Making an Observable from a callback

Question

I have an auth guard that needs an asynchronous response true/false when the site is visited and the user is already logged in.

I'm using Firebase's onAuthStateChanged (link to docs) and it uses a callback function. How can I turn my isLoggedIn() method into something that can return Observable<boolean>?

Typscript:

get isLoggedIn(): Observable<boolean> {

    // want something like this:
    return Observable.fromCallback(firebase.auth().onAuthStateChanged).map(user => !!user);

    // this returns () => boolean, but I need a promise or observable
    return firebase
      .auth()
      .onAuthStateChanged((user) => {
        return !!user;
      });

}

Answer 1

Note: bindCallback and Observable.create were deprecated,
you could use new Observable instead:

...
return new Observable((subscriber) => {
  firebase
    .auth()
    .onAuthStateChanged((user) => {
      subscriber.next(user);
    })
}

Answer 2

You can do it like this.

get isLoggedIn(): Observable<boolean> {

  // want something like this:
  return Observable.create(
    observer => firebase
        .auth()
        .onAuthStateChanged((user) => {
          observer.next(!!user)
        });
    );
}