How to call different api resource from paging source or remote mediator Kotlin

Question

Hey I want to call two different api for my Paging Library 3. I want to ask what is best suit for me to use Paging Source or Remote Mediator?. What is the use case of both? Can someone please explain me.

For 1st api call only for single time

@GET("/movie?min=20")

Above api call returns this response

data class movie(
  var id: Int?,
  var name: String?,
  var items : List<Genre>?
}

Now for 2nd api call its loop to call again and again

@GET("/movie?count=20&&before={time}")

Above api call retrun this

data class movie(
  var items : List<Genre>?
}

Genre

data class Genre(
   var type: String?,
   var date: String?,
   var cast: String?
}

Genre have data in both api call. I tried to google this and found this Example. But inside this both api return same data. But in my case both returns little bit different. Also id, name is only used in UI component else list will go to adapter. But I didn't understand how to achieved this. I am new in Flow, it too difficult to understand, to be honest I am trying to learning CodeLab. Another important thing when 1st time api call, in which the last item contains date will send to 2nd api call in time parameter and then 2nd api last item date call again 2nd api, this will go in loop. So how can I track this again in loop condition. Third I want to update data at top of list, can we store data in memory than we can update value on that list? Thanks for advance. Sorry for my wrong english.

UPDATE

After @dlam suggestion, I tried to practice some code

MainActivity

class MainActivity : AppCompatActivity() {

    private lateinit var binding: ActivityMainBinding
    private val viewModel by viewModels<ActivityViewModel>()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        lifecycleScope.launchWhenCreated {
            viewModel.getMovie().collectLatest {
//                setupAdapter()
            }
        }

    }
}

ActivityViewModel

class ActivityViewModel(app: Application) : AndroidViewModel(app) {

    fun getMovie(): Flow<PagingData<Genre>> {
        return Pager(
            config = PagingConfig(
                pageSize = 20
            ),
            pagingSourceFactory = {
                MultiRequestPagingSource(DataSource())
            }
        ).flow
    }
}

MultiRequestPagingSource

class MultiRequestPagingSource(private val dataSource: DataSource) : PagingSource<String, Genre>() {

    override fun getRefreshKey(state: PagingState<String, Genre>): String? {
        return state.anchorPosition?.let { anchorPosition ->
            state.closestPageToPosition(anchorPosition)?.nextKey
        }
    }

    override suspend fun load(params: LoadParams<String>): LoadResult<String, Genre> {
        val key = params.key ?: ""

        return try {
            val data = when (params) {
                is LoadParams.Refresh -> {
                    dataSource.fetchInitialMovie()
                }
                is LoadParams.Append -> {
                    dataSource.fetchMovieBefore(key)
                }
                is LoadParams.Prepend -> null
            }

            LoadResult.Page(
                data = data.result,
                prevKey = null,
                nextKey = data?.nextKey,
            )
        } catch (exception: IOException) {
            LoadResult.Error(exception)
        }
    }
}

I am getting error on data = data.result

Type mismatch.
Required:
List<TypeVariable(Value)>
Found:
ArrayDeque<Genre>?

DataSource

package com.example.multirequestpaging

class DataSource {

    data class MovieResult(
        val result: ArrayDeque<Genre>?,
        val nextKey: String?
    )

    fun fetchInitialMovie(): MovieResult {
        val response = ApiInterface.create().getMovieResponse(20)

        return MovieResult(
            addInArrayDeque(response),
            response.items?.last()?.date
        )
    }

    fun fetchMovieBefore(key: String): MovieResult {
        val response = ApiInterface.create().getMovieResponseBefore(20, key)

        return MovieResult(
            addInArrayDeque(response),
            response.items?.last()?.date
        )
    }

    private fun addInArrayDeque(response: MovieResponse): ArrayDeque<Genre> {
        val result: ArrayDeque<Genre> = ArrayDeque()
        response.items?.forEach {
            result.add(it)
        }

        return result
    }

}

For Full code Project Link

1. I want to add an item to the top of the list. How can I use invalidate function? Sorry I didn't understand where I can use.

2. I want to use id,name in other place so how can i get those variable value in my activity class.

3. Is my code structure is good?. Do I need to improved, please give an example. It will also help beginner, who is learning Paging Library.

Thanks

Answer 1

PagingSource is the main driver for Paging, it's responsible for loading items that get displayed and represents the single source of truth of data.

RemoteMediator is for layered sources, it is essentially a callback which triggers when PagingSource runs out of data, so you can fetch from a secondary source. This is primarily useful in cases where you fetching from both DB + Network, where you want locally cached data to power Paging, and then use RemoteMediator as a callback to fetch more items into the cache from network.

In this scenario you have two APIs, but they both fetch from the same Network source, so you only need PagingSource here. If I'm understanding correctly, you essentially want to call the first API on initial load and the second API on subsequent prepend / append page loads, which you can check / switch on by the type of LoadParams you get. See the subtypes here: https://developer.android.com/reference/kotlin/androidx/paging/PagingSource.LoadParams