Converting loop with condition into RxJava stream

Question

I have code that does blocking operation in while loop (downloads some data from a server). Client does not know how many items are going to be returned in each step. Loop breaks when N items are downloaded.

val n = 10
val list = ArrayList<T>()

while (list.size < n) {
    val lastItemId = list.last()?.id ?: 0
    val items = downloadItems(lastItemId)
    list.addAll(items)
}

downloadItems performs blocking HTTP call and returns list. Now let's assume downloadItems changes and new return type is Observable<Item>. How could I change the code to use RxJava without performing something like blockingGet?

Anatolii · Accepted Answer · 2018-07-05 18:24:13Z

10

You could use repeatUntil to achieve this:

var totalItems = 0    
var id = 0
Observable.fromCallable {
            downloadItems(id)
        }
        .flatMap {
            list ->
                totalItems += list.size
                id = list.last()?.id ?: 0
                Observable.just(list)
        }
        .repeatUntil({totalItems > n})
        .subscribe({result -> System.out.println(result) })

edited Jul 5, 2018 at 18:24

answered Jul 5, 2018 at 8:21

Anatolii

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Comments

Tuby · Accepted Answer · 2018-07-05 15:26:47Z

1

I think this is elegant way

int n = 10;
Observable.range(0,n)
        .flatMap(i -> downloadItems(i))
        .toList()
        .subscribe(itemsList -> itemsList.forEach(item -> System.out.println(item)));

answered Jul 5, 2018 at 15:26

Tuby

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2 Comments

x2bool Over a year ago

That would call downLoadItems 10 times. I just want to download 10 items. Each call of downloadItems could return more than 0 items, exact number of items is unknown.

Tuby Over a year ago

I found question that is similar to yours stackoverflow.com/questions/50827046/…

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