async def preprocess_websocket(
        self, websocket_context: Optional[WebsocketContext]=None,
    ) -> Optional[ResponseReturnValue]:
        """Preprocess the websocket i.e. call before_websocket functions.

        Arguments:
            websocket_context: The websocket context, optional as Flask
                omits this argument.
        """
        websocket_ = (websocket_context or _websocket_ctx_stack.top).websocket
        blueprint = websocket_.blueprint
        processors = self.url_value_preprocessors[None]
        if blueprint is not None:
            processors = chain(processors, self.url_value_preprocessors[blueprint])  # type: ignore
        for processor in processors:
            processor(websocket_.endpoint, websocket_.view_args)

        functions = self.before_websocket_funcs[None]
        if blueprint is not None:
            functions = chain(functions, self.before_websocket_funcs[blueprint])  # type: ignore
        for function in functions:
            result = await function()
            if result is not None:
                return result
        return None
Preprocess the websocket i.e. call before_websocket functions.

        Arguments:
            websocket_context: The websocket context, optional as Flask
                omits this argument.