Typescript 如何将 Observable 转换为 BehaviorSubject

我想把一个可观察的东西变成一个行为主体.这样地:

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

我也try 过:

a$ = new Observable()
b$ = new BehaviorSubject(a$, 123)
// ?

以及:

a$ = new Observable()
b$ = a$.asBehaviorSubject(123)
// ?

以及:

a$ = new Observable()
b$ = a$.pipe(
  toBehaviorSubject(123)
)
// ?

但这些都不管用.目前，我必须这样实施:

a$ = new Observable()
b$ = new BehaviorSubject(123)
a$.subscribe(b$)
// ?

这在课堂上会有点难看:

class Foo() {
  a$ = new Observable() // Actually, a$ is more complicated than this.
  b$ = new BehaviorSubject(123)

  constructor() {
    this.a$.subscribe(this.b$)
  }
}

那么，有没有一种更简单的方法可以在不使用类构造函数的情况下将可观察对象转换为行为主体呢？

这是我的真实情况:

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`
  }
}