Android Firebase - “onDataChange” And “onCancelled” Not Being Called With No Internet Connection

Question

In my app, I simply try to retrieve a reading passage from my Firebase database by adding a ListenerForSingleValueEvent in the following code:

Answer 1

Firebase separates the flow of data events (such as onDataChange()) from other things that might happen. It will only call onCancelled when there is a server-side reason to do so (currently only when the client doesn't have permission to access the data). There is no reason to cancel a listener, just because there is no network connection.

What you seem to be looking for is a way to detect whether there is a network connection (which is not a Firebase-specific task) or whether the user is connected to the Firebase Database back-end. The latter you can do by attaching a listener to .info/connected, an implicit boolean value that is true when you're connected to the Firebase Database back-end and is false otherwise. See the section in the document on detecting connection state for full details.

Answer 2

Hope that my solution can help somebody else (I assume that you already did something else)

Besides to set the keepSynced to true in my database reference:

databaseRef.keepSynced(true)

And add to my Application class:

FirebaseDatabase.getInstance().setPersistenceEnabled(true)

I've added two listeners, one for offline mode and other one for online:

override fun create(data: BaseObject): Observable<String> {
    return Observable.create { observer ->
        val objectId = databaseRef.push().key
        objectId?.let { it ->
            data.id = it

            val valueEvent = object : ValueEventListener {
                override fun onCancelled(e: DatabaseError) {
                    observer.onError(e.toException())
                }

                override fun onDataChange(dataSnapshot: DataSnapshot) {
                    observer.onComplete()
                }
            }
            // This listener will be triggered if we try to push data without an internet connection
            databaseRef.addListenerForSingleValueEvent(valueEvent)

            // The following listeners will be triggered if we try to push data with an internet connection
            databaseRef.child(it)
                    .setValue(data)
                    .addOnCompleteListener {
                        observer.onComplete()
                        // If we are at this point we don't need the previous listener
                        databaseRef.removeEventListener(valueEvent)
                    }
                    .addOnFailureListener { e ->
                        if (!observer.isDisposed) {
                            observer.onError(e)
                        }
                        databaseRef.removeEventListener(valueEvent)
                    }
        } ?: observer.onError(FirebaseApiNotAvailableException("Cannot create new $reference Id"))
    }
}

To be honest, I don't like the idea to attach two listeners very much, if somebody else has a better and elegant solution, please let me know, I have this code temporarily until I find something better.