React Hooks useState 数组为空，组件中呈现状态

Question

我有这样一种情况，我可以使用 reducer 成功分派我的状态，并且可以在我的组件中渲染它

这里是相关代码

在我的 action/index.js

export const receivedLeaguesList = json => ({
  type: RECEIVE_LEAGUES_LIST,
  json: json
});

export function fetchLeaguesList() {
  return function(dispatch) {
    dispatch(requestLeaguesList());
      return axios
        .get("https://www.api-football.com/demo/v2/leagues/")
        .then(res => {
          let leagues = res.data.api.leagues;
          dispatch(receivedLeaguesList(leagues));
        })
        .catch(e => {
          console.log(e);
        });
    }
}

我的reducers/index.js

import { REQUEST_LEAGUES_LIST, RECEIVE_LEAGUES_LIST } from "../actions";

const initialState = {
  leaguesList: [],
  isLeagueListLoading: false
};

const reducer = (state = initialState, action) => {
  switch (action.type) {
    case REQUEST_LEAGUES_LIST:
      return { ...state, isLeagueListLoading: true };
    case RECEIVE_LEAGUES_LIST:
      return { ...state, leaguesList: action.json, isLeagueListLoading: false };
    default:
      return state;
  }
};

在我的组件中 component/Leagues.js

let Leagues = ({ leaguesList, loading, getList }) => {
  useEffect(() => {
    getList();
  }, [getList]);


  const [itemsLeagues] = useState([leaguesList]);

  console.log("league list", itemsLeagues);

const mapDispatchToProps = {
  getList: fetchLeaguesList
};

我在这里复制了演示 => https://codesandbox.io/s/select-demo-71u7h?

我可以在我的组件中渲染我的 leaguesList 状态，但为什么

  const [itemsLeagues] = useState([leaguesList]);

  console.log("league list", itemsLeagues);

returns 空数组 ?

看图

Answer 1

您设置的 useState 的初始值有误：

 const [itemsLeagues] = useState(leaguesList);

而不是

 const [itemsLeagues] = useState([leaguesList]);

useState 的 return 值不是值本身，而是值和 mutator 的数组：

const [value, setValue] = useState([42, 43])
// here's value equals [42, 43]

因此，如果您试图解构传递给 useState() 的包装数组，您应该像这样使用它（尽管您不需要它）：

const [[itemsLeagues]] = useState([leaguesList]);

React Hooks useState 数组为空，组件中呈现状态

React Hooks useState Array empty with states rendered in the component

javascript

reactjs

react-hooks

use-state