replay

确保观察者接收到同样的序列，即使是在 Observable 发出元素后才订阅

可被连接的 Observable 和普通的 Observable 十分相似，不过在被订阅后不会发出元素，直到 connect 操作符被应用为止。这样一来你可以控制 Observable 在什么时候开始发出元素。

replay 操作符将 Observable 转换为可被连接的 Observable，并且这个可被连接的 Observable 将缓存最新的 n 个元素。当有新的观察者对它进行订阅时，它就把这些被缓存的元素发送给观察者。

演示

let intSequence = Observable<Int>.interval(1, scheduler: MainScheduler.instance)
    .replay(5)

_ = intSequence
    .subscribe(onNext: { print("Subscription 1:, Event: \($0)") })

DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
    _ = intSequence.connect()
}

DispatchQueue.main.asyncAfter(deadline: .now() + 4) {
  _ = intSequence
      .subscribe(onNext: { print("Subscription 2:, Event: \($0)") })
}

DispatchQueue.main.asyncAfter(deadline: .now() + 8) {
  _ = intSequence
      .subscribe(onNext: { print("Subscription 3:, Event: \($0)") })
}

输出结果：

Subscription 1:, Event: 0
Subscription 2:, Event: 0
Subscription 1:, Event: 1
Subscription 2:, Event: 1
Subscription 1:, Event: 2
Subscription 2:, Event: 2
Subscription 1:, Event: 3
Subscription 2:, Event: 3
Subscription 1:, Event: 4
Subscription 2:, Event: 4
Subscription 3:, Event: 0
Subscription 3:, Event: 1
Subscription 3:, Event: 2
Subscription 3:, Event: 3
Subscription 3:, Event: 4
Subscription 1:, Event: 5
Subscription 2:, Event: 5
Subscription 3:, Event: 5
Subscription 1:, Event: 6
Subscription 2:, Event: 6
Subscription 3:, Event: 6
...