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

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

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

7. DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
8. _ = intSequence.connect()
9. }

11. DispatchQueue.main.asyncAfter(deadline: .now() + 4) {
12. _ = intSequence
13. .subscribe(onNext: { print("Subscription 2:, Event: \($0)") })
14. }

16. DispatchQueue.main.asyncAfter(deadline: .now() + 8) {
17. _ = intSequence
18. .subscribe(onNext: { print("Subscription 3:, Event: \($0)") })
19. }

输出结果：

1. Subscription 1:, Event: 0
2. Subscription 2:, Event: 0
3. Subscription 1:, Event: 1
4. Subscription 2:, Event: 1
5. Subscription 1:, Event: 2
6. Subscription 2:, Event: 2
7. Subscription 1:, Event: 3
8. Subscription 2:, Event: 3
9. Subscription 1:, Event: 4
10. Subscription 2:, Event: 4
11. Subscription 3:, Event: 0
12. Subscription 3:, Event: 1
13. Subscription 3:, Event: 2
14. Subscription 3:, Event: 3
15. Subscription 3:, Event: 4
16. Subscription 1:, Event: 5
17. Subscription 2:, Event: 5
18. Subscription 3:, Event: 5
19. Subscription 1:, Event: 6
20. Subscription 2:, Event: 6
21. Subscription 3:, Event: 6
22. ...