Example of Firebase LiveData mechanism

FirebaseLiveData.kt

class FirebaseLiveData<T>(
        private val reference: DatabaseReference,
        private val resourceType: Class<T>
) : LiveData<Resource<T>>(), ValueEventListener {

    override fun onActive() {
        super.onActive()
        if (value == null) {
            value = Resource.loading()
        }
        reference.addValueEventListener(this)
    }

    override fun onInactive() {
        super.onInactive()
        reference.removeEventListener(this)
    }

    override fun onCancelled(error: DatabaseError) {
        value = Resource.error(error.message)
    }

    override fun onDataChange(data: DataSnapshot) {
        value = if (!data.exists()) {
            Resource.error("404 Resource ${resourceType.simpleName} not found")
        } else {
            Resource.success(data.getValue(resourceType))
        }
    }
}