code: def _handle(self, request: Request, response: Response) -> TypeGenerator[Any, None, None]:
          
          # request.start_time = datetime.now().timestamp()
          # 创建一个新的会话上下文
          ctx = self._context(
              cast(asyncio.AbstractEventLoop, self._loop),
              request,
              response,
              self,
          )
          request.app = self
          response.app = self
          request.ctx = ctx
          response.ctx = ctx
          request.response = response
          response.request = request
  
          # 把当前注册的中间件转为迭代器
          middleware_iter = iter(self._middleware)
          # 通过迭代器的模式生成一个执行下一个中间的调用方法
          next_call = self._next_middleware(middleware_iter, ctx)
          # 顺序执行中间件
          yield from self._middleware_call(middleware_iter, ctx, next_call)
          # 设置 cookies
          cookies_headers = ctx.cookies.headers()
          if cookies_headers is not None:
              ctx.response.set("Set-Cookie", cookies_headers)
          # 写出 headers
          ctx.response.flush_headers()
          # 写出 body
          ctx.response.flush_body()
docs: request 解析后的回调，调用中间件，并处理 headers, body 发送。