.BasePlaybackProvider):
 
         return super(SpotifyPlaybackProvider, self).pause()
 
-    def play(self, track):
-        if track.uri is None:
-            return False
-
+    def change_track(self, track):
+        self.audio.set_uri('appsrc://').get()
+        self.audio.set_metadata(track).get()
         try:
             self.backend.spotify.session.load(
                 Link.from_string(track.uri).as_track())
             self.backend.spotify.session.play(1)
-
-            self.audio.prepare_change()
-            self.audio.set_uri('appsrc://')
-            self.audio.start_playback()
-            self.audio.set_metadata(track)
-
-            self._timer.play()
-
-            return True
         except SpotifyError as e:
             logger.info('Playback of %s failed: %s', track.uri, e)
             return False
+        self._timer.play()
+        return True
 
     def resume(self):
         time_position = self.get_time_position()