Trigger callbacks for ``attr`` on this object.

        Args:
            attr (str) :
            old (object) :
            new (object) :

        Returns:
            None

def trigger(self, attr, old, new, hint=None, setter=None):
        ''' Trigger callbacks for ``attr`` on this object.

        Args:
            attr (str) :
            old (object) :
            new (object) :

        Returns:
            None

        '''
        def invoke():
            callbacks = self._callbacks.get(attr)
            if callbacks:
                for callback in callbacks:
                    callback(attr, old, new)
        if hasattr(self, '_document') and self._document is not None:
            self._document._notify_change(self, attr, old, new, hint, setter, invoke)
        else:
            invoke()