設定 Context

import createContext，並設定 createContext 的預設值，
範例預設為: 訂單陣列 orders、增加訂單函式 addOrder

Provider（提供者）是用來提供 context 值

Consumer（消費者）是用來使用 context 值

/src/context/order.js

import { createContext } from "react";
export const { Provider, Consumer } = createContext({
  orders: [],
  addOrder: () => {},
});

設定 Provider，contextValue 傳入 context

在外層設定值與函式，傳入 context

在需使用到的組件外，包一層 Provider，傳入需要的 value，使底下的組件都可以使用 value 裡的值

App.js

import React, { Component } from 'react';
import Header from "./Header";
import ProductList from "./ProductList";
import { Provider } from "./context/order.js";

class App extends Component {
  state = {
    orders: []
  }
  addOrder = order => {
    this.setState({
      orders: [...this.state.orders, order]
    })
  }
  render() {
    const { orders } = this.state;
    const contextValue = {
      orders,
      addOrder: this.addOrder
    }
    return (
      <div>
        <Provider value={contextValue}>
          <Header />
          <ProductList />
        </Provider>
      </div>
    );
  }
}

export default App;

Consumer 使用 context

在需要使用 context value 的地方包一層 Consumer，需注意 value 必須用函式傳入

這麼一來就可以取到 context 的值囉！

Header.js

import React from 'react';
import { Consumer } from "./context/order.js";

const Header = () => {
  return (
    <header>
      <Consumer>
       { value => (`購物車(${value.orders.length})`)}
      </Consumer>
      <hr/>
    </header>
  );
}

export default Header;

ProductList.js

import React from 'react';
import Product from "./Product";
const menu = [
  { id: 0, name: '雞肉鍋'},
  { id: 1, name: '豬肉鍋'},
  { id: 2, name: '牛肉鍋'},
  { id: 3, name: '海鮮鍋'},
  { id: 4, name: '泡菜鍋'},
]

const ProductList = () => {
  return(
    <ul>
      {menu.map(item => <Product key={item.id} {...item} />)}
    </ul>
  )
}

export default ProductList;

Product.js

import React from 'react';
import { Consumer } from "./context/order.js";

const Product = ({ id, name }) => {
  return (
    <li>
      <label>{name}</label>
      <Consumer>
        {
          value => (
            <button onClick={() => {value.addOrder(id)}}>+</button>
          )
        }
      </Consumer>
    </li>
  );
}

export default Product;