firebase_read.kt

//Get path to data according to selected tab
val databaseRef = FirebaseDatabase.getInstance().getReference(ref)
val topics = ArrayList<Topic>()
databaseRef.orderByChild("index").addValueEventListener(object : ValueEventListener {

    override fun onDataChange(data: DataSnapshot) {
        /*
            In the database a child contains more fields
            than Topic model. So we have to choose which one we want to match.
         */
        data.children.forEach {
            val item = Topic(
                    it.child("name").value?.toString(),
                    it.child("image").value?.toString(),
                    it.child("description").value?.toString()
                    )
            topics.add(item)
        }
        callback.onSuccess(topics)
    }

    override fun onCancelled(error: DatabaseError) {
        error.let {
            callback.onFailure(error.toException().fillInStackTrace())
        }
    }
})