Combine Publisher sending one value on each subscription. Unlike Just, the value is determined at subscription time.

DeferredJust.swift

//    MIT License
//
//    Copyright (c) 2020 Manuel Maly
//    Permission is hereby granted, free of charge, to any person obtaining a copy
//    of this software and associated documentation files (the "Software"), to deal
//    in the Software without restriction, including without limitation the rights
//    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
//    copies of the Software, and to permit persons to whom the Software is
//    furnished to do so, subject to the following conditions:
//
//    The above copyright notice and this permission notice shall be included in all
//    copies or substantial portions of the Software.
//
//    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
//    SOFTWARE.

import Combine
import Foundation

public struct DeferredJust<Output>: Publisher {

    public typealias Failure = Never

    let value: () -> Output

    public func receive<S: Subscriber>(subscriber: S) where S.Input == Output, S.Failure == Never {
        subscriber.receive(
            subscription: Sub(subscriber, value: value)
        )
    }

    private class Sub<Downstream: Subscriber>: Subscription where Downstream.Input == Output, Downstream.Failure == Never {

        private var downstream: Downstream?
        private let value: () -> Output

        init(_ downstream: Downstream, value: @escaping () -> Output) {
            self.downstream = downstream
            self.value = value
        }

        func request(_ demand: Subscribers.Demand) {
            guard demand > 0, let downstream = downstream else {
                cancel()
                return
            }
            _ = downstream.receive(value())
            downstream.receive(completion: .finished)
        }

        func cancel() {
            downstream = nil
        }
    }
}