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kotlinlang

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.

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.

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

LimitOffsetDataSource from Room is in room-runtime though

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

So I ended up exposing `Flow&lt;PagingData&lt;..&gt;&gt;`, 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?

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

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

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

This's a method of my `LocalDataSource`:

```override fun findPosts(pageSize: Int): Flow&lt;PagingData&lt;Post&gt;&gt; {
    return Pager(
        config = PagingConfig(pageSize = pageSize, enablePlaceholders = false),
        pagingSourceFactory = { postsDao.findPosts() }
    ).flow.map { pagingData -&gt;
           pagingData.map(PostEntity::toPost)
    }
}```
`findPosts` is a Room method that returns `PagingSource&lt;Int, PostEntity&gt;` and
I'm exposing `Flow&lt;PagingData&lt;Post&gt;&gt;` since I'm unable to map a `PagingSource&lt;Int, PostEntity&gt;` to a `PagingSource&lt;Int, Post&gt;` .

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?

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.

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

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