Android - ViewPager2 resets adapter position to 0 on calling Diffutil.dispatchUpdatesTo

Question

I have a ViewPager2 which is set to swipe horizontally. I idea is to build an infinite scrolling pager. The class looks something like this:

class SwipeAdapter(fragmentActivity: FragmentActivity): FragmentStateAdapter(fragmentActivity)

I have overridden the usual createFragment(position: Int): Fragment where I am returning a new Fragment

I have a list of Ids that I am operating on which is the adapter data set.

private var contentIds = mutableListOf<Int>()

I have also implemented

override fun containsItem(itemId: Long): Boolean {
    return contentIds.contains(itemId.toInt())
}

override fun getItemId(position: Int): Long {
    return contentIds[position].toLong()
}

My DiffUtil looks like this:

inner class ContentIdsDiffCallback(private val oldItems: List<Int>, private val newItems: List<Int>) : DiffUtil.Callback() {
    override fun getOldListSize(): Int {
        return oldItems.size
    }

    override fun getNewListSize(): Int {
        return newItems.size
    }

    override fun areItemsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
        return oldItems[oldItemPosition] == newItems[newItemPosition]
    }

    override fun areContentsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
        return areItemsTheSame(oldItemPosition, newItemPosition)
    }
}

Nothing fancy happening here since I am dealing with Ids only.

How I am setting the data?

fun setData(data: MutableList<Int>) {
    val diffCallback = ContentIdsDiffCallback(contentIds, data)
    val diffResult = DiffUtil.calculateDiff(diffCallback, true)
    contentIds.addAll(data)
    diffResult.dispatchUpdatesTo(this)
}

So the logic is, I have registered a `registerOnPageChangeCallback` and overridden the method `onPageSelected`.

Over there, I check how many pages has the user swiped and when the user has swiped 60% of the items (val sixtyPercentItems: Int = ceil(viewpager2Adapter.itemCount - 1 * 0.6).toInt()) I call my API to fetch the next set of items. When that APi returns, I call adapter.setData (the method shown above).

**Surprisingly when that happens, my adapter resets to zero!** My list is not resetting i,e., it is *adding* the items, I haven't kept any special variable which would influence this behaviour. I am kind of in a fix. That was the whole point of DiffUtil right? Has anybody faced this?

I have the same issue, could you found a solution? – AndroidRuntimeException Jul 09 '20 at 16:16 — AndroidRuntimeException, Jul 09 '20 at 16:16

score 1 · Answer 1 · answered Jul 09 '20 at 16:33

I found a simple workaround, I am not happy with the solution but works for now.

val recyclerView = viewPager2.getChildAt(0) as RecyclerView
val state = recyclerView.layoutManager?.onSaveInstanceState()
yourAdapter.items = newItems //Update your items here
recyclerView.layoutManager?.onRestoreInstanceState(state)

Android - ViewPager2 resets adapter position to 0 on calling Diffutil.dispatchUpdatesTo

1 Answers1