I'm trying to use `Room` with `Paging 3`. I'll exp...

# room

[Mantas Varnagiris]

I'm trying to use Room with Paging 3. I'll explain my setup first so the problem is a bit clearer
• database (android module) has Room . It has a Dao with fun getItems(): PagingSource<Int, ItemEntity>
• ItemEntity is internal only to database module - I don't want to use it in domain. It is converted to Item
• core (kotlin module) has Paging 3. It wants to do paging using PagingSource<Int, Item> (not ItemEntity as it has no knowledge of it)
• database module implements interface that gives PagingSource<Int, Item>
Now my problem is that I cannot convert from PagingSource<Int, ItemEntity> to PagingSource<Int, Item> . At least I could not find a way to do it. I don't want to expose ItemEntity but I want to do paging with converted model Item . How can I do that?e

[Mantas Varnagiris]

The only way I could make it work now is if Dao exposes DataSource.Factory<Int, ItemEntity> and then use .map { it.toItem() }..asPagingSourceFactory().invoke()

[Mantas Varnagiris]

So I have to use DataSource.Factory from Paging 2 to map values

[Jorge R]

What I could see so far is that Paging 3 is coupling all your architecture with its own structure. As you are saying, if you need to get a PagingSource from Room, either it provides you with a way to map your entity types to your model class, or you need to define your paging source in a different layer.
What I would do, take a flow out of Room, then define an intermediate layer implementing PagingSource and using that flow to emit your model type using the PagingSource. Then use this new layer in your repository.
I guess Room returning a PagingSource will work if you don't need mapping entities to model objects... Who would like to have a clean arch with layer isolation nowadays, uh? :D

[Mantas Varnagiris]

Well it's still in alpha so hopefully mapping is coming. I was hoping to avoid creating my own PagingSource but it does seem like this is the best approach now. Although my "hack" of using Paging 2 DataSource.Factory that supports mapping and conversion to PagingSource kind of works as well 🙂

[Jorge R]

Yeah, nice one. Although I would be sceptical if you need to use something from Paging2 that is not in Paging3...
Fingers crossed about release version. In my opinion, Paging 3 is bringing back some of the problems that LiveData had before flow, coupling all your layers with its own type and structure.

[Dustin Lam]

Can you file an FR against our issue tracker? I'm happy to bring this up for 3.1 timeline. We're still working out the lower level APIs for Paging3 and need to make sure it works well for all cases.

https://issuetracker.google.com/issues/new?component=413106&template=1096385

Btw, you can wrap a PagingSource to do the mapping like so:

class MappedPagingSource<Key : Any, R : Any, T :Any>(
  val originalPagingSource: PagingSource<Key, T>
  val block: suspend (T) -> R
): PagingSource<Key, R> {
  override suspend fun load(params) {
    val page = originalPagingSource.load(params)
    return LoadResult.Page(
      ...
      data = page.data.map(block),
      ...
  }
}

[Dustin Lam]

Actually since this should be a relatively straightforward addition, we're also taking contributions to paging via github and I'm happy to help you get setup and point you in the right direction if you're interested in contributing this kind of wrapper / helper

[Dustin Lam]

Otherwise I'm happy to bring it up in discussion separately, but most important thing for paging to focus on is to make sure the base apis are super solid and work for all cases before building the higher level stuff - thanks for your patience while we work out the kinks in alpha!

[Mantas Varnagiris]

Ah nice I'll take a look. Thanks for your response.

[Dustin Lam]

Sorry it took awhile for me to see this 🙂

[Mantas Varnagiris]

Should I still file an FR?

[Jorge R]

Ey @Dustin Lam, you mean something like this?
https://github.com/mathias21/FlickrInterestingDay/blob/master/app/src/main/java/co[…]/flickrinterestingday/datasource/remote/picture/PagingSource.kt

[Dustin Lam]

Yes, in Mantas' case he started with a PagingSource impl from Room so the sample I put just wraps that, but if you're querying for results directly then that works too

[Dustin Lam]

I would generally recommend to use the .map operator on PagingData, but I understand they live in different layers so you can make your own cost-benefit decision there