Initial placeholder with Paging Architecture Components

Question

I'm using the Paging Architecture Components in my project to load a list from the network (no database usage yet). My DataSource is a PositionalDataSource subclass, and my PagedList.Config looks like this:

PagedList.Config config = new PagedList.Config.Builder()
        .setPageSize(10)
        .setInitialLoadSizeHint(20)
        .setPrefetchDistance(20)
        .setEnablePlaceholders(true)
        .build();

Placeholders are enabled, and I handle null ViewHolders in my PagedListAdapter subclass to display placeholder items differently. The placeholders work fine when loading additional items at the end of the list, but I also want to display a placeholder item before the first few items are loaded (like a regular loading indicator).

Is there a way to do this with the Paging library? I tried to call LoadInitialCallback.onResult() with a position of 1 instead of 0, but this only creates a placeholder item before the first real item, and that placeholder item never disappears.

Answer 1

I realize this question is rather old but I answer this, since this is where I ended up from googling the issue . In v3 there are LoadStateAdapter which you can add to Header and Footer for Prepend and Append load states. Initial load will be the refresh state which does not seem to be included but we can simply copy the functionality and extend it to include Refresh:

I simply copied withLoadStateHeaderAndFooter and added a third adapter that could be used for refresh

private fun PagingAdapter.withLoadStateAll(
        header: LoadStateAdapter<*>,
        footer: LoadStateAdapter<*>,
        refresh: LoadStateAdapter<*>
    ): ConcatAdapter {
        addLoadStateListener { loadStates ->
            header.loadState = loadStates.prepend
            footer.loadState = loadStates.append
            refresh.loadState = loadStates.refresh
        }
        return ConcatAdapter(refresh,header, this, footer)
    }

then you can simply register this to your Recycler as per the documentation and it'll show during initial load.

recycler.adapter = adapter.withLoadStateAll(LoadAdapter(),LoadAdapter(),LoadAdapter())

If you need to show more than one item during initial load simply copy LoadStateAdapter and change the number of items from 1 to n:

class InitialLoadAdapter(private val num : Int) : RecyclerView.Adapter<LoadViewHolder>() {

    var loadState: LoadState = LoadState.NotLoading(endOfPaginationReached = false)
        set(loadState) {
            if (field != loadState) {
                val oldItem = displayLoadStateAsItem(field)
                val newItem = displayLoadStateAsItem(loadState)

                if (oldItem && !newItem) {
                    notifyItemRangeRemoved(0,num)
                } else if (newItem && !oldItem) {
                    notifyItemRangeInserted(0,num)
                } else if (oldItem && newItem) {
                    notifyItemRangeChanged(0,num)
                }
                field = loadState
            }
        }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): LoadViewHolder {
        return LoadViewHolder(parent)
    }
    override fun onBindViewHolder(holder: LoadViewHolder, position: Int) {}

    override fun getItemViewType(position: Int): Int = getStateViewType(loadState)

    override fun getItemCount(): Int = if (displayLoadStateAsItem(loadState)) num else 0

    private fun getStateViewType(loadState: LoadState): Int = 0

    private fun displayLoadStateAsItem(loadState: LoadState): Boolean {
        return loadState is LoadState.Loading || loadState is LoadState.Error
    }
}