问如何将多个主题合并为可观察到的，并在订阅时发出

EN

export interface IFilterData {
    foo: string;
    bar: string;
}

@Injectable({
    providedIn: 'root'
})
export class FilterService {
    public foo$: Observable<string>;
    public bar$: Observable<string>;
    public filterChanged$: Observable<IFilterData>;

    private fooSubject$: Subject<string>;
    private barSubject$: Subject<string>;
    private _foo: string;
    private _bar: string;

    constructor() {
        this._foo = 'InitialFoo';
        this._bar = 'InitialBar';

        this.foo$ = this.fooSubject$.asObservable();
        this.bar$ = this.barSubject$.asObservable();

        this.filterChanged$ = merge(this.fooSubject$, this.barSubject$).pipe(
            switchMap(() => {
                return of({
                    foo: this._foo,
                    bar: this._bar
                });
            })
        );
    }

    set foo(value: string) {
        this._foo = value;
        this.fooSubject$.next(value);
    }
    get foo(): string {
        return this._foo;
    }

    set bar(value: string) {
        this._bar = value;
        this.barSubject$.next(value);
    }
    get bar(): string {
        return this._bar;
    }
}