a.kt

open class FirebaseResourceLiveData(ref: DatabaseReference) : LiveData<Resource<DataSnapshot>> {
    init { 
        value = Resource.Loading() // setValue() of LiveData
    }
    
    private val listener = MyValueEventListener()

    override fun onActive() { // we have observers!
        value = Resource.Loading(value?.extractData)
        query.addValueEventListener(listener)
    }

    override fun onInactive() { // no observers
        query.removeEventListener(listener)
    }

    private inner class MyValueEventListener : ValueEventListener {
        override fun onDataChange(dataSnapshot: DataSnapshot) {
            value = Resource.Success(dataSnapshot)
        }
        override fun onCancelled(databaseError: DatabaseError) {
            val e = databaseError.toException()
            value = Resource.Failure(e)
        }
    }
}