Modifying PagedList in Android Paging Architecture library

Question

I'm currently looking into incorporating the Paging Architecture library (version 2.1.0-beta01 at the time of writing) into my app. One components is a list which allows the user to delete individual items from it. This list is network-only and caching localy with Room does not make sense.

PagedList is immutable and does not support modification. I have read that having a copy of the list which is than modified and returned as the new one is the way to go. The documentation states the same:

If you have more granular update signals, such as a network API signaling an update to a single item in the list, it's recommended to load data from the network into memory. Then present that data to the PagedList via a DataSource that wraps an in-memory snapshot. Each time the in-memory copy changes, invalidate the previous DataSource, and a new one wrapping the new state of the snapshot can be created.

I currently have the basic recommended implementation to show a simple list. My DataSource looks like this:

class MyDataSource<SomeItem> : PageKeyedDataSource<Int, SomeItem>() {

    override fun loadInitial(params: LoadInitialParams<Int>, callback: LoadInitialCallback<Int, SomeItem>) {
        // Simple load from API and notification of `callback`.
    }

    override fun loadAfter(params: LoadParams<Int>, callback: LoadCallback<Int, SomeItem>) {
        // Simple load from API and notification of `callback`.
    }

    override fun loadBefore(params: LoadParams<Int>, callback: LoadCallback<Int, SomeItem>) {
        // Simple load from API and notification of `callback`.
    }
}

How would a concrete implementation of an in-memory cache (without Room and without invalidating the entire dataset) as referenced in the documentation look like?

Answer 1

If you want to modify your list without going all the way down to the data layer, you will need to override submitList in your adapter, and then set a callback on your PagedList object. Whenever the PagedList changes, you can then copy those changes to your local dataset. This is not recommended but it's a pretty minimal hack to get working.

Here's an example:

class MyListAdapter : PagedListAdapter<MyDataItem, MyViewHolder>(MyDiffCallback()) {

    /**
     * This data set is a bit of a hack -- we are copying everything the PagedList loads into our
     * own list.  That way we can modify it.  The docs say you should go all the way down to the
     * data source, modify it there, and then bubble back up, but I don't think that will actually
     * work for us when the changes are coming from the UI itself.
     */
    private val dataSet = arrayListOf<MyDataItem>()

    override fun onBindViewHolder(holder: MyViewHolder, position: Int) {
        //Forces the next page to load when we reach the bottom of the list
        getItem(position)

        dataSet.getOrNull(position)?.let {
            holder.populateFrom(it)
        }
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
        val view = parent.inflate(R.layout.my_view_holder)
        return MyViewHolder(view)
    }

    class MyDiffCallback : DiffUtil.ItemCallback<MyDataItem>() {

        override fun areItemsTheSame(oldItem: MyDataItem, newItem: MyDataItem) =
                oldItem.id == newItem.id

        override fun areContentsTheSame(oldItem: MyDataItem, newItem: MyDataItem) =
                oldItem == newItem
    }

    override fun submitList(pagedList: PagedList<MyDataItem>?) {
        pagedList?.addWeakCallback(listOf(), object : PagedList.Callback() {
            override fun onChanged(position: Int, count: Int) {
                dataSet.clear()
                dataSet.addAll(pagedList)
            }

            override fun onInserted(position: Int, count: Int) {
                dataSet.clear()
                dataSet.addAll(pagedList)
            }

            override fun onRemoved(position: Int, count: Int) {
                dataSet.clear()
                dataSet.addAll(pagedList)
            }
        })
        super.submitList(pagedList)
    }
}