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

replay - 图1

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. ...