Firebase Offline Capabilities and addListenerForSingleValueEvent

Solution 1:

Update (2021): There is a new method call (get on Android and getData on iOS) that implement the behavior you'll like want: it first tries to get the latest value from the server, and only falls back to the cache when it can't reach the server. The recommendation to use persistent listeners still applies, but at least there's a cleaner option for getting data once even when you have local caching enabled.

How persistence works

The Firebase client keeps a copy of all data you're actively listening to in memory. Once the last listener disconnects, the data is flushed from memory.

If you enable disk persistence in a Firebase Android application with:

Firebase.getDefaultConfig().setPersistenceEnabled(true); 

The Firebase client will keep a local copy (on disk) of all data that the app has recently listened to.

What happens when you attach a listener

Say you have the following ValueEventListener:

ValueEventListener listener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot snapshot) {
        System.out.println(snapshot.getValue());
    }

    @Override
    public void onCancelled(FirebaseError firebaseError) {
        // No-op
    }
};

When you add a ValueEventListener to a location:

ref.addValueEventListener(listener); 
// OR
ref.addListenerForSingleValueEvent(listener); 

If the value of the location is in the local disk cache, the Firebase client will invoke onDataChange() immediately for that value from the local cache. If will then also initiate a check with the server, to ask for any updates to the value. It may subsequently invoke onDataChange() again if there has been a change of the data on the server since it was last added to the cache.

What happens when you use addListenerForSingleValueEvent

When you add a single value event listener to the same location:

ref.addListenerForSingleValueEvent(listener);

The Firebase client will (like in the previous situation) immediately invoke onDataChange() for the value from the local disk cache. It will not invoke the onDataChange() any more times, even if the value on the server turns out to be different. Do note that updated data still will be requested and returned on subsequent requests.

This was covered previously in How does Firebase sync work, with shared data?

Solution and workaround

The best solution is to use addValueEventListener(), instead of a single-value event listener. A regular value listener will get both the immediate local event and the potential update from the server.

A second solution is to use the new get method (introduced in early 2021), which doesn't have this problematic behavior. Note that this method always tries to first fetch the value from the server, so it will take longer to completely. If your value never changes, it might still be better to use addListenerForSingleValueEvent (but you probably wouldn't have ended up on this page in that case).

As a workaround you can also call keepSynced(true) on the locations where you use a single-value event listener. This ensures that the data is updated whenever it changes, which drastically improves the chance that your single-value event listener will see the current value.

Solution 2:

So I have a working solution for this. All you have to do is use ValueEventListener and remove the listener after 0.5 seconds to make sure you've grabbed the updated data by then if needed. Realtime database has very good latency so this is safe. See safe code example below;

public class FirebaseController {

private DatabaseReference mRootRef;
private Handler mHandler = new Handler();

private FirebaseController() {
    FirebaseDatabase.getInstance().setPersistenceEnabled(true);

    mRootRef = FirebaseDatabase.getInstance().getReference();
}

public static FirebaseController getInstance() {
    if (sInstance == null) {
        sInstance = new FirebaseController();
    }
    return sInstance;
}

Then some method you'd have liked to use "addListenerForSingleEvent";

public void getTime(final OnTimeRetrievedListener listener) {
    DatabaseReference ref = mRootRef.child("serverTime");
    ref.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            if (listener != null) {
                // This can be called twice if data changed on server - SO DEAL WITH IT!
                listener.onTimeRetrieved(dataSnapshot.getValue(Long.class));
            }
            // This can be called twice if data changed on server - SO DEAL WITH IT!
            removeListenerAfter2(ref, this);
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            removeListenerAfter2(ref, this);
        }
    });
}

// ValueEventListener version workaround for addListenerForSingleEvent not working.
private void removeListenerAfter2(DatabaseReference ref, ValueEventListener listener) {
    mHandler.postDelayed(new Runnable() {
        @Override
        public void run() {
            HelperUtil.logE("removing listener", FirebaseController.class);
            ref.removeEventListener(listener);
        }
    }, 500);
}

// ChildEventListener version workaround for addListenerForSingleEvent not working.
private void removeListenerAfter2(DatabaseReference ref, ChildEventListener listener) {
    mHandler.postDelayed(new Runnable() {
        @Override
        public void run() {
            HelperUtil.logE("removing listener", FirebaseController.class);
            ref.removeEventListener(listener);
        }
    }, 500);
}

Even if they close the app before the handler is executed, it will be removed anyways. Edit: this can be abstracted to keep track of added and removed listeners in a HashMap using reference path as key and datasnapshot as value. You can even wrap a fetchData method that has a boolean flag for "once" if this is true it would do this workaround to get data once, else it would continue as normal. You're Welcome!