2个重要的Subject

Subjects 是一种遵循 Subject 协议的特殊的发布者。

这个协议要求subjects 有一个 .send(_:) 方法，来允许开发者发送特定的值给订阅者或管道。

Combine 中有两种内建的 subject : CurrentValueSubject 和 PassthroughSubject。

Example 1

PassthroughSubject的订阅者无法接收之前的消息

let relay = PassthroughSubject<String, Never>()

relay.send("Hello")
relay.send("World!")

let subscription = relay
    .sink { value in
        print("subscription1 received value: \(value)")
}

//relay.send("Hello")
//relay.send("World!")

//: 之前发送的2条消息无法被subscription接收

Example 2

发送新的subject给发布者，之前定义的subscription即可接收到消息

let publisher = ["Here","we","go!"].publisher

publisher.subscribe(relay)

输出：

subscription1 received value: Here
subscription1 received value: we
subscription1 received value: go!

Example 3

CurrentValueSubject发送的消息无论早晚都会全部发送给订阅者

let variable = CurrentValueSubject<String, Never>("")

variable.send("Initial text")

let subscription2 = variable.sink { value in
    print("subscription2 received value: \(value)")
}

variable.send("More text")

输出：

subscription2 received value: Initial text
subscription2 received value: More text

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