Swift Couple：多播（_:) 会为每个订阅者创建一个新主题

Question

在这段代码中，为什么当有 2 个订阅者时

debug: multicast triggered..

 只打印一次。既然苹果说

multicast(_:)

为每个订阅者创建一个新的发布者，那么打印语句不应该被触发两次吗？

import Foundation
import Combine

var cancellables = Set<AnyCancellable>()
let upstreamPublisher = Range(1...3).publisher.delay(for: 1, scheduler: DispatchQueue.main)

let sharedSubject = CurrentValueSubject<Int, Never>(-1)

// Create a multicast publisher and apply autoconnect
let multicastPublisher = upstreamPublisher
    .multicast({
        print("debug: multicast triggered..")
        return sharedSubject
    })
    .autoconnect()

// Subscriber 1
multicastPublisher
    .sink { value in
        print("Subscriber 1 received value: \(value)")
    }
    .store(in: &cancellables)

// Subscriber 2
multicastPublisher
    .sink { value in
        print("Subscriber 2 received value: \(value)")
    }
    .store(in: &cancellables)

// Output
//debug: multicast triggered..
//Subscriber 1 received value: -1
//Subscriber 2 received value: -1
//Subscriber 1 received value: 1
//Subscriber 2 received value: 1
//Subscriber 1 received value: 2
//Subscriber 2 received value: 2
//Subscriber 1 received value: 3
//Subscriber 2 received value: 3

Answer 1

经过多次测试，我没有看到它多次调用

multicast(_:)

 闭包。 （“每次订阅者附加到多播发布者时创建一个新主题”的整个概念似乎完全违背了多播的目的，即您只需要调用一次发布者的 

receive<S>(subscriber:)

 方法。）

在我看来，这看起来像是文档中的一个错误：

multicast(subject:)

和

multicast(_:)

之间的区别似乎很简单，一个接受一个

Subject

参数，一个接受一个闭包。

---

我个人会避免在这种情况下使用

autoconnect()

，而是推迟 

connect

，直到添加了所有订阅者：

var cancellables = Set<AnyCancellable>()
let upstreamPublisher = Range(1...3).publisher.delay(for: 1, scheduler: DispatchQueue.main)

// Create a multicast publisher and apply autoconnect
let multicastPublisher = upstreamPublisher
    .multicast {
        print("debug: multicast triggered..")
        return PassthroughSubject<Int, Never>()
    }

// Subscriber 1
multicastPublisher
    .sink { value in
        print("Subscriber 1 received value: \(value)")
    }
    .store(in: &cancellables)

// Subscriber 2
multicastPublisher
    .sink { value in
        print("Subscriber 2 received value: \(value)")
    }
    .store(in: &cancellables)

// Connect
multicastPublisher.connect()
    .store(in: &cancellables)

multicast

函数返回“可连接”发布者的要点是，您可以将连接推迟到代码中的某个未来点。

这也消除了对

CurrentValueSubject

 的需要（尽管如果需要，您应该随意使用它）。但是，这就是为什么我使用上面的

PassThroughSubject

。

结果：

debug: multicast triggered..
Subscriber 2 received value: 1
Subscriber 1 received value: 1
Subscriber 2 received value: 2
Subscriber 1 received value: 2
Subscriber 2 received value: 3
Subscriber 1 received value: 3