Combine 基本概念

public protocol Publisher {
    // 1
    associatedtype Output

    // 2
    associatedtype failure: Error

    // 4
    func receive<S>(subscriber: S) where S: Subscriber, Self.Failure == S.Failure, Self.Output == S.Input
}

extension Publisher {
    // 3
    public func subscribe<S>(_ subscriber: S) where S: Subscriber, Self.Failure == S.Failure, Self.Output == S.Input 
}

public protocol Subscriber: CustomCombineIdentifierConvertible {
    // 1
    associatedtype Input

    // 2
    associatedtype Failure: Error

    // 3
    func receive(subscription: Subscription)

    // 4
    func receive(_ input: Self.Input) -> Subscribers.Demand

    // 5
    func receive(completion: Subscribers.Completion<Self.Failure>)    
}

public protocol Subscription: Cancellable, CustomCombineIdentifierconvertible {
    func request(_ demand: Subscribers.Demand)
}

// 1 
let publisher = (1...6).publisher

// 2
final class IntSubscriber: Subscriber {
    // 3
    typealias Input = Int
    typealias Failuse = Never

    // 4
    func reveive(subscription: Subscription) {
        subscription.request(.max(3))
    }

    // 5
    func receive(_ input: Int) -> Subscribers.Demand {
        print("Recevied value", input)
        // return .unlimited
        return .none // 等于 .max(0)
    }

    // 6
    func receive(completion: Subscribers.Completion<Never>) {
        print("Received completion", completion)
    }
}

// 1
var subscriptions = Set<AnyCancellable>()

// 2
let subject = CurrentValueSubject<Int, Never>(0)

// 3
subject
    .sink(receiveValue: {print($0)})
    .store(in: &subscriptions) // 4 

// 1
let subject = PassthroughSubject<Int, Never>

// 2
let publisher = subject.eraseToAnyPublisher()

// 3
publisher
    .sink(receiveValue {print($0)})
    .store(in: &subscriptions)

// 4
subject.send(0)