问题
According to Firebase docs:
ChildAdded is triggered once for each existing child and then again every time a new child is added to the specified path
So, I have an app, that has a little banner at the top that pops up every time a user gets a new message. As you could have guessed, these messages are stored in a child in the user object in Firebase. So, here's the problem, when I load the app, it pops up for EVERY message the user has. Is it possible to have this observe event ONLY be called when a new child is added? I don't want it to be triggered for every single existing child, only when a new one is added. I would hate to have to store message references in core data, and do a check for every child to see if it already exists in core data :/
回答1:
A few ways to do this:
- Use ref.queryLimitedToLast(1) when you start. See Firebase, only get new children.
- keep track of the most recent key you've got in your local storage and then query with queryOrderByKey()
.queryStartingAtValue(latestKey) - Add a timestamp to the items and then queryOrderByChild("timestamp")
.queryStartingAt(now)for items with timestamp >= now. See how to discard initial data in a Firebase DB
来源：https://stackoverflow.com/questions/39584091/firebase-observe-with-type-childadded-retrieves-all-my-information-every-time