android – Update a Single Item in a PagedList using Google's Paging Library-ThrowExceptions

Exception or error:

I have a simple list of Pokemon in a RecyclerView with just the Pokemon’s name and a “Favorite” ToggleButton. I’m using the Paging Library from Android JetPack with a PageKeyedDataSource to retrieve small chunks of Pokemon and display them to the user. I only want the data to persist as long as the Activity is not destroyed (i.e. I do NOT want to persist the data to Room or a database, but rather have it persist as long as the ViewModel is alive).

Screenshot of app

I want to be able to click the heart button on any Pokemon item and update the “isFavorite” field in the SimplePokemon model to true or false. From my understanding, if I wanted to change a single item from that PagedList, I would need to invalidate the DataSource and that should, theoretically, generate a new LiveData of a PagedList that could be fed to the Adapter and shown on screen.

Question: How can I update a single item from a PagedList using the Paging Library without the need for Room or some other database?

In the future, I want to scale this solution to a social media feed where users can like posts, but I don’t know if storing social feed items in a database such as Room is necessary (or efficient) since those feed items are constantly changing. So I opted for storing them in the ViewModel and then clearing them every time the user quits the app.

Here is my code so far:

SimplePokemon.kt:

data class SimplePokemon(
    @SerializedName("name") val name: String,
    @SerializedName("url") val url: String,
    var isFavorite: Boolean = false
)

PokemonViewModel.kt:

class PokemonViewModel(application: Application) : AndroidViewModel(application) {

    private val config = PagedList.Config.Builder()
        .setPageSize(20)
        .setEnablePlaceholders(false)
        .build()

    private fun initializedPagedListBuilder(config: PagedList.Config): LivePagedListBuilder<String, SimplePokemon> {
        val dataSourceFactory = object : DataSource.Factory<String, SimplePokemon>() {
            override fun create(): DataSource<String, SimplePokemon> {
                return PokemonDataSource()
            }
        }
        return LivePagedListBuilder<String, SimplePokemon>(dataSourceFactory, config)
    }

    fun pokemonPagedListLiveData(): LiveData<PagedList<SimplePokemon>> {
        return initializedPagedListBuilder(config).build()
    }
}

PokemonAdapter.kt:

class PokemonAdapter :
    PagedListAdapter<SimplePokemon, PokemonAdapter.PokemonViewHolder>(PokemonDiffUtil()) {

    inner class PokemonViewHolder(v: View) : RecyclerView.ViewHolder(v) {
        private val pokemonNameTextView: TextView = v.findViewById(R.id.pokemon_name_text_view)
        private val pokemonFavoriteToggle: ToggleButton =
            v.findViewById(R.id.pokemon_favorite_toggle_button)

        fun bind(data: SimplePokemon) {
            pokemonNameTextView.text = data.name
            pokemonFavoriteToggle.isChecked = data.isFavorite
        }
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): PokemonViewHolder {
        val view =
            LayoutInflater.from(parent.context).inflate(R.layout.item_simple_pokemon, parent, false)
        return PokemonViewHolder(view)
    }

    override fun onBindViewHolder(holder: PokemonViewHolder, position: Int) {
        val item = getItem(position)
        item?.let { holder.bind(it) }
    }
}

PokemonDataSource.kt:

class PokemonDataSource : PageKeyedDataSource<String, SimplePokemon>() {

    private val api = NetworkService.pokemonNetworkInterface

    override fun loadInitial(
        params: LoadInitialParams<String>,
        callback: LoadInitialCallback<String, SimplePokemon>
    ) {
        api.getPokemon().enqueue(object : Callback<PokeResponse<List<SimplePokemon>>> {

            override fun onFailure(call: Call<PokeResponse<List<SimplePokemon>>>?, t: Throwable?) {
                Log.e("PokemonDataSource", "Failed to fetch data!")
            }

            override fun onResponse(
                call: Call<PokeResponse<List<SimplePokemon>>>?,
                response: Response<PokeResponse<List<SimplePokemon>>>
            ) {
                val listing = response.body()
                val pokemon = listing?.results
                callback.onResult(pokemon ?: listOf(), listing?.previous, listing?.next)
            }
        })
    }

    override fun loadAfter(
        params: LoadParams<String>,
        callback: LoadCallback<String, SimplePokemon>
    ) {
        api.getPokemon(url = params.key)
            .enqueue(object : Callback<PokeResponse<List<SimplePokemon>>> {
                override fun onFailure(
                    call: Call<PokeResponse<List<SimplePokemon>>>?,
                    t: Throwable?
                ) {
                    Log.e("PokemonDataSource", "Failed to fetch data! Oh Noooo!")
                }

                override fun onResponse(
                    call: Call<PokeResponse<List<SimplePokemon>>>?,
                    response: Response<PokeResponse<List<SimplePokemon>>>
                ) {
                    val listing = response.body()
                    val pokemon = listing?.results
                    callback.onResult(pokemon ?: listOf(), listing?.next)
                }
            })
    }

    override fun loadBefore(
        params: LoadParams<String>,
        callback: LoadCallback<String, SimplePokemon>
    ) {
        api.getPokemon(url = params.key)
            .enqueue(object : Callback<PokeResponse<List<SimplePokemon>>> {
                override fun onFailure(
                    call: Call<PokeResponse<List<SimplePokemon>>>?,
                    t: Throwable?
                ) {
                    Log.e("PokemonDataSource", "Failed to fetch data! Oh Noooo!")
                }

                override fun onResponse(
                    call: Call<PokeResponse<List<SimplePokemon>>>?,
                    response: Response<PokeResponse<List<SimplePokemon>>>
                ) {
                    val listing = response.body()
                    val pokemon = listing?.results
                    callback.onResult(pokemon ?: listOf(), listing?.previous)
                }
            })
    }

I also want to make sure that the RecyclerView does not jump to the top every time the DataSource is updated.

The ideal scenario would be to keep a List of Pokemon as long as the Activity is alive and be able to update individual Pokemon items locally. Theoretically, I would also send a POST request to the backend to update the Pokemon in the back-end, but I’m just trying to keep the question simple.

Any help would be sincerely appreciated.

How to solve:

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