React 新版官方文档 (一) useReducer 用法详解

useReducer

useReducer 是一个可以让你向组件中添加 reducer 的 Hook

const [state, dispatch] = useReducer(reducer, initialArg, init?)

基本用法

const reducer = (state, action) => {
...
}
const MyComponent = () => {
    const [state, dispatch] = useReducer(reducer, {age: 42})
    const handleClick = () => {
        dispatch({type:'increase'})
    }
   ...
}

比 useState 多了一个处理函数，该函数可以根据不同的分发状态来对应的改变状态

注意：state 不可修改 不能这样写， reducer 函数应当返回一个新对象

function reducer(state, action) {
  switch (action.type) {
    case 'incremented_age': {
      //  Don't mutate an object in state like this:
      state.age = state.age + 1;
      return state;
    }
function reducer(state, action) {
  switch (action.type) {
    case 'incremented_age': {
      // ✅ Instead, return a new object
      return {
        ...state,
        age: state.age + 1
      };
    }
    

不要重新执行初始函数 第一种写法会导致每次渲染时候都调用 createInitialState 函数 造成性能浪费

function createInitialState(username) {
  // ...
}
// ❌
function TodoList({ username }) {
  const [state, dispatch] = useReducer(reducer, createInitialState(username));
  
// ✅
function TodoList({ username }) {
  const [state, dispatch] = useReducer(reducer, username, createInitialState);
  // ...

dispatch 后如何使用最新的状态值

调用 dispatch 函数不会更改运行代码中的状态 ，使用最新的状态值应该是这样写

function handleClick() {
  console.log(state.age);  // 42

  dispatch({ type: 'incremented_age' }); // Request a re-render with 43
  console.log(state.age);  // Still 42!

  setTimeout(() => {
    console.log(state.age); // Also 42!
  }, 5000);
}

const action = { type: 'incremented_age' };
dispatch(action);
const nextState = reducer(state, action);
console.log(state);     // { age: 42 }
console.log(nextState); // { age: 43 }

参考

• https://react.dev/reference/react/useReducer