wrap function in mobx computed() and convert to rxjs observable

toObservable.ts

import { Observable } from 'rxjs';
import { computed, IValueDidChange } from 'mobx';
import isNil from 'lodash/isNil';
import omit from 'lodash/omit';
import { IEqualsComparer } from 'mobx/lib/internal';

interface ToObservableOptions<T> {
    initial?: boolean;
    equals?: IEqualsComparer<T>;
}
  
/***
 * wrap function in mobx computed() and convert to observable
 *
 * @param observable - mobx observable property
 * @param options - initial: whether to fire an initial event if the property is non-empty
 */
export function toObservable<T>(observable: () => T, options?: ToObservableOptions<T>): Observable<IValueDidChange<T>> {
    const optionsToUse: ToObservableOptions<T> = { initial: true, ...options };
    return new Observable((observer) => {
        const fireInitial = isNil(optionsToUse.initial) || optionsToUse.initial;
        const item = fireInitial && observable();
        if (item) {
            observer.next({
                newValue: item,
                object: item,
                type: 'update',
                oldValue: undefined,
            });
        }
        computed(observable, omit(optionsToUse, ['initial'])).observe((item) => observer.next(item));
    });
}