onInitMixin.ts

export const onInit = <T extends Constructor>(base: T = class {} as T) =>
  class OnInitSubject extends base implements OnInit, AfterViewInit {
    private _init = new BehaviorSubject<void>(null);
    onInit$ = this._init.asObservable();

    ngOnInit(): void {
      this._init.next();
      // tslint:disable-next-line:no-unused-expression
      super['ngOnInit'] && super['ngOnInit']();
    }

    ngAfterViewInit(): void {
      this._init.complete();
      // tslint:disable-next-line:no-unused-expression
      super['ngAfterViewInit'] && super['ngAfterViewInit']();
    }
  };