Firebase update on disconnect

问题

I have a node on firebase that lists all the players in the game. This list will update as and when new players join. And when the current user ( me ) disconnects, I would like to remove myself from the list.

As the list will change over time, at the moment I disconnect, I would like to update this list and update firebase.

This is the way I am thinking of doing it, but it doesn't work as .update doesnt accept a function. Only the object. But if I create the object beforehand, when .onDisconnect calls, it will not be the latest object... How should I go about doing this?

payload.onDisconnect().update( () => {
            const withoutMe = state.roomObj
            const index = withoutMe.players.indexOf( state.userObj.name )
            if ( index > -1 ) {
                withoutMe.players.splice( index, 1 )
            }
            return withoutMe
        })

回答1:

The onDisconnect handler was made for this use-case. But it requires that the data of the write operation is known at the time that you set the onDisconnect. If you think about it, this should make sense: since the onDisconnect happens after your client is disconnected, the data of the data of that write operation must be known before the disconnect.

It sounds like you're building a so-called presence system: a list that contains a node for each user that is currently online. The Firebase documentation has an example of such a presence system. The key difference from your approach is that it in the documentation each user only modifies their own node.

So: when the user comes online, they write a node for themselves. And then when they get disconnected, that node gets removed. Since all users write their node under the same parent, that parent will reflect the users that are online.

The actual implementation is a bit more involved since it deals with some edge cases too. So I recommend you check out the code in the documentation I linked, and use that as the basis for your own similar system.

来源：https://stackoverflow.com/questions/46710850/firebase-update-on-disconnect