Making custom Deffered Future Publisher in Swift Combine?-CodePudding

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?

CodePudding user response：

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)
        }
    }
}