added app.register_listener method w/test

sanic/app.py

@@ -117,6 +117,21 @@ class Sanic:
 
         return decorator
 
+    def register_listener(self, listener, event):
+        """
+        Register the listener for a given event.
+
+        Args:
+            listener: callable i.e. setup_db(app, loop)
+
+            event: when to register listener i.e. 'before_server_start'
+
+        Returns: listener
+
+        """
+
+        return self.listener(event)(listener)
+
     # Decorator
     def route(self, uri, methods=frozenset({'GET'}), host=None,
               strict_slashes=None, stream=False, version=None, name=None):

tests/test_server_events.py

@@ -49,6 +49,23 @@ def test_single_listener(listener_name):
     assert random_name_app.name + listener_name == output.pop()
 
 
+@pytest.mark.parametrize('listener_name', AVAILABLE_LISTENERS)
+def test_register_listener(listener_name):
+    """
+    Test that listeners on their own work with
+    app.register_listener method
+    """
+    random_name_app = Sanic(''.join(
+        [choice(ascii_letters) for _ in range(choice(range(5, 10)))]))
+    output = list()
+    # Register listener
+    listener = create_listener(listener_name, output)
+    random_name_app.register_listener(listener,
+                                      event=listener_name)
+    start_stop_app(random_name_app)
+    assert random_name_app.name + listener_name == output.pop()
+
+
 def test_all_listeners():
     random_name_app = Sanic(''.join(
         [choice(ascii_letters) for _ in range(choice(range(5, 10)))]))