React Hooks useState Array 為空，在組件中呈現狀態

Question

我有一種情況，我可以使用減速器成功調度我的狀態，並且我可以在我的組件中渲染它

這里是相關代碼

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;
  }
};

在我的組件component/Leagues.js

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


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

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

const mapDispatchToProps = {
  getList: fetchLeaguesList
};

我在這里復制了演示 => https://codesandbox.io/s/select-demo-71u7h ？

我可以在執行 map 的組件中渲染我的leaguesList狀態，但是為什么

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

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

返回一個空數組？

看圖片

Answer 1

您將 useState 的初始值設置錯誤：

 const [itemsLeagues] = useState(leaguesList);

代替

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

useState 的返回值不是 value 本身，而是 value 和 mutator 的數組：

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

所以如果你試圖解構你傳遞給 useState() 的包裝數組，你應該像這樣使用它（盡管你不需要它）：

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