request 解析后的回调，调用中间件，并处理 headers, body 发送。

def _handle(self, request: Request, response: Response) -> TypeGenerator[Any, None, None]:
        """
        request 解析后的回调，调用中间件，并处理 headers, body 发送。
        """
        # 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()