Store: Problems when working with a List

I’m trying to setup a pretty basic Store that works with a List<T> that is backed by a (Room) SoT, but I am running into a few problems, outlined below. The crux of the problem is that a call to the store’s get() method never returns in some cases when the list is empty.

My setup is fairly straight forward:

val myStore = StoreBuilder.from(
        fetcher = Fetcher.of { postType: PostType ->

            // Retrofit api call that returns a list of posts
            val response = myApi.getPostsByType(postType) 

            // convert the api response json into a List<Post>
            return when {
                response.isSuccessful -> Post.fromWordpress(response.body())
                else -> listOf()
            }
        },
        sourceOfTruth = SourceOfTruth.of(
            reader = { postType ->
                //Dao method returns a Flow<List<Post>> from the "posts" table
                db.postsDao().postsByTypeObservable(postType)
            },
            writer = { postType, postList ->
                // writing the saved list to the Room DB is a bit more complicated, 
                // because we have some many-many relationships with categories...
                db.postsDao().withTransaction {
                    val posts = postList.map { it.post }
                    val categories = postList.flatMap { it.categories }.distinct()
                    val postCategories = postList.flatMap { it.postCategories }.distinct()

                    db.postsDao().upsert(posts)
                    db.categoriesDao().upsert(categories)
                    db.postCategoriesDao().upsert(postCategories)
                }
            },
            delete = db.postsDao()::deletePostsByType,
            deleteAll = db.postsDao()::deleteAllPosts
        )
    ).build()

This setup works OK, unless/until there are no posts to work with. I’ve specifically run into 2 problems:

1. If you call get() with this setup and there is no data in the SoT, the fetcher will not be called.

From reading the source code, it appears that the Fetcher only gets called if the reader returns null. But when working with a list, the reader will return an empty list, and not null. Because of that, the fetcher never gets called.

We can actually fix this problem fairly easily by converting an empty list to null in the reader:

 reader = { postType ->
     //Dao method returns a Flow<List<Post> from the "posts" table
     db.postsDao().postsByTypeObservable(postType).map { entries ->
         when {
             entries.isEmpty() -> null
             else -> entries
     }
 },

Now, an empty list is treated as null, and thus the fetcher gets called.

1. But now we have a second problem. If you call get() and there are no entries in the SoT, and the Fetcher also returns no entries, the suspended get() method will never finish/complete.

That is because once the fetcher finishes up, there are no new entries to put into the database. The get() is ultimately (via a stream) observing the Flow from the reader, and that flow will not emit anything (since there were no writes). Because of that, there is never any value emitted on the Flow (due to the filters setup in the get()) method. And since there is no value ever emitted, the suspended get() method never finishes/completes.

Is there a way to setup a Store that works with a List<T> and ensure that the get() method will always return?