Sends the widget state, or a piece of it, to the front-end, if it exists.

        Parameters
        ----------
        key : unicode, or iterable (optional)
            A single property's name or iterable of property names to sync with the front-end.

def send_state(self, key=None):
        """Sends the widget state, or a piece of it, to the front-end, if it exists.

        Parameters
        ----------
        key : unicode, or iterable (optional)
            A single property's name or iterable of property names to sync with the front-end.
        """
        state = self.get_state(key=key)
        if len(state) > 0:
            if self._property_lock:  # we need to keep this dict up to date with the front-end values
                for name, value in state.items():
                    if name in self._property_lock:
                        self._property_lock[name] = value
            state, buffer_paths, buffers = _remove_buffers(state)
            msg = {'method': 'update', 'state': state, 'buffer_paths': buffer_paths}
            self._send(msg, buffers=buffers)