backends: Add BackendListener interface with playlists_loaded() event

docs/api/backends.rst

@@ -33,6 +33,13 @@ Library provider
     :members:
 
 
+Backend listener
+================
+
+.. autoclass:: mopidy.backends.listener.BackendListener
+    :members:
+
+
 .. _backend-implementations:
 
 Backend implementations

mopidy/backends/listener.py

@@ -0,0 +1,32 @@
+from __future__ import unicode_literals
+
+import pykka
+
+
+class BackendListener(object):
+    """
+    Marker interface for recipients of events sent by the backend actors.
+
+    Any Pykka actor that mixes in this class will receive calls to the methods
+    defined here when the corresponding events happen in the core actor. This
+    interface is used both for looking up what actors to notify of the events,
+    and for providing default implementations for those listeners that are not
+    interested in all events.
+
+    Normally, only the Core actor should mix in this class.
+    """
+
+    @staticmethod
+    def send(event, **kwargs):
+        """Helper to allow calling of backend listener events"""
+        listeners = pykka.ActorRegistry.get_by_class(BackendListener)
+        for listener in listeners:
+            getattr(listener.proxy(), event)(**kwargs)
+
+    def playlists_loaded(self):
+        """
+        Called when playlists are loaded or refreshed.
+
+        *MAY* be implemented by actor.
+        """
+        pass

tests/backends/listener_test.py

@@ -0,0 +1,13 @@
+from __future__ import unicode_literals
+
+from mopidy.backends.listener import BackendListener
+
+from tests import unittest
+
+
+class CoreListenerTest(unittest.TestCase):
+    def setUp(self):
+        self.listener = BackendListener()
+
+    def test_listener_has_default_impl_for_playlists_loaded(self):
+        self.listener.playlists_loaded()