state-def.ts

import { BehaviorSubject, of } from 'rxjs';
import { distinctUntilChanged, concatMap, map, take } from 'rxjs/operators';

export class StateDef<T> extends BehaviorSubject<T> {
  setState(setter: (state: T) => T) {
    this.pipe(take(1)).subscribe(state => {
      super.next(setter(state));
    });
  }

  select<R>(selector: (state?: T) => R) {
    return this.pipe(map(selector), distinctUntilChanged());
  }

  selectOnce<R>(selector: (state?: T) => R) {
    return this.select(selector).pipe(take(1));
  }

  next() {
    throw new Error('Nice Try, Use setState instead of next.');
  }
}