TypeError：未定义不是函数（正在评估userItems.map）

Question

编辑：现在我正在使用“ react-native-navigation @ latest”，一切都很好。

当我第一次打开应用程序时， 它可以正确处理数据，但是在发生第二次刷新错误之后

TypeError：未定义不是函数（正在评估“ userItems.map”）

该错误位于：
在用户中（由Connect（Users）创建）；
....

....

组件/Users.js

  componentWillMount(){
    this.props.fetchUsers();
  }

  render() {
    const userItems = this.props.users || [];
    const abc = userItems.map((user,index) => (
      <Text key={index}>
      {user.email}
      </Text>
    ));
    return (
      <View>
        {abc}
      </View>
    );
  }
}
const mapStateToProps = state => ({
  users: state.UserReducer.items
})

const mapDispatchToProps = {
  fetchUsers
};


export default connect(mapStateToProps, mapDispatchToProps)(Users);

动作

export function fetchUsers() {
  return dispatch => {
    fetch("http://example.com/v1/users")
      .then(res => res.json())
      .then(users =>
        dispatch({
          type: FETCH_USERS,
          payload: users
        })
      );
  };
}

reducer / userReducer.js

import { FETCH_USERS } from "../Actions/types";

    const initialState = {
      items: []
    };

    const userReducer= (state = initialState, action) => {
        switch(action.type){
            case FETCH_USERS:
                return {
                    ...state,
                    items:action.payload
                }
            default:
                return state;
        }
    }
export default userReducer;

reducer / index.js

import UserReducer from './userReducer';
const AppReducer = combineReducers({
  ...
  UserReducer,
  ...
  ...
  ...
});

export default AppReducer;

后端很好，我和邮递员在一起

用户后端

  const router = new express.Router();

  router.route("/users").get((req, res) => {
    User.find({},{password:0}, (err, users) => {
      if (err) {
        return res.status(404).send({message: "error"});
        console.log(err);
      } else {
        return res.status(200).send({users});
      }
    });
  });

响应后端

{
    "users": [
        {
            "dateCreated": "..",
            "dateModified": "...",
            "lastLogin": "...",
            "_id": "...",
            "email": "...",
            "__v": 0
        },
        {
            "dateCreated": "..",
            "dateModified": "...",
            "lastLogin": "...",
            "_id": "...",
            "email": "...",
            "__v": 0
        }
    ]
}

package.json依赖项

 "dependencies": {
    "asyncstorage": "^1.5.0",
    "es6-promise": "^4.2.4",
    "react": "16.3.1",
    "react-native": "0.55.3",
    "react-native-vector-icons": "^4.6.0",
    "react-navigation": "v1.0.0-beta.26",
    "react-redux": "^5.0.7",
    "redux": "^3.7.2",
    "redux-logger": "^3.0.6",
    "redux-persist": "^5.9.1",
    "redux-thunk": "^2.2.0"
  },

Answer 1

您确定要在MapDispatchToProps中映射动作吗？ （理想情况下，此帖子将是评论，但我还没有评论权限）。

const mapDispatchToProps = (dispatch) => {
    return {
        fetchUsers: () => dispatch(fetchUsers());
    };    
};

Answer 2

如果/users响应是这样的：

{
    "users": [
        {
            "dateCreated": "..",
            "dateModified": "...",
            "lastLogin": "...",
            "_id": "...",
            "email": "...",
            "__v": 0
        }
    ]
}

然后，在调度FETCH_USERS ，您应该这样做：（注意payload属性）

export function fetchUsers() {
  return dispatch => {
    fetch("http://example.com/v1/users")
      .then(res => res.json())
      .then(data =>
        dispatch({
          type: FETCH_USERS,
          payload: data.users
        })
      );
  };
}

或者，您可以在化简器中执行此操作（请注意items属性）：

const userReducer= (state = initialState, action) => {
    switch(action.type){
        case FETCH_USERS:
             return {
                  ...state,
                  items: action.payload.users 
             }
             default:
             return state;
    }
}

Answer 3

看起来您的代码正在使用来自给定代码示例的react-native-router-flux（具有react-navigation作为依赖项）。 github上列出了一个未解决的问题。 如该SO答案中所述，当前的解决方案是使用beta26版本的react-native router-flux。 我怀疑这会解决您的问题。 如果没有，那么值得一试！ ..

Hth ..