Wsrtjtyk

I'm trying to convert an Observable into a BehaviorSubject. Like this:

a$ = new Observable()

b$ = BehaviorSubject.create(new BehaviorSubject(123), a$)

// 🔴

I have also tried:

a$ = new Observable()

b$ = new BehaviorSubject(a$, 123)

// 🔴

And:

a$ = new Observable()

b$ = a$.asBehaviorSubject(123)

// 🔴

And:

a$ = new Observable()

b$ = a$.pipe(

  toBehaviorSubject(123)

)

// 🔴

But none of these works. For now I have to implement like this:

a$ = new Observable()

b$ = new BehaviorSubject(123)

a$.subscribe(b$)

// 🔵

This would be a little bit ugly in a class:

class Foo() {

  a$ = new Observable() // Actually, a$ is more complicated than this.

  b$ = new BehaviorSubject(123)



  constructor() {

    this.a$.subscribe(this.b$)

  }

}

So, is there a simpler way to convert a Observable to a BehaviorSubject without using class constructor?

This is my real case:

export class Foo {

  autoCompleteItems$ = new BehaviorSubject<string>(null)

  autoCompleteSelected$ = new BehaviorSubject<number>(-1)

  autoCompleteSelectedChange$ = new Subject<'up'|'down'>()



  constructor() {

    this.autoCompleteItems$.pipe(

      switchMap((items) => {

        if (!items) return EMPTY

        return this.autoCompleteSelectedChange$.pipe(

          startWith('down'),

          scan<any, number>((acc, value) => {

            if (value === 'up') {

              if (acc <= 0) {

                return items.length - 1

              } else {

                return acc - 1

              }

            } else {

              if (acc >= items.length - 1) {

                return 0

              } else {

                return acc + 1

              }

            }

          }, -1)

        )

      })

    ).subscribe(this.autoCompleteSelected$)

  }



  doAutoComplete = () => {

    const item = this.autoCompleteItems$.value[this.autoCompleteSelected$.value]

    // do something with `item`

  }

}

I have pretty concerns about the use case too. But here it comes a solution, feel free vote down as long you leave feedback too. Since BehaviourSubject and any other Subject are Observables,

import { BehaviorSubject, from } from 'rxjs'; 

import { map, mergeMap } from 'rxjs/operators';





const source$ = from([1,2,3,4,5,6,7,8,9]);

const bs = new BehaviorSubject('start')

    .pipe(

        mergeMap(() => source$)

    );



bs.subscribe(console.log);

This is how I convert my Observables to BehaviorSubjects:

import { Observable } from 'rxjs/Observable';

import { BehaviorSubject } from 'rxjs/BehaviorSubject';



export function convertObservableToBehaviorSubject<T>(observable: Observable<T>, initValue: T): BehaviorSubject<T> {

    const subject = new BehaviorSubject(initValue);



    observable.subscribe(

        (x: T) => {

            subject.next(x);

        },

        (err: any) => {

            subject.error(err);

        },

        () => {

            subject.complete();

        },

    );



    return subject;

}