在一些需要连续更新状态的场景下，useReducer 会比 useState 更加的合适

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

React 官网的 useReducer 

```javascript
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();
  }
}

function Counter() {
  const [state, dispatch] = useReducer(reducer, initialState);
  return (
    <>
      Count: {state.count}
      <button onClick={() => dispatch({type: 'decrement'})}>-</button>
      <button onClick={() => dispatch({type: 'increment'})}>+</button>
    </>
  );
}
```

最简的 useReducer 实践

```javascript
const [state, dispatch] = useReducer((state, action) => (
	{ ...state, action }),
	initState
)
```

在这个最简单的事件中，useReducer 就充当了 useState 的替代，dispatch 修改状态，返回最新的 state