audio: Add state_changed(old_state, new_state) event

mopidy/audio/listener.py

@@ -28,3 +28,18 @@ class AudioListener(object):
         *MAY* be implemented by actor.
         """
         pass
+
+    def state_changed(self, old_state, new_state):
+        """
+        Called after the playback state have changed.
+
+        Will be called for both immediate and async state changes in GStreamer.
+
+        *MAY* be implemented by actor.
+
+        :param old_state: the state before the change
+        :type old_state: string from :class:`mopidy.core.PlaybackState` field
+        :param new_state: the state after the change
+        :type new_state: string from :class:`mopidy.core.PlaybackState` field
+        """
+        pass

tests/audio/listener_test.py

@@ -0,0 +1,16 @@
+from __future__ import unicode_literals
+
+from mopidy import audio
+
+from tests import unittest
+
+
+class AudioListenerTest(unittest.TestCase):
+    def setUp(self):
+        self.listener = audio.AudioListener()
+
+    def test_listener_has_default_impl_for_reached_end_of_stream(self):
+        self.listener.reached_end_of_stream()
+
+    def test_listener_has_default_impl_for_state_changed(self):
+        self.listener.state_changed(None, None)