我们什么时候应该使用Combine的多播autoconnect()

Question

我有这个代码

import Foundation
import Combine

let upstreamPublisher = Just("Hello, World!")
let sharedSubject = CurrentValueSubject<String, Never>("initial value")

// Create a multicast publisher and apply autoconnect
let multicastPublisher = upstreamPublisher
    .multicast(subject: sharedSubject)
    .autoconnect()

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

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

输出为：

Subscriber 1 received value: initial value
Subscriber 1 received value: Hello, World!

我从一篇文章中读到

The first time you subscribe to it, it connects to the upstream publisher and starts the work immediately. This is useful in scenarios where the upstream publisher emits a single value and you can use a CurrentValueSubject to share it with subscribers.

。第一部分对我来说有意义，但我不知道这句话的第二部分的真正含义。我们如何与当前值主题和自动连接共享结果？

Answer 1

我们如何与当前值主题和自动连接共享结果？

这已经是你的代码正在做的事情了！

我假设您希望订阅者 2 也收到初始值和“Hello, World!”，即这样的输出

Subscriber 1 received value: initial value
Subscriber 2 received value: initial value
Subscriber 2 received value: Hello, World!
Subscriber 1 received value: Hello, World!

这不会发生，因为

Just

同步发布。一旦您拨打

sink

，订阅者就会自动连接到

multicast

（因为

autoconnect

）。结果，

Just

发布了它的值并立即完成。当您添加第二个订阅者时，一切都已完成，并且不会收到任何内容。

您可以通过在

.print("Subscriber 2")

之前添加

sink

来看到这种情况的发生。输出是：

Subscriber 2: receive subscription: (Multicast)
Subscriber 2: request unlimited
Subscriber 2: receive finished

如果将上游变成不异步发布值的发布者，例如网络请求、计时器、

delay

等，然后两个订阅者都会获得值：

let upstreamPublisher = Just("Hello, World!")
    .delay(for: 1, scheduler: DispatchQueue.main)

确保您将

AnyCancellable

的引用存储在某处，否则发布者将在“Hello, World!”之前被取消。已发布！

---

当然，总是可以选择手动调用

connect

。