AsyncSubject

最后，我们将看到另一个类型的 Subject -- AsyncSubject，观察了 AsyncSubject 的 Observer，只会在 AsyncSubject 完成时才能观测到值：

const subject = new Rx.BehaviorSubject(0);

const observerA = {
  next: x => console.log(`A next ${x}`),
  error: e => console.error(`A error ${e}`),
  complete: () => console.log('A done')
};

subject.subscribe(observerA);
console.log('observerA subscribed');

const observerB = {
  next: x => console.log(`B next ${x}`),
  error: e => console.error(`B error ${e}`),
  complete: () => console.log('B done')
};

setTimeout(() =>subject.next(1), 100);
setTimeout(() =>subject.next(2), 200);
setTimeout(() =>subject.next(3), 300);

setTimeout(() => {
  subject.subscribe(observerB);
  console.log('observerB subscribed');
}, 400);

/* 程序输出：
  "observerA subscribed"
  "A next 3"
  "A done"
  "B next 3"
  "B done"
  "observerB subscribed"
 */

查看例子

AsyncSubject 就和他的名字一样，类似于同样用来处理异步任务的 Promise。在 Promise 中，我们可以通过 resolve(value) 声明任务完成，并将获得的值发送出去，然后再 Promise.then() 方法中处理得到的值。