PublishReplay 如何工作

它在内部创建了一个 ReplaySubject 并使其与 multicast 兼容。ReplaySubject 的最小重放值是 1 发射。这导致以下结果：

• 首次订阅将触发 publishReplay(1) 内部订阅源流并通过 ReplaySubject 管理所有排放，有效缓存最后 n （= 1）个排放
• 如果在源仍处于活动状态时启动第二个订阅，则 multicast() 将我们连接到相同的 replaySubject，我们将接收所有下一个排放，直到源流完成。
• 如果在源已经完成之后启动订阅，则 replaySubject 已缓存最后 n 个排放，并且它将仅在完成之前接收那些。

const source = Rx.Observable.from([1,2])
  .mergeMap(i => Rx.Observable.of('emission:'+i).delay(i * 100))
  .do(null,null,() => console.log('source stream completed'))
  .publishReplay(1)
  .refCount();

// two subscriptions which are both in time before the stream completes
source.subscribe(val => console.log(`sub1:${val}`), null, () => console.log('sub1 completed'));
source.subscribe(val => console.log(`sub2:${val}`), null, () => console.log('sub2 completed'));

// new subscription after the stream has completed already
setTimeout(() => {
  source.subscribe(val => console.log(`sub_late-to-the-party:${val}`), null, () => console.log('sub_late-to-the-party completed'));
}, 500);

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