Building a list from two observable sources in RxKotlin/RxJava using collectInto

Question

I have a Category data class and a Plan data class. Each Category has a list of plan ids. There are Categories and Plans stored via Room. I am trying to construct a local List<Any> where I add each category to a list, and then add each of it's plans.

So for each category, add the category to the list, then add each plan that belongs to that category.

The final result would look something like this...

0 -> a Category
1 -> a Plan
2 -> a Plan
3 -> a Plan
4 -> a Category
5 -> a Plan

etc.

The following calls sucessfully return an Observable<List<Category>> and an Observable<Plan>

AppDatabase
   .getDatabase(context)
   .categoryDao()
   .getAll()

AppDatabase.getDatabase(context).planDao().getPlan(planId)

Here I am trying to build my list, but it actually never emits when I subscribe to it. No completion, or error. Everything else in the stream get's hit. Why can't I get my final result?

    fun fetchCategoriesAndPlans() {
    val items = mutableListOf<Any>()
    AppDatabase
        .getDatabase(context)
        .categoryDao()
        .getAll()
        .concatMap { listOfCategories ->
            listOfCategories.toObservable()
        }
        .doOnNext { category ->
            items.add(category)
        }
        .concatMap { category ->
            category.getPlanIds()!!.toObservable()
        }
        .flatMap { planId ->
            AppDatabase.getDatabase(context).planDao().getPlan(planId)
        }.collectInto(items, BiConsumer{ list, i ->
            Log.d(TAG, "Collect into")
            list.add(i)
        })
        .subscribeBy(
            onSuccess = {
                Log.d(TAG, "Got the list")
            },
            onError = {
                Log.e(TAG, "Couldn't build list ${it.message}", it)
            })
}

The reason the code wouldn't work is the `collectInto()`. `collectInto()` does not emit anything until the source stream completes. `.categoryDao().getAll()` on Room returns an infinite stream which never completes. The given answer would work because the answer uses `Single`, instead of `Observable`. — Sanlok Lee, Jul 16 '19 at 07:26
`val items = mutableListOf(); doOnNext { items.add(it) }` wouldn't have worked. — EpicPandaForce, Jul 16 '19 at 09:32

score 1 · Accepted Answer · answered Jul 16 '19 at 06:38

I make a demo base on your case which help emit both Category and Plan

override fun onCreate(savedInstanceState: Bundle?) {
    ...

    getCategories()
        .flattenAsObservable { it }
        .flatMap { getPlanWithCategory(it) }
        .toList()
        .subscribe({
            for (item in it) {
                Log.i("TAG", " " + item.javaClass.canonicalName)
            }
        }, {

        })
}

fun getPlanWithCategory(category: Category): Observable<Any> {
    val getPlansObservable = Observable.fromArray(category.planIds).flatMapIterable {
        it
    }.flatMap {
        getPlan(it).toObservable()
    }
    return Observable.concat(Observable.just(category), getPlansObservable)
}


fun getPlan(planId: String): Single<Plan> {
    return Single.just(Plan())
}

fun getCategories(): Single<List<Category>> {
    val categories = arrayListOf<Category>()
    categories.add(Category(arrayListOf("1", "2", "3")))
    categories.add(Category(arrayListOf("1", "2")))
    return Single.just(categories)
}

class Category(val planIds: List<String>)

class Plan

Out put

 I/TAG:  Category
 I/TAG:  Plan
 I/TAG:  Plan
 I/TAG:  Category
 I/TAG:  Plan
 I/TAG:  Plan

Hope it help

Sorry. I don't know, I try to take your code and testing to find the problem but it always have combine error (and I can not fix it), then I make a new way base on my knowledge. — Linh, Jul 16 '19 at 07:24

Building a list from two observable sources in RxKotlin/RxJava using collectInto

1 Answers1