Hey everybody, What's the recommended approach for using Paging 3 and Room with Clean Architecture?...

# android

[Ahmed Mourad]

Hey everybody,

What's the recommended approach for using Paging 3 and Room with Clean Architecture? Since the domain layer is Java or Kotlin only, my repository or usecases can't return a PagingSource and implementing it in the presentation layer is far from ideal.

[Ahmed Mourad]

It could be possible to create an ItemsPager interface in the domain layer that the PagingSource also implements and have a mapper that maps ItemPager to PagingSource, but I've never seen that done anywhere and it would also mean that I don't get to use Room's Paging support.

[Dustin Lam]

PagingSource comes from paging-common which does not have android dependency

[Dustin Lam]

LimitOffsetDataSource from Room is in room-runtime though

[Anshulupadhyay03]

It is a coincidence that I am doing some r&d on it. I will also try to apply the suggestions here and update my finding

[Ahmed Mourad]

So I ended up exposing Flow<PagingData<..>>, because it doesn't seem to be possible to map the items of a PagingSource directly.

However, this thing doesn't seem to be testable, whatsoever, if your PagingSource is created by Room. Is that the case?

[Dustin Lam]

What are you interested in testing?

[Dustin Lam]

we have a pretty in depth doc here: https://developer.android.com/topic/libraries/architecture/paging/test

[Dustin Lam]

in terms of transformations, you need to collect the state of PagingData into a presenter api like the differ

[Dustin Lam]

then use .snapshot(), we'll be adding that sample soon

[Ahmed Mourad]

This's a method of my `LocalDataSource`:

override fun findPosts(pageSize: Int): Flow<PagingData<Post>> {
    return Pager(
        config = PagingConfig(pageSize = pageSize, enablePlaceholders = false),
        pagingSourceFactory = { postsDao.findPosts() }
    ).flow.map { pagingData ->
           pagingData.map(PostEntity::toPost)
    }
}

findPosts is a Room method that returns PagingSource<Int, PostEntity> and
I'm exposing Flow<PagingData<Post>> since I'm unable to map a PagingSource<Int, PostEntity> to a PagingSource<Int, Post> .

I have a problem with both creating a fake of PagingData and with testing it.

I need to create a fake of LocalDataSource where I replace the method mentioned above with a fake version of it, the way to do that seems to be to create a PagingSource just for testing, which is more code than the actual code being tested.

The other problem is testing the PagingData flow, there seems to be no way to know what data PagingData contains, so even if I create a PagingSource , how do I know it contains the correct data and delivers them correctly?

[Ahmed Mourad]

I've seen the docs you mentioned but they assume I'm able to expose the PagingSource directly and that I own it, both of which are not the case.

[Dustin Lam]

As I mentioned earlier, you need to collect PagingData's state into a presenter API like a differ which you can make a simple no-op implementation of

[Dustin Lam]

For your PagingSource from Room, I would just use Room's test utils and get an in-memory impl from room you can control rather than mocking it