使用来自上下文提供者的函数反应 Context Api

Question

我正在尝试使用 contextApi 来处理购物车中的商品列表。

我已经有一个项目中的身份验证上下文并且它工作正常，但是这个新的购物车上下文给我带来了问题。

我有一个名为 addItem(item: ShoppingItem) 的函数，当我尝试在我的组件上使用此函数时，我收到一个错误，指出 addItem 不是函数。我将在下面显示我的代码。谢谢

购物车上下文：

import { createContext, useState } from 'react';

export interface ShoppingCartContextType {
    list: number;
    addItem(item: ShoppingItem): Promise<void>;
    logout(): void;
}; 

export interface ShoppingItem {
    id: string
}

const ShoppingCartContext = createContext<ShoppingCartContextType>({} as ShoppingCartContextType);

export const ShoppingCartProvider: React.FC = ({children}) => {
    const [shoppingCart, setShoppingCart] = useState<ShoppingItem[]>([{id: "item de merda"}]);

    async function addItem(item: ShoppingItem): Promise<void>{
        console.log('adding an item')
        setShoppingCart(current => [...current, item])
       
        return new Promise((resolve) => {
           resolve();
        });
    }

    function logout() {
      localStorage.clear();
    }

  return (
    <ShoppingCartContext.Provider 
        value= {{ list: shoppingCart.length,  addItem, logout}}>
        {children}
    </ShoppingCartContext.Provider>
  );
};

export default ShoppingCartContext;

使用上下文的组件：

import { useContext } from 'react';

import './styles.css';
import { toast } from 'react-toastify';
import 'react-toastify/dist/ReactToastify.css';  

import { FaShoppingCart } from 'react-icons/fa';
import AuthContext from '../../contexts/auth';
import ShoppingCartContext, { ShoppingItem } from '../../contexts/shoppingCart';
import { NonAuthRoutes } from '../../helpers/Authentication/authenticationRoutes';
import { useHistory } from 'react-router-dom';

interface Props {
    productId: string | undefined;
    productName: string | undefined;
    price: number | undefined;
    quantity: number | undefined;
}

function BuyButton({productId, productName, price, quantity}: Props) {

    
    const history = useHistory();
    const {signed} = useContext(AuthContext);
    const {addItem} = useContext(ShoppingCartContext);

    async function handleBuy(e: React.FormEvent<HTMLButtonElement>) {
        e.stopPropagation();
        addItem({id: productId} as ShoppingItem);
        if (!signed) {
            toast.warn("Você esta sendo redirecionado para a página de login", {autoClose:15000});
            setTimeout(() => {
                history.push(NonAuthRoutes.login);
            }, 800);
            return ;
        }
    }

    return (
        <div className="button-container">
            <button className="button" onClick={handleBuy}>
                <FaShoppingCart />
                COMPRAR
            </button>
        </div>
    );
}

export default BuyButton;

Answer 1

此错误通常发生是因为BuyButton 不是ShoppingCartProvider 的子对象，并且您在初始化上下文时没有为addItem 方法提供默认值。

您可以通过为ShoppingCartContext 指定addItem 的默认值来支持这一点，如下所示：

const ShoppingCartContext = createContext<ShoppingCartContextType>({
    addItem: () => alert('test')
});

以这个 CodeSandbox 为例：

https://codesandbox.io/s/nifty-williams-ziysn?file=/src/App.tsx