code: def addCallback(cls, eventType, func, record=None, once=False):
          
          callbacks = cls.callbacks()
          callbacks.setdefault(eventType, [])
          callbacks[eventType].append((func, record, once))
docs: Adds a callback method to the class.  When an event of the given type is triggered, any registered
          callback will be executed.
  
          :param  eventType: <str>
          :param  func: <callable>