Convert Promise to Observable

I am trying to wrap my head around observables. I love the way observables solve development and readability issues. As I read, benefits are immense.

Observables on HTTP and collections seem to be straight forward. How can I convert something like this to observable pattern.

This is from my service component, to provide authentication. I'd prefer this to work like other HTTP services in Angular2 - with support for data, error and completion handlers.

firebase.auth().createUserWithEmailAndPassword(email, password)
  .then(function(firebaseUser) {
    // do something to update your UI component
    // pass user object to UI component
  })
  .catch(function(error) {
    // Handle Errors here.
    var errorCode = error.code;
    var errorMessage = error.message;
    // ...
  });

Any help here would be much appreciated. The only alternative solution I had was to create EventEmitters. But I guess that's a terrible way to do things in services section

If you are using RxJS 6.0.0:

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

try this:

import 'rxjs/add/observable/fromPromise';
import { Observable } from "rxjs/Observable";

const subscription = Observable.fromPromise(
    firebase.auth().createUserWithEmailAndPassword(email, password)
);
subscription.subscribe(firebaseUser => /* Do anything with data received */,
                       error => /* Handle error here */);

you can find complete reference to fromPromise operator here.

1 Direct Execution / Conversion

Use from to directly convert a previously created Promise to an Observable.

import { from } from 'rxjs';

// getPromise() is called once, the promise is passed to the Observable
const observable$ = from(getPromise());

observable$ will be a hot Observable that effectively replays the Promises value to Subscribers.

It's a hot Observable because the producer (in this case the Promise) is created outside of the Observable. Multiple subscribers will share the same Promise. If the inner Promise has been resolved a new subscriber to the Observable will get its value immediately.

2 Deferred Execution On Every Subscribe

Use defer with a Promise factory function as input to defer the creation and conversion of a Promise to an Observable.

import { defer } from 'rxjs';

// getPromise() is called every time someone subscribes to the observable$
const observable$ = defer(() => getPromise());

observable$ will be a cold Observable.

It's a cold Observable because the producer (the Promise) is created inside of the Observable. Each subscriber will create a new Promise by calling the given Promise factory function.

This allows you to create an observable$ without creating and thus executing a Promise right away and without sharing this Promise with multiple subscribers. Each subscriber to observable$ effectively calls from(promiseFactory()).subscribe(subscriber). So each subscriber creates and converts its own new Promise to a new Observable and attaches itself to this new Observable.

3 Many Operators Accept Promises Directly

Most RxJS operators that combine (e.g. merge, concat, forkJoin, combineLatest ...) or transform observables (e.g. switchMap, mergeMap, concatMap, catchError ...) accept promises directly. If you're using one of them anyway you don't have to use from to wrap a promise first (but to create a cold observable you still might have to use defer).

// Execute two promises simultaneously
forkJoin(getPromise(1), getPromise(2)).pipe(
  switchMap(([v1, v2]) => v1.getPromise(v2)) // map to nested Promise
)

Check the documentation or implementation to see if the operator you're using accepts ObservableInput or SubscribableOrPromise.

type ObservableInput<T> = SubscribableOrPromise<T> | ArrayLike<T> | Iterable<T>;
// Note the PromiseLike ----------------------------------------------------v
type SubscribableOrPromise<T> = Subscribable<T> | Subscribable<never> | PromiseLike<T> | InteropObservable<T>;

The difference between from and defer in an example: https://stackblitz.com/edit/rxjs-6rb7vf

const getPromise = val => new Promise(resolve => {
  console.log('Promise created for', val);
  setTimeout(() => resolve(`Promise Resolved: ${val}`), 5000);
});

// the execution of getPromise('FROM') starts here, when you create the promise inside from
const fromPromise$ = from(getPromise('FROM'));
const deferPromise$ = defer(() => getPromise('DEFER'));

fromPromise$.subscribe(console.log);
// the execution of getPromise('DEFER') starts here, when you subscribe to deferPromise$
deferPromise$.subscribe(console.log);

The correct pattern to transform a promise into an observable is using defer and from operators:

import { defer, from } from 'rxjs';
    
const observable$ = defer(() => from(myPromise()));

Why we need the defer operator?

Promises are eager, this means that when called they fire instantly. This is the opposite from how observables work. Observables are lazy, they are only fired when .subscribe() is called. This is the reason we need to always wrap it into a defer operator. The from operator doesn't do this work, so defer is always needed.