# useReducer

## 基本

 `useState` 的替代方案。它接收一个形如 `(state, action) => newState` 的 reducer，并返回当前的 state 以及与其配套的 `dispatch` 方法。（如果你熟悉 Redux 的话，就已经知道它如何工作了。）

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

## 示例

```jsx
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>
    </>
  );
}
```