CODEWITHSUNDEEP
swiftreactive-programmingrx-swift
HitMan
HitMan

Reputation: 195

RxSwift - How to concatenate observables of collection

I'm trying to concatenate two observables of collection with concat() operator, however, it's not working as expected.

I've two observables:

let first = Observable<[Int]>.create { observer in
    observer.onNext([1, 2])
    observer.onCompleted()
    return Disposables.create()
}

let second = PublishSubject<[Int]>()

Using concat():

let items = Observable.concat([first, second])

items.subscribe(onNext: {
    print($0)
})

second.onNext([3, 4, 5])

Output:

[1, 2]
[3, 4, 5]

What I want:

[1, 2]
[1, 2, 3, 4, 5]

Upvotes: 3

Views: 2892

Answers (1)

Daniel T.
Daniel T.

Reputation: 33967

So you don't just want to concat two observables, you also want to concat the arrays that are in the events that are produced by the two observables. You aren't going far enough to get what you want.

Imagine you had two Array<[Int]> instead of two Observable<[Int]>. Concat-ing the two (as in arr1 + arr2) would not produce [[1, 2], [1, 2, 3, 4, 5]], instead it would produce [[1, 2], [3, 4, 5]]. Your Observables are behaving the same way.

To both concat and combine them, you need scan, as in:

let items = Observable.concat([first, second])
    .scan([], accumulator: +)

Which will produce two events:

[1, 2]
[1, 2, 3, 4, 5]

FYI, with arrays, we don't have a scan operator, but we can approximate it with reduce. For arrays, it would be:

let arr1: Array<[Int]> = [[1, 2]]
let arr2: Array<[Int]> = [[3, 4, 5]]
let itemsArr = (arr1 + arr2).reduce([], { result, element in
    return result + [(result.last ?? []) + element]
})
print(itemsArr)

Upvotes: 2

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