How to use the cachier.pickle_core._PickleCore.CacheChangeHandler function in cachier

def wait_on_entry_calc(self, key):
        with self.lock:
            self._reload_cache()
            entry = self._get_cache()[key]
            if not entry['being_calculated']:
                return entry['value']
        event_handler = _PickleCore.CacheChangeHandler(
            filename=self._cache_fname(), core=self, key=key
        )
        observer = Observer()
        event_handler.inject_observer(observer)
        observer.schedule(
            event_handler, path=self.expended_cache_dir, recursive=True
        )
        observer.start()
        observer.join(timeout=1.0)
        if observer.isAlive():
            # print('Timedout waiting. Starting again...')
            return self.wait_on_entry_calc(key)
        # print("Returned value: {}".format(event_handler.value))
        return event_handler.value