def callWhenActivated(self, callback):
        """Externally register for notification when this publishable has received all relevant data.
        """
        if self.isActivated:
            callback(self)
        else:
            self._activationListeners.append(callback)
Externally register for notification when this publishable has received all relevant data.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/spread/publish.py