observable-host-listener-3.ts

import { HostListener } from '@angular/core';
import { UnaryFunction, Observable } from 'rxjs';

export function ObservableHostListener(
  eventName: string, 
  args?: string[], 
  operator?: UnaryFunction<Observable<any>, Observable<any>>
): PropertyDecorator {

  // Calling HostListener will return a decorator, ready to
  // be used by TypeScript.
  const preparedDecorator: (target: any, key: string) => void = HostListener(eventName, args);

  // Returning a PropertyDecorator here, in which preparedDecorator
  // will have to be called to register the event name with Angular's
  // change detection mechanism.
  return (target: any, key: string) => {
    // what to do here?
  }
}