React useReducer 清空 initialState

Question

我正在尝试在 initialState 中使用 axios 获取的一些数据，以便稍后将其与 useReducer 一起使用。但是一旦我将它作为我的第二个参数的 initialState 传递它 returns 一个没有对象的空数组。请帮忙...:(

 const [prods, setProds] = useState([]);
  const getProds = () => {
    axios.get(`API`).then((res) => {
      const data = res.data;
      setProds(data.products);
    });
  };


  const result = prods.map(function (el) {
    const quantity = Object.assign({}, el);
    quantity.count = 0;
    return quantity;
  });
  
  const initialState = {
    products: [result],
  };
  useEffect(() => {
    getProds();
  }, []);

console.log(initialState);

  const [countState, dispatch] = useReducer(counterReducer, initialState);

  console.log(countState);

第一个和第二个日志的结果如下：

useReducer empties initialState

API 将计数添加到每个调用后的结果如下所示：

enter image description here

这里是我的减速器的代码，我检查了很多次但看起来不错:

export const counterReducer = (state, action) => {
  switch (action.type) {
    case "increment":
      return {
        ...state,
        products: state.products.map((product) =>
          product.id === action.id
            ? { ...product, count: product.count + 1 }
            : product
        ),
      };

    case "decrement":
      return {
        ...state,
        products: state.products.map((product) =>
          product.id === action.id
            ? {
                ...product,
                count: product.count !== 1 ? product.count - 1 : 1,
              }
            : product
        ),
      };
    default:
      return state;
  }
};

Answer 1

useReducer 仅在第一次渲染时（在 axios 完成之前）消耗 initialState，在所有后续的重新渲染中，它只会 return 来自内存单元的状态。你要做的是在顶部调用useReducer

const [countState, dispatch] = useReducer(counterReducer, {products: []});

然后在 getProds

中调用 dispatch

const getProds = () => {
    axios.get(`API`).then((res) => {
      const data = res.data;
      setProds(data.products);
      dispatch({ type: "PRODUCTS_FETCHED", payload: data.products});
    });
  };

并在你的 reducer 中添加相应的 case 来设置状态。

export const counterReducer = (state, action) => {
  switch (action.type) {
    case "increment":
      return {
        ...state,
        products: state.products.map((product) =>
          product.id === action.id
            ? { ...product, count: product.count + 1 }
            : product
        ),
      };

    case "decrement":
      return {
        ...state,
        products: state.products.map((product) =>
          product.id === action.id
            ? {
                ...product,
                count: product.count !== 1 ? product.count - 1 : 1,
              }
            : product
        ),
      };
    case "PRODUCTS_FETCHED":
      return {
        ...state,
        products: action.payload
      };
    default:
      return state;
  }
};

React useReducer 清空 initialState

React useReducer empties initialState

javascript

reactjs

use-reducer