Remove Firebase listener doesn't work Android

Question

I'm adding an event value listener to a reference and when I want to remove it, but it doesn't work.

override fun onStart() {
    super.onStart()
    stateListener =dbReference.stateRef.addValueEventListener(object : ValueEventListener {

        override fun onCancelled(p0: DatabaseError?) {
            TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
        }

        override fun onDataChange(snapshot: DataSnapshot?) {
           //there is my functions
        })
}

override fun onStop() {
    super.onStop()
    dbReference.stateRef.removeEventListener(stateListener)
    Log.d("My_TAG", "$stateListener ")
}

This is the only place I'm adding the event.

after editing (still don't work)

override fun onStart() {
    super.onStart()
    stateListener = object : ValueEventListener {
        override fun onCancelled(p0: DatabaseError?) {
            TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
        }
        //One of the phone got flipped
        override fun onDataChange(snapshot: DataSnapshot?) {
        }

    }
    dbReference.stateRef.addValueEventListener(stateListener)

}

override fun onStop() {
    super.onStop()
    Log.d("Life", "onStop")
    dbReference.stateRef.removeEventListener(stateListener)
}

Answer 1

In order to remove a listener, you first need to create it and then attach it to a DatabaseReference object.

val valueEventListener = ValueEventListener() {
    //override fun onDataChange
    //override fun onCancelled
}
dbReference.stateRef.addValueEventListener(eventListener)

So to remove the listener, you need to do it accordingly to the life-cycle of your activity like this:

dbReference.stateRef.removeEventListener(valueEventListener);

• If you have added the listener in onStart you have to remove it in onStop.
• If you have added the listener in onResume you have to remove it in onPause.
• If you have added the listener in onCreate you have to remove it in onDestroy.

But remember onDestroy is not always called.

Answer 2

addValueEventListener doesn't return anything. You just created the listener anonymously. Therefore, your stateListener isn't referring to your listener.

Change your code as below:

class YourClassName:Activity(){    
private lateinit var statelistener: ValueEventListener

override fun onStart() {
    stateListener = ValueEventListener {
        [...]
    }
    dbReference.stateRef.addValueEventListener(stateListener)
}


override fun onStop() {
    super.onStop()
    dbReference.stateRef.removeEventListener(stateListener)
    Log.d("My_TAG", "$stateListener ")
}
}

Now, your unsubscribe code should work. I am not very comfortable with Kotlin yet so pardon the syntax issues but the code above should work.