listener: Speed up event emitting and improve error reporting

(cherry picked from commit 655b7badf4)

mopidy/listener.py

@@ -17,7 +17,21 @@ def send(cls, event, **kwargs):
     listeners = pykka.ActorRegistry.get_by_class(cls)
     logger.debug('Sending %s to %s: %s', event, cls.__name__, kwargs)
     for listener in listeners:
-        listener.proxy().on_event(event, **kwargs)
+        # Save time by calling methods on Pykka actor without creating a
+        # throwaway actor proxy.
+        #
+        # Because we use `.tell()` there is no return channel for any errors,
+        # so Pykka logs them immediately. The alternative would be to use
+        # `.ask()` and `.get()` the returned futures to block for the listeners
+        # to react and return their exceptions to us. Since emitting events in
+        # practise is making calls upwards in the stack, blocking here would
+        # quickly deadlock.
+        listener.tell({
+            'command': 'pykka_call',
+            'attr_path': ('on_event',),
+            'args': (event,),
+            'kwargs': kwargs,
+        })
 
 
 class Listener(object):