+ Context api

Question

我是相当新的反应钩子和上下文api。我试图在react中创建一个全局状态管理(没有redux)，但是我遇到了这个错误。

TypeError: undefined is not iterable (cannot read property Symbol(Symbol.iterator))

我刚开始这个项目，只是尝试建立一个购物车(它将使用全局CartItem来跟踪总项目)。

GlobalStates.jsx (我的全局状态管理器文件)

import React from 'react';

export const CartContext = React.createContext(0)

const GlobalContextWrapper = (props) => {
    const [cartItem,incCartItem] = React.useState(0);
    return(
        <CartContext.Provider value={[cartItem,incCartItem]}>
            {props.children}
        </CartContext.Provider>
    )
}

export default GlobalContextWrapper

index.js

import React from 'react';
import ReactDOM from 'react-dom';

import GlobalContextWrapper from "./GlobalStates.jsx"

import Test from "./Test.jsx"

const Main = () => {
    return(
      <GlobalContextWrapper>
        <Test/>
      </GlobalContextWrapper>
    )
}

ReactDOM.render(<Main/>,document.getElementById('root'));

Test.jsx (试图访问上下文全局状态的文件)

import React from 'react';
import CartContext from "./GlobalStates"

const Test = () => {
    const [cartItem,incCartItem] = React.useContext(CartContext)
    return(
        <h1>The cart total is {cartItem}</h1>
    )
}

export default Test

Answer 1

您的代码工作正常，但是在测试中您需要正确导入上下文：import {CartContext} from "./GlobalStates"

​

const CartContext = React.createContext(0);

const GlobalContextWrapper = (props) => {
  const [cartItem, incCartItem] = React.useState(0);
  return (
    <CartContext.Provider value={[cartItem, incCartItem]}>
      {props.children}
    </CartContext.Provider>
  );
};
const Test = () => {
  const [cartItem, incCartItem] = React.useContext(
    CartContext
  );
  return <h1>The cart total is {cartItem}</h1>;
};

const Main = () => {
  return (
    <GlobalContextWrapper>
      <Test />
    </GlobalContextWrapper>
  );
};

ReactDOM.render(<Main />, document.getElementById('root'));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.4/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.4/umd/react-dom.production.min.js"></script>
<div id="root"></div>

​