onSnapshot() shared query results

[Wojciech Fiderek]

Hi everyone, I wonder if multiple onSnapshot listeners are sharing same data source (so database will not receive doubled read request).

Here is shortened example use case (when you want to have separate wrappers for onAdded and onRemoved):

firestoreDb.colection('colName').onSnapshot( (querySnap) => {
    querySnap.docChanges( handleAdded )
});
firestoreDb.colection('colName').onSnapshot( (querySnap) => {
    querySnap.docChanges( handleRemoved )
});

If yes then what are constraints where queries data results are not shared between multiple onSnapshot listeners (eg. using where conditons OR documentSnapshots that not using querySnapshot result)?

[Sam Stern]

Hi there,

Assuming those are on the same device, yes they share the same listener and the query will only happen once.  This is only the case because the queries are exactly the same.  If you have two different but potentially overlapping listeners, they will be totally separate read requests on the server.  They share the same offline cache, but that does not affect how the online reads are billed.

- Sam

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