如何执行graphql订阅？

Question

我正在向我的React应用添加订阅。这将使我能够获取实时更新。尝试这样做时出现以下错误：

TypeError：link.request未定义

export const httpLink = new ApolloClient({
  uri: "https://{graphql endpoint}",
  cache: new InMemoryCache(),
});

export const wsLink = new WebSocketLink({
  uri: "wss://{graphql endpoint}", 
  options: {
    reconnect: true,
  },
});

const link = split(
  // split based on operation type
  ({ query }) => {
    const { kind, operation } = getMainDefinition(query);
    return kind === "OperationDefinition" && operation === "subscription";
  },
  wsLink,
  httpLink
);

可能是什么错误？