observable-host-listener-4.ts

// Creating an internal subject that will recieve values from
// Angular calling the shadowed method with new values.
const subject = new Subject<any>();

// We need a super secret string to monkey patch a new method onto
// the target class.
const newKey = `__keyForObservableHostListener__${key}`;

Object.defineProperties(target, {
  [newKey]: {
    // here, we just next() the recieved values into the internal subject.
    value: (val: any) => subject.next(val),
    configurable: false,
    enumerable: false,
    writable: false
  },
  // And here, we're assigning our internal to the original key.
  [key]: {
    get() {
      if (operator) {
        return subject.pipe(operator);
      } else {
        return subject.asObservable();
      }
    }
  }
});