Laravel GraphQl 出现无法连接到 websocket 端点错误

Question

实现一个简单的GraphQl查询（例如 simple）Mutation后Subscription，当我尝试将其运行subscription到graphql-playground：

subscription {
   userCreated{
     name
     username
     password
   }
}

我收到此错误：

{
  "error": "Could not connect to websocket endpoint ws://127.0.0.1:8000/graphql. 
Please check if the endpoint url is correct."
}

GraphQl 架构：

#...
type User {
    id: Int!
    name: String
    email: String
    product : [Product!]
}

type Mutation {
 createUser(
    name: String!
    password: String!
    email : String!
  ): User @field(resolver:"UserMutator@create") @broadcast(subscription: "userCreated")
}

type Subscription {
  userCreated: User
}

UserMutator班级：

subscription {
   userCreated{
     name
     username
     password
   }
}

UserCreated订阅

{
  "error": "Could not connect to websocket endpoint ws://127.0.0.1:8000/graphql. 
Please check if the endpoint url is correct."
}