Firebase Offline Capabilities and addListenerForSingleValueEvent

Whenever I use addListenerForSingleValueEvent with setPersistenceEnabled(true), I only manage to get a local offline copy of DataSnapshot and NOT the updated DataSnapshot from the server.

However, if I use addValueEventListener with setPersistenceEnabled(true), I can get the latest copy of DataSnapshot from the server.

Is this normal for addListenerForSingleValueEvent as it only searches DataSnapshot locally (offline) and removes its listener after successfully retrieving DataSnapshot ONCE (either offline or online)?

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.

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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 https://stackoverflow.com/questions/33260450/how-does-work-firebase-sync-with-share-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.

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!

You can create transaction and abort it, then onComplete will be called when online (nline data) or offline (cached data)

I previously created function which worked only if database got connection lomng enough to do synch. I fixed issue by adding timeout. I will work on this and test if this works. Maybe in the future, when I get free time, I will create android lib and publish it, but by then it is the code in kotlin:

/**
     * @param databaseReference reference to parent database node
     * @param callback callback with mutable list which returns list of objects and boolean if data is from cache
     * @param timeOutInMillis if not set it will wait all the time to get data online. If set - when timeout occurs it will send data from cache if exists
     */
    fun readChildrenOnlineElseLocal(databaseReference: DatabaseReference, callback: ((mutableList: MutableList<@kotlin.UnsafeVariance T>, isDataFromCache: Boolean) -> Unit), timeOutInMillis: Long? = null) {

        var countDownTimer: CountDownTimer? = null

        val transactionHandlerAbort = object : Transaction.Handler { //for cache load
            override fun onComplete(p0: DatabaseError?, p1: Boolean, data: DataSnapshot?) {
                val listOfObjects = ArrayList<T>()
                data?.let {
                    data.children.forEach {
                        val child = it.getValue(aClass)
                        child?.let {
                            listOfObjects.add(child)
                        }
                    }
                }
                callback.invoke(listOfObjects, true)
            }

            override fun doTransaction(p0: MutableData?): Transaction.Result {
                return Transaction.abort()
            }
        }

        val transactionHandlerSuccess = object : Transaction.Handler { //for online load
            override fun onComplete(p0: DatabaseError?, p1: Boolean, data: DataSnapshot?) {
                countDownTimer?.cancel()
                val listOfObjects = ArrayList<T>()
                data?.let {
                    data.children.forEach {
                        val child = it.getValue(aClass)
                        child?.let {
                            listOfObjects.add(child)
                        }
                    }
                }
                callback.invoke(listOfObjects, false)
            }

            override fun doTransaction(p0: MutableData?): Transaction.Result {
                return Transaction.success(p0)
            }
        }

In the code if time out is set then I set up timer which will call transaction with abort. This transaction will be called even when offline and will provide online or cached data (in this function there is really high chance that this data is cached one). Then I call transaction with success. OnComplete will be called ONLY if we got response from firebase database. We can now cancel timer (if not null) and send data to callback.

This implementation makes dev 99% sure that data is from cache or is online one.

If you want to make it faster for offline (to don't wait stupidly with timeout when obviously database is not connected) then check if database is connected before using function above:

DatabaseReference connectedRef = FirebaseDatabase.getInstance().getReference(".info/connected");
connectedRef.addValueEventListener(new ValueEventListener() {
  @Override
  public void onDataChange(DataSnapshot snapshot) {
    boolean connected = snapshot.getValue(Boolean.class);
    if (connected) {
      System.out.println("connected");
    } else {
      System.out.println("not connected");
    }
  }

  @Override
  public void onCancelled(DatabaseError error) {
    System.err.println("Listener was cancelled");
  }
});

When workinkg with persistence enabled, I counted the times the listener received a call to onDataChange() and stoped to listen at 2 times. Worked for me, maybe helps:

private int timesRead;
private ValueEventListener listener;
private DatabaseReference ref;

private void readFB() {
    timesRead = 0;
    if (ref == null) {
        ref = mFBDatabase.child("URL");
    }

    if (listener == null) {
        listener = new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                //process dataSnapshot

                timesRead++;
                if (timesRead == 2) {
                    ref.removeEventListener(listener);
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
            }
        };
    }
    ref.removeEventListener(listener);
    ref.addValueEventListener(listener);
}