84 lines
2.1 KiB
JavaScript
84 lines
2.1 KiB
JavaScript
import { useReducer, useRef } from "react";
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// 定义一个初始化的状态
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const initialState = { count: 1 };
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/**
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* reducer
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* @param {*} state 状态
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* @param {*} action 数据变化的描述对象
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*/
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function counter(state, action) {
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switch (action.type) {
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case "INCREMENT":
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return { count: state.count + action.payload };
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case "DECREMENT":
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return { count: state.count - action.payload };
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default:
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return state;
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}
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}
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// 惰性初始化函数
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function init(initialState){
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// 有些时候我们需要基于之前的初始化状态做一些操作，返回新的处理后的初始化值
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// 重新返回新的初始化状态
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return {
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count : initialState.count * 10
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}
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}
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function App() {
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// const [num, setNum] = useState(0);
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// 后期要修改值的时候，都是通过 dispatch 来进行修改
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const [state, dispatch] = useReducer(counter, initialState, init);
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const selRef = useRef();
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const increment = () => {
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// 做自增操作
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// 1. 你要增加多少？
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const num = selRef.current.value * 1;
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// setNum(num);
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dispatch({
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type: "INCREMENT",
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payload: num,
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});
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};
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const decrement = () => {
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const num = selRef.current.value * 1;
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dispatch({ type: "INCREMENT", payload: num });
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};
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const incrementIfOdd = () => {
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const num = selRef.current.value * 1;
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if (state.count % 2 !== 0) {
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dispatch({ type: "INCREMENT", payload: num });
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}
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};
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const incrementAsync = () => {
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const num = selRef.current.value * 1;
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setTimeout(() => {
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dispatch({ type: "INCREMENT", payload: num });
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}, 1000);
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};
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return (
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<div>
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<p>click {state.count} times</p>
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<select ref={selRef}>
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<option value="1">1</option>
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<option value="2">2</option>
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<option value="3">3</option>
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</select>
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<button onClick={increment}>+</button>
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<button onClick={decrement}>-</button>
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<button onClick={incrementIfOdd}>increment if odd</button>
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<button onClick={incrementAsync}>increment async</button>
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</div>
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);
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}
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export default App;
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