Understanding The RxJS Autorun Process

Hey, RxJS streamers! 🙋‍♂️

Today we're going to review a small library that re-evaluates an expression based on updates in streams it uses.

tl;dr: docs and package at github.com/kosich/rxjs-autorun 🔗

Let's boldly explore it!

It's a sweetened countdown

Our first example.

Say, we want to prettify each value on a timer stream. So we'll write such an expression:

import { timer } from 'rxjs';
import { computed, $ } from 'rxjs-autorun';

// timer would emit every second
const a = timer(0, 1_000);
// expression would concat every emission with a cake
const result = computed(() => $(a) + ' 🍰');
// now we should subscribe to the resulting stream
result.subscribe(x => console.log(x));
// > 0 🍰
// > 1 🍰
// > 2 🍰
// > …

Explanation: computed takes a function that uses some streams and re-evaluates it when those streams update. It returns an observable that you can manipulate further. And $(a) indicates that a is a stream and its updates should be listened to.

So technically this expression is equivalent to

a.pipe( map(x => x + '🍰') )

But let's keep discovering what else this tiny lib can do:

Infinite monkey theorem needs infinite bananas

Here we'll combine a timer that would represent a queue of our little 🐒 fellas with a stream of fetched bananas 🍌:

import { timer, of } from 'rxjs';
import { delay } from 'rxjs/operators';
import { computed, $ } from 'rxjs-autorun';

const a = timer(0, 1_000); // get some monkeys
const b = of('🍌').pipe(delay(2_000)); // fetch some treats
const result = computed(() => '🐒 #' + $(a) + ' gets ' + $(b)); // mix
result.subscribe(x => console.log(x)); // listen to result stream
// ...2s gap...
// > 🐒 #1 gets 🍌
// > 🐒 #2 gets 🍌
// > 🐒 #3 gets 🍌
// > …

Not hard at all, is it?

This expression is similar to

combineLatest(a, b).pipe( map(([x,y]) => x + y) )

Let's review another multiple streams example:

Who's up to some pizza?

The last trick we're gonna learn today is an ability to read the latest values without tracking their updates:

import { Subject } from 'rxjs';
import { computed, $, _ } from 'rxjs-autorun';

const a = new Subject(); // neighbours
const b = new Subject(); // food
computed(() => $(a) + ' likes ' + _(b))
  .subscribe(x => console.log(x));
a.next('🐈'); // nothing: b is still empty
b.next('🥛'); // > 🐈 likes 🥛
a.next('🐭'); // > 🐭 likes 🥛
b.next('🍕'); // nothing: _(b) doesn't trigger re-runs
a.next('🙋‍♂️'); // 🙋‍♂️ likes 🍕

Explanation:

function indicates that we need to take one value from a stream, but we don't want to recalculate our expression when this particular stream emits. So if an expression uses

$(a)

and

_(b)

— it would react only to updates.

This also means that computed(() => _(a)) expression would emit one value and immediately complete.

Okay, one really the last thing before we wrap-up:

Transformation

This time, try to guess what it is:

import { timer, of } from 'rxjs';
import { computed, $, _ } from 'rxjs-autorun';

const a = timer(0, 1_000);
const b = of('💧');
const c = of('❄');
const result = computed(() => $(a) % 2 ? _(b) : _(c));
result.subscribe(x => console.log(x));

Indeed, this is capricious weather 🙂

Actually, this expression is somewhat similar to switchMap

Outro 😳

All the examples you can try online.

And there's more to the library, go explore it yourself!

In the following articles, we'll review how to filter emissions and how to manage subscriptions inside rxjs-autorun expressions. Not to miss those and other RxJS posts — follow me here and on twitter!

Thank you for reading this article! Stay reactive and have a nice day 🙂

I want to thank fkrasnowski for lengthy discussions of this idea, ryansolid for giving it a try, and Johan for collaborating with me on this! 🙏

Psst.. need something more to read?

I got you covered:

See you!

Also published at https://dev.to/rxjs/rxjs-autorun-cop