How to convert an Observable into a BehaviorSubject?

Question

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 an 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`   } } 

Answer 1

No need to convert it.

Just create a subject and attach observable to it with : obs.subscribe(sub)

example:

var obs = new rxjs.Observable((s) => {setTimeout(()=>{s.next([1])} , 500)}) //observable var sub = new rxjs.BehaviorSubject([0]) //create subject obs.subscribe(sub) //<----- HERE ----- attach observable to subject setTimeout(() => {sub.next([2, 3])}, 1500) //subject updated sub.subscribe(a => console.log(a)) //subscribe to subject 

Note: obs.subscribe(sub) is equivalent to :

obs.subscribe({   next: v => sub.next(v),   error: v => sub.error(v),   complete: () => sub.complete() }) 

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