在 Firebase 模拟器中一次更新后多次读取 Firestore 文档

Question

I am using Firestore to save users data. Each user in my app has a stream that listens to the user's document. I am running Firestore emulator and noticed that SOMETIMES after the user document is updated, document is read twice or more. I have attached an image from the Firestore emulator that shows that. Is there a reason why I am getting multiple reads after a single update. I am not doing any read to the user document outside the stream.

I am using Flutter mobile app for watching the user document ".snapshots()". The print statement in this code prints only one read. But if I am reading the user document only in the stream, then how do I end up with 2 reads in the emulator?

  Stream<MyProfile?> watchMyProfile() {
    return _firestore
        .doc(PathService.user(uid))
        .withConverter<MyProfile>(
          fromFirestore: (snapshot, _) {
            return MyProfile.fromJson(snapshot.data()!);
          },
          toFirestore: (myProfile, _) => myProfile.toJson(),
        )
        .snapshots()
        .map((doc) => doc.data())
        .map(
      (myProfile) {
        // THIS ONLY PRINTS ONCE.
        debugPrint('watchMyProfile - read my profile');
        return myProfile;
      },
    );
  }

UPDATE 1: To make sure that I am not reading any user document after update in firestore.rules file, I used something like this:

allow update: if true;
allow get: if true;
allow list: if true;

Unfortunately, I am still seeing lots of reads after an update to the user document.

Answer 1

I have reproduced the issue at my end and found the same behavior. As you can see in the image provided, reads have increased significantly.

This is due to constructing the stream inside a stream. It will get re-created on each call to build(), up to 60 times per second if there's an animation on the page.

I recommend you to construct the stream in a private variable then use it in the stream so it will not get re-created on each build method and will be called only once.

Something like this

class SomeClass extends State<Some_State>{
    final Stream<MyProfile> _usersStream =             
    FirebaseFirestore.instance.collection('users').doc('ABC123').snapshots();
    
@override
  Widget build(BuildContext context) {
    return StreamBuilder<MyProfile>(
      stream: _usersStream,
        builder: (context, snapshot) {
        //…
  }
}

For reference, Real time updates on a document is done by StreamBuilder as shown in here