Is it possible to make `ReplaySubject` to run a closure on being subscribed to?

Question

I want to create a cold observable that would only start doing expensive operation if there is an actual subscription. ReplaySubject would fit nicely except for the part that I need to be able to start an expensive background operation when the actual subscription is made and not on create of the observable. Is there a way to do so? Some sort of onSubscribed { ... } method.

Answer 1

Here are a couple of options:

Adding the expensive operation to a doOn(onSubscribe:) that's in between the Observable and the subscription:

let observable = Observable.of(1, 2)
    .doOn(onSubscribe: { _ in
        expensiveOperation()
    })

observable
    .subscribeNext { e in
        print(e)
    }

Making the Observable connectable and separating the doOn(onSubscribe:):

let observable = Observable.of(1, 2)
    .publish()

observable
    .doOn(onSubscribe: { _ in
        expensiveOperation()
    })
    .subscribe()

observable
    .subscribeNext { e in
        print(e)
    }

observable.connect()