Rxjs create an observable that emits when a function is called

Question

I'm likely missing something simple but can't quite puzzle this out from the docs. I'd simply like to have a function that when called will emit a value on an rxjs observable.

Psuedocode:

const myFunction = () => true //or whatever value
const myObservable = ... emit true whenever myFunction is called

...use myFunction somewhere where it being called is a useful event so 
observers of myObservable can process it

What's the standard pattern to emit a value when a function is called with rxjs?

[bindCallback](https://rxjs-dev.firebaseapp.com/api/index/function/bindCallback) might be helpful. — Andrei Gătej, Jun 03 '20 at 07:56

score 4 · Accepted Answer · answered Jun 03 '20 at 05:33

You need a Subject for that https://www.learnrxjs.io/learn-rxjs/subjects

Here is the reworked example from the documentation:

import { Subject } from 'rxjs';

const myObservable = new Subject<number>();

const myFunction = () => {
  subject.next(1);
  subject.next(2);
}

myObservable.subscribe({
  next: (v) => console.log(`observerA: ${v}`)
});
myObservable.subscribe({
  next: (v) => console.log(`observerB: ${v}`)
});

// ...use myFunction somewhere where it being called is a useful event so 
myFunction();

Normally, you don't even need myFunction. Calling subject.next() would be enough.

score 1 · Answer 2 · answered Jun 03 '20 at 05:31

You need to use Subject.


const subject = new Subject(); // a subject to notify
const myObservable = subject.asObservable();

const myFunction = () => {
  subject.next(true);
};

myObservable.subscribe(console.log);

myFunction();
myFunction();
myFunction();

Rxjs create an observable that emits when a function is called

2 Answers2