Observable 中的 Async/Await

Question

How can I use async/await inside the Observable?? With this code I'm unable to trigger the unsubscribe function within observable thus interval is not cleared.

const { Observable } = require("rxjs");

const test = () => new Observable(async (subscriber) => {
  await Promise.resolve();

  const a = setInterval(() => {
    subscriber.next(Math.random());
    console.log("zz");
  }, 500);

  return () => {
    console.log("asdsad");
    clearInterval(a);
  };
});

const xyz = test().subscribe(console.log);


setTimeout(() => {
  xyz.unsubscribe();
}, 3000);

Answer 1

Async/Await inside an observable is not supported. However, it can be done with a behavior subject and an asynchronous nested function.

Create a behavior subject, convert it to an observable ( .asObservable() ), execute the asynchronous nested function, return the observable. Here's an example.

function getProgress() {

    // Change this value with latest details
    const value = new BehaviorSubject('10%');
    const observable = value.asObservable();

    // Create an async function
    const observer = async() => {
      // Perform all tasks in here
      const wait1 = await new Promise(resolve => setTimeout(resolve, 3000));
      value.next('66%');

      const wait2 = await new Promise(resolve => setTimeout(resolve, 3000));
      value.next('100%');

      // Complete observable
      value.complete();
    }

    // Call async function & return observable
    observer();
    return observable;
  }

It's very readable and works like a charm.

Answer 2

First of all, subscriber passed to observable contructor cannot be async function. There is no support for that.

If you need to create observable from promise, use from :

import { from } from 'rxjs';
const observable = from(promise);

But considering your scenario.

Because there is no way to cancel native js promise, you cannot realy unsubscribe from such created observable, so:

const obs = from(new Promise(resolve => {
  setTimeout(() => {
    console.log('gonna resolve');
    resolve('foo');
  }, 1000);
}));

const sub = obs.subscribe(console.log);
setTimeout(() => sub.unsubscribe(), 500);

will print:

gonna resolve
gonna resolve
gonna resolve
(...)

so yeah: gonna resolve will be printed in the cosole all the time, but nothing more - result passed to resolve will be ignored - just not logged.

From the other hand, if you remove that unsubscribtion ( setTimeout(() => sub.unsubscribe(), 500); ) this time you will see:

gonna resolve
foo
gonna resolve
gonna resolve
gonna resolve
(...)

There is one way that maybe will help you - defer - but it's not strictly related with your question.

import { defer } from 'rxjs';

defer(async () => {
  const a = await Promise.resolve(1);
  const b = a + await Promise.resolve(2);

  return a + b + await Promise.resolve(3);
}).subscribe(x => console.log(x)) // logs 7