Full Supplier code

ViewHolderSupplier.kt

typealias ViewHolderProducer = (parent: ViewGroup, viewType: Int) -> RecyclerView.ViewHolder
typealias ViewHolderConsumer = (viewHolder: RecyclerView.ViewHolder, creationTimeNanos: Long) -> Unit


abstract class ViewHolderSupplier(
    context: Context,
    private val viewHolderProducer: ViewHolderProducer
) {

    internal lateinit var viewHolderConsumer: ViewHolderConsumer

    private val fakeParent: ViewGroup by lazy { FrameLayout(context) }
    private val mainHandler: Handler = Handler(Looper.getMainLooper())
    private val itemsCreated: MutableMap<Int, Int> = ConcurrentHashMap<Int, Int>()
    private val itemsQueued: MutableMap<Int, Int> = ConcurrentHashMap<Int, Int>()
    private val nanoTime: Long get() = System.nanoTime()

    abstract fun start()

    abstract fun enqueueItemCreation(viewType: Int)

    abstract fun stop()

    protected fun createItem(viewType: Int) {
        val created = itemsCreated.getOrZero(viewType) + 1
        val queued = itemsQueued.getOrZero(viewType)
        if (created > queued) return

        val holder: RecyclerView.ViewHolder
        val start: Long
        val end: Long

        try {
            start = nanoTime
            holder = viewHolderProducer.invoke(fakeParent, viewType)
            end = nanoTime
        } catch (e: Exception) {
            return
        }
        holder.setItemViewType(viewType)
        itemsCreated[viewType] = itemsCreated.getOrZero(viewType) + 1

        mainHandler.postAtFrontOfQueue { viewHolderConsumer.invoke(holder, end - start) }
    }

    internal fun setPrefetchBound(viewType: Int, count: Int) {
        if (itemsQueued.getOrZero(viewType) >= count) return
        itemsQueued[viewType] = count

        val created = itemsCreated.getOrZero(viewType)
        if (created >= count) return

        repeat(count - created) { enqueueItemCreation(viewType) }
    }

    internal fun onItemCreatedOutside(viewType: Int) {
        itemsCreated[viewType] = itemsCreated.getOrZero(viewType) + 1
    }

    private fun Map<Int, Int>.getOrZero(key: Int) = getOrElse(key) { 0 }
}