'add_listener(callback, mask=EVENT_ALL)
Adds a listener for scheduler events. When a matching event occurs, ``callback`` is executed with the event
object as its sole argument. If the ``mask`` parameter is not provided, the callback will receive events of all
types.
:param callback: any callable that takes one argument
:param int mask: bitmask that indicates which events should be listened to
.. seealso:: :mod:`apscheduler.events`
.. seealso:: :ref:`scheduler-events`'

def add_listener(self, callback, mask=EVENT_ALL):

    with self._listeners_lock:
        self._listeners.append((callback, mask))
