如何避免对使用带有 firestore 的 snapshotListener 删除的消息调用 onEvent(QuerySnapshot snapshots, FirestoreException error)？

Question

I am using Firestore in Java with the method:

 firestore.collection("myCollection").addSnapshotListener(listener);

My listener is:

 EventListener<QuerySnapshot> listener = new EventListener<QuerySnapshot>() {
        WriteBatch wb = firestore.batch();
        @Override
        public void onEvent(QuerySnapshot snapshots, FirestoreException error) {
           snapshots.forEach(doc -> {
                // do something with the doc and then delete it
                ...
                wb.delete(doc.getReference()
        }
        wb.commit(); 
}

The problem is that when I delete the documents I call again the onEvent method. There is a way to avoid it?

Answer 1

There is a way to avoid it?

Sure there is. What you are doing is adding a real-time listener which is triggered every time you make a change at your myCollection location. Doesn't matter if it's a delete, write or even an update operation, the onEvent() method fires. To avoid this, you should get the data only once by using Query's get() method. This is the correspondent addListenerForSingleValueEvent() method from Firebase Realtime Database.

Edit:

According to your comment:

There is a way to use the listener and having the onEvent fired only with create (or create and update) operations?

Yes, it is, you can view the changes between snapshots using a switch statement, as explained the official documentation regarding changes between snapshots .

It is often useful to see the actual changes to query results between query snapshots, instead of simply using the entire query snapshot. For example, you may want to maintain a cache as individual documents are added, removed, and modified.