Making custom Deffered Future Publisher in Swift Combine?

Question

Like the title says I would like to make custom publisher that will basically function like deffered future. Normally when I want to encapsulate code in some Future, but want it to execute on subscription, I would need to write something like this:

Deffered {
    Future { promise in
    }
}

Now I was thinking of making custom publisher, something along the lines DefferedFuture that will have exact same functionality as Future, but will execute promise only on subscription?

Since that is exactly what deferred future does already, what new functionality do you need? — matt, Dec 08 '21 at 21:29
I want to write it as one publisher, not Deffered { Future {} }, just NewDefferedFuture {}. — MegaManX, Dec 08 '21 at 22:03

score 2 · Accepted Answer · answered Dec 09 '21 at 22:26

The most obvious answer is this:

func single<Output, Failure>(_ promise: @escaping (@escaping (Result<Output, Failure>) -> Void) -> Void) -> Deferred<Future<Output, Failure>> where Failure: Error {
    Deferred {
        Future<Output, Failure>(promise)
    }
}

If it absolutely must be a type rather than a function then:

extension Publishers {
    struct Single<Output, Failure>: Publisher where Failure: Error {
        let promise: (@escaping (Result<Output, Failure>) -> Void) -> Void
        
        func receive<S>(subscriber: S) where S: Subscriber, Failure == S.Failure, Output == S.Input {
            Deferred { Future(promise) }
            .subscribe(subscriber)
        }
    }
}

For a more generic solution, you can use [Ian Keen's AnyPubliser extension](https://gist.github.com/IanKeen/934258d2d5193160391e14d25e54b084) which will allow you to emit more than one value before completing. Something Future doesn't allow. — Daniel T., Dec 10 '21 at 00:00

Making custom Deffered Future Publisher in Swift Combine?

1 Answers1