How to wait for multiple Firestore Streams to return Data in Flutter

Question

I want to query two Firestore collections, but only want to return the data of the collection which returns a result quicker then the other one.

My approach was to use 2 streams and to wait till one of them gives back data, which I then can return.

I used the onData Parameter, but the compiler never jumps in the onData Method even if there is data in the collections which matches the query.

    var someId;

    CollectionReference collection1 = _db.collection('collection1');
    CollectionReference collection2 = _db.collection('collection2');

    Query collection1Query = collection1.where('users', arrayContains: uid);
    Query collection2Query = collection2.where('users', arrayContains: uid);

    var resultStream1 = collection1Query.snapshots().listen((doc) {});
    var resultStream2 = collection2Query.snapshots().listen((doc) {});

    while(someId == null){
      resultStream1.onData((data) {
        var someId = data.docs.first.id;
      });
      resultStream2.onData((data) async {
        var someId = data.docs.first.id;
        await doOneMoreThing(someId);
      });
    }
    resultStream1.cancel();
    resultStream2.cancel();

    return someId;

Solution

For that check the async package there is a class named StreamGroup, and you can combine N streams on that class writing:

final newStream = StreamGroup.merge([streamOne, streamTwo]);

Then you can use that newStream as a common stream.

Comments

[pskink]

check StreamGroup / StreamZip