DiffUtil and registerAdapterDataObserver

Question

I have a RecyclerView adapter with DiffUtil functionality. I wanted to use DataObserver to notify fragment when DiffUtil finishes its magic. However, looks like DiffUtil doesn't call the observers.

Adapter

private val items: MutableList<Message>
private val updater = PublishSubject.create<MutableList<Message>>()

init {
    items = ArrayList()

    updater
        .debounce(500, TimeUnit.MILLISECONDS)
        .subscribeOn(Schedulers.computation())
        .map { Pair(it, DiffUtil.calculateDiff(DiffUtilCallback(it, items), true)) }
        .doOnNext { setItems(it.first) }
        .map { it.second }
        .observeOn(AndroidSchedulers.mainThread())
        .subscribe { diffResult.dispatchUpdatesTo(this) }
}

fun updateData(msgs: MutableList<Message>) = updater.onNext(msgs)

private fun setItems(newItems: MutableList<Message>) {
    items.clear()
    items.addAll(newItems)
}

Fragment

adapter.registerAdapterDataObserver(object : RecyclerView.AdapterDataObserver() {
    override fun onChanged() {
        //is not called
    }
})

How do I make the AdapterDataObserver work? What is wrong with my (trying to be reactive) approach?

Answer 1

I found out the problem. I assumed onChanged() is called everytime something changes. But it is 1:1 with notify methods.

So notifyDatasetChanged() invokes onChanged(), but DiffUtil never calls it (that is the purpose of DiffUtil), which means all the other methods (onItemRangeRemoved(), onItemRangeMoved(), etc.) gotta be implemented.