'Called whenever playback state is changed.
*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'

def playback_state_changed(self, old_state, new_state):

    pass
