在Firebase iOS SDK中，每次删除子项时都会触发.childAdded。我怎么能阻止这个？

I am storing some data under a node called "notifications" within my Firebase database. When I add an event observer to "notifications" and set the event type to ".childAdded" the completion handler is called every time a child is added to the "notifications" node AND every time a child is deleted from the "notifications" node. First of all, I don't understand why this is happening as nothing is being added, only deleted. Is there some way to avoid this? I am storing some data under a node called "not