RxSwift merge different kind of Observables

Question

How should I merge 2 different types of Observables in RxSwift?

For example:

var a: Observable<Int>
var b: Observable<Void>

Observable.of(a,b).merge() is not possible because of type parameter difference.

Libau

Answer 1

To merge them, they need to have the same type for their Element.

So, one option is to throw away their type information and cast to AnyObject. Now they can be merged:

let stringSubject = PublishSubject<String>()
let stringObservable = stringSubject.asObservable().map { $0 as AnyObject }

let intSubject = PublishSubject<Int>()
let intObservable = intSubject.asObservable().map { $0 as AnyObject }

Observable.of(stringObservable, intObservable).merge()
    .subscribeNext { print($0) }
    .addDisposableTo(disposeBag)

stringSubject.onNext("a")
stringSubject.onNext("b")
intSubject.onNext(1)
intSubject.onNext(2)
stringSubject.onNext("c")

Output:

a
b
1
2
c

Another option would be to wrap then in an enum:

enum Container {
    case S(String)
    case I(Int)
}

let stringSubject = PublishSubject<String>()
let stringObservable = stringSubject.asObservable().map { Container.S($0) }

let intSubject = PublishSubject<Int>()
let intObservable = intSubject.asObservable().map { Container.I($0) }

Observable.of(stringObservable, intObservable).merge()
    .subscribeNext { e in
        switch e {
        case .S(let str):
            print("next element is a STRING: \(str)")
        case .I(let int):
            print("next element is an INT: \(int)")
        }
    }
    .addDisposableTo(disposeBag)

stringSubject.onNext("a")
stringSubject.onNext("b")
intSubject.onNext(1)
intSubject.onNext(2)
stringSubject.onNext("c")

Output:

next element is a STRING: a
next element is a STRING: b
next element is an INT: 1
next element is an INT: 2
next element is a STRING: c

As for the other operators that can combine Observables of varying types (like zip and combineLatest), none work quite like merge. However, check those out. They might be better suited to your requirements.

Viyella

Answer 2

In case you want to merge two observables but you don't need the value of Element and want a way to get rid of the Expression of type 'AnyObject' is unused when using the .map { $0 as AnyObject } option:

var a = PublishSubject<Int>()
var b = PublishSubject<Void>()

let observable = Observable.merge(a.map { _ in return Void() }, b)

Twink