Paging3 - How to do Reverse Pagination in Chat App - android-paging-3

Question

I am recently migrate to Paging3. However, I noticed that most of the tutorials and guides are mainly on normal pagination (from top to bottom).
I need to implement the REVERSE pagination as user scroll to top boundary, will load for page 2,3,4..
Is there any tutorial/guide for this?
PS: Now the initial loading is working fine, but when I scroll to top-most, I have no idea how to load Page 2 data.
My current approach
PagingSource
class ChatPagingSource(
private val apiService: ApiService,
private val roomId: String
): PagingSource<Int, Message>() {
override fun getRefreshKey(state: PagingState<Int, Message>): Int? = null
override suspend fun load(params: LoadParams<Int>): LoadResult<Int, Message> {
return try{
val page = params.key?: 1
val pageSize = params.loadSize
val call = apiService.getMessageFeedPaging(
room_id = roomId,
page = page,
max = pageSize,
exclude = EXCLUDE_TYPE
)
val repoItems = call.messages
val prevKey = if(page > 1) page - 1 else null
val nextKey = if(repoItems.isNotEmpty()) page + 1 else null
if(repoItems.isNotEmpty()) {
val messageList = mutableListOf<Message>()
for (i in repoItems) {
val replyData = Converters.convertReplyDataAPItoReplyData(i.reply_data)
val msg = Converters.convertMessageAPItoMessage(replyData, i, hasError = false)
messageList.add(msg)
}
LoadResult.Page(messageList, prevKey, nextKey)
} else {
LoadResult.Page(listOf(), prevKey, nextKey)
}
}catch (e: Exception) {
LoadResult.Error(e)
}
}
}
Repository
fun loadRemoteMessageStream(roomId: String): LiveData<PagingData<Message>> {
return Pager(
config = PagingConfig(20),
pagingSourceFactory = { ChatPagingSource(apiService, roomId) }
).liveData
}
ViewModel
private val _remoteMessage = chatRepository.loadRemoteMessageStream(currentRoomId)
.cachedIn(viewModelScope)
.let { it as MutableLiveData<PagingData<Message>> }
val remoteMessage: LiveData<PagingData<Message>> = _remoteMessage
Fragment
chatViewModel.remoteMessage.observe(viewLifecycleOwner, {
chatAdapter.submitData(viewLifecycleOwner.lifecycle, it)
})

Answer 1

In case this helps anyone, I will post out my own answer.
The key is to reverse prevKey and nextKey and fixed the pageSize that your API required (in my case is 20).
override suspend fun load(params: LoadParams<Int>): LoadResult<Int, Message> {
val pageSize = 20
val prevKey = if(repoItems.isNotEmpty()) page + 1 else null
val nextKey = if(page > 1) page - 1 else null
}
Then, in the recyclerview, you should use stackFromEnd = true so that initially the recyclerview will auto-scroll to bottom.