RxSwift 系列(七) -- Connectable Operators

前言

本篇文章将要学习RxSwift中连接操作符。
Connectable Observable在订阅时不发射事件消息，而是仅当调用它们的connect()方法时才发射消息，这样就可以等待所有我们想要的订阅者都已经订阅了以后，再开始发出事件消息，这样能保证我们想要的所有订阅者都能接收到事件消息。其实也就是等大家都就位以后，开始发出消息。

在开始学习Connectable Observable之前，让我们来看一个non-connectable operator:

let intervar = Observable<Int>.interval(1, scheduler: MainScheduler.instance)

_ = intervar.subscribe(onNext: {print("subscribe:1, event:($0)")})

delay(5, closure: { (_) in
        _ = intervar.subscribe(onNext: {print("subscribe:2, event:($0)")})
    })

Connectable Observable在RxSwift中包括了：

• publish
• replay
• multicast

publish

将一个源Observable sequence转变为一个connectable sequence。

let intervar = Observable<Int>.interval(1, scheduler: MainScheduler.instance).publish()

_ = intervar.subscribe(onNext: {print("subscribe:1, event:($0)")})

_ = intervar.subscribe(onNext: {print("subscribe:2, event:($0)")})
    
intervar.connect()

你可以试试把intervar.connect()注释掉，会发现并没有任何响应。

replay

将一个正常的sequence转换成一个connectable sequence，然后和replaySubject相似，能接收到订阅之前的事件消息。

let intSequence = Observable<Int>.interval(1, scheduler: MainScheduler.instance)
        .replay(5)
    
_ = intSequence
    .subscribe(onNext: { print("Subscription 1:, Event: ($0)") })
    
delay(2) { _ = intSequence.connect() }
    
delay(4) {
    _ = intSequence
        .subscribe(onNext: { print("Subscription 2:, Event: ($0)") })
}
    
delay(8) {
    _ = intSequence
        .subscribe(onNext: { print("Subscription 3:, Event: ($0)") })
}

multicast

将一个正常的sequence转换成一个connectable sequence，并且通过特性的subject发送出去，比如PublishSubject，或者replaySubject，behaviorSubject等。不同的Subject会有不同的结果。

let subject = PublishSubject<Int>()
    
_ = subject
    .subscribe(onNext: { print("Subject: ($0)") })
    
let intSequence = Observable<Int>.interval(1, scheduler: MainScheduler.instance)
    .multicast(subject)
    
_ = intSequence
    .subscribe(onNext: { print("	Subscription 1:, Event: ($0)") })
    
delay(2) { _ = intSequence.connect() }
    
delay(4) {
    _ = intSequence
        .subscribe(onNext: { print("	Subscription 2:, Event: ($0)") })
}
    
delay(6) {
    _ = intSequence
        .subscribe(onNext: { print("	Subscription 3:, Event: ($0)") })
}

致谢

如果发现有错误的地方，欢迎评论，感谢！