问了解beast::tcp_stream中超时的用法吗？

EN

    void on_resolve(beast::error_code ec, tcp::resolver::results_type results)
    {
        if(ec) return fail(ec, "resolve");
        // Set the timeout for the operation
        beast::get_lowest_layer(ws_).expires_after(std::chrono::seconds(30));

        // Make the connection on the IP address we get from a lookup
        beast::get_lowest_layer(ws_).async_connect(
            results, beast::bind_front_handler(
                &session::on_connect, shared_from_this()));
    }

    void on_connect(beast::error_code ec, tcp::resolver::results_type::endpoint_type ep)
    {
        if(ec) return fail(ec, "connect");

        // Turn off the timeout on the tcp_stream, because
        // the websocket stream has its own timeout system.
        // beast::get_lowest_layer(ws_).expires_never(); // Note: do NOT call this line for this question!!!

...
        host_ += ':' + std::to_string(ep.port());
        // Perform the websocket handshake
        ws_.async_handshake(host_, "/",
            beast::bind_front_handler(&session::on_handshake, shared_from_this()));
    }

void func_async_calling()
{
  // set some timeout here(i.e. XXXX seconds)
  Step 1> beast::get_lowest_layer(ws_).expires_after(std::chrono::seconds(XXXX));
  Step 2> ws_.async_operation(..., func_async_callback, )
  Step 3> beast::get_lowest_layer(ws_).expires_never();
}

void func_async_callback()
{
  Step 1>Either call 
    // Disable the timeout for the next logical operation.
    beast::get_lowest_layer(ws_).expires_never(); 
  or
    // Enable a new timeout
    beast::get_lowest_layer(ws_).expires_after(std::chrono::seconds(YYYY));

  Step 2> call another asynchronous function
  Step 3> beast::get_lowest_layer(ws_).expires_never();
}