Combine<Int,Never>.swift

//
/* 
 *		Created by 游宗諭 in 2021/9/2
 *		
 *		Using Swift 5.0
 *		
 *		Running on macOS 12.0
 */


import Foundation

// MARK: - Interface Define
protocol MySubscription {
    func askValue()
}

protocol MyPublisher {
    func register(to subscriber: MySubscriber)
}

enum AskAgain {
    case yes, no
}
protocol MySubscriber {
    func takeAction(for value: Int) -> AskAgain
    func register(to subscription: MySubscription)
}

// MARK: - Test Helper

class PlainPublisher: MyPublisher {
    func register(to subscriber: MySubscriber) {
        let mySubscription = StubSubscription(mySubscriber: subscriber)
        subscriber.register(to: mySubscription)
    }
}

class StubSubscription: MySubscription {
    
    let mySubscriber: MySubscriber
    var value = 0
    init(mySubscriber: MySubscriber) {
        self.mySubscriber = mySubscriber
    }
    func askValue() {
        while true {
            value += 1
            let askAgain = mySubscriber.takeAction(for: value)
            if askAgain == .no {break}
            
        }
    }
}


class StubSubscriber: MySubscriber {
    var _takeAction: (Int) -> Void
    var askAgain:() -> AskAgain
    
    internal init(_takeAction: @escaping (Int) -> Void, askAgain: @escaping () -> AskAgain) {
        self._takeAction = _takeAction
        self.askAgain = askAgain
    }

    func takeAction(for value: Int) -> AskAgain {
        _takeAction(value)
        return askAgain()
    }
    func register(to subscription: MySubscription) {
        subscription.askValue()
    }
}

// MARK: - TestCase
import XCTest

final class CombineProtocolsTests: XCTestCase {
    func testShowHowTheCombineProtocolsInteract() throws {
        var history:[Int] = []
        let publisher = PlainPublisher()
        var count = 0
        let scriber = StubSubscriber {
            history.append($0)
        } askAgain: {
            count += 1
            return count < 3 ? .yes : .no
        }
        
        publisher.register(to: scriber)
        XCTAssertEqual(history, [1, 2, 3])
    }
}

Idea from Open Combine Just

https://github.com/OpenCombine/OpenCombine/blob/master/Sources/OpenCombine/Publishers/Just.swift