React的hooks---useReducer

useReducer 作为 useState 的代替方案，在某些场景下使用更加适合，例如 state 逻辑较复杂且包含多个子值，或者下一个 state 依赖于之前的 state 等。

使用 useReducer 还能给那些会触发深更新的组件做性能优化，因为父组件可以向自组件传递 dispatch 而不是回调函数

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

使用：

import React, { useReducer } from 'react'

const initialState = { count: 0 };

function reducer(state, action) {
  switch (action.type) {
    case 'increment':
      return {count: state.count + 1};
    case 'decrement':
      return {count: state.count - 1};
    default:
      throw new Error();
  }
}

export default function Counter() {
  const [state, dispatch] = useReducer(reducer, initialState);

  return (
    <>
      <p>Count: {state.count}</p>
      <button onClick={() => dispatch({type: 'decrement'})}>-</button>
      <button onClick={() => dispatch({type: 'increment'})}>+</button>
    </>
  );
}

初始化 state：

useReducer 初始化 sate 的方式有两种

// 方式1
const [state, dispatch] = useReducer(
	reducer,
  {count: initialCount}
);

// 方式2
function init(initialClunt) {
  return {count: initialClunt};
}

const [state, dispatch] = useReducer(reducer, initialCount, init);