React Hooks之useReducer

useReducer ：接受类型为 (state, action) => newState 的reducer，并返回与 dispatch 方法配对的当前状态。（如果你熟悉Redux，你已经知道它是如何工作的。）

下面定义两个计数器Counter组件 和 CounterTest组件，使用 useReducer 使它们共享一个count state，当一个组件的 count state发生变化时，另一个组件的count state也会相应的改变。

1. reducer.js

import { useReducer } from "react";

const initialState = 0;

function reducer(state, action) {
  switch (action.type) {
    case "reset":
      return initialState;
    case "increment":
      return { count: state.count + 1 };
    case "decrement":
      return { count: state.count - 1 };
    default:
      // A reducer must always return a valid state.
      // Alternatively you can throw an error if an invalid action is dispatched.
      return state;
  }
}

export default reducer;

2. Counter.js

import React from "react";

function Counter(props) {
  return (
    

      Count: {props.state.count}
       props.dispatch({ type: "reset" })}>Reset
       props.dispatch({ type: "increment" })}>+
       props.dispatch({ type: "decrement" })}>-
    
  );
}

export default Counter;

3. CounterTest.js

import React from "react";

function CounterTest(props) {
  return (
    

      Count: {props.state.count}
       props.dispatch({ type: "reset" })}>Reset
       props.dispatch({ type: "increment" })}>+
       props.dispatch({ type: "decrement" })}>-
    
  );
}

export default CounterTest;

4. index.js

import React, { useReducer } from "react";
import reducer from "./reducer";
import Counter from "./Counter";
import CounterTest from "./CounterTest";

const App = () => {
  const [state, dispatch] = useReducer(reducer, { count: 0 });

  return (
    

      
      
    
  );
};

const rootElement = document.getElementById("root");
ReactDOM.render(, rootElement);

ps：此处还有两种方法对代码进行优化：
方法一：将共同的状态 state 和 dispatch 通过 React.createContext存入到context中，再通过useContext API获取context中的内容。
方法二：直接采用React Context API。
接下来会对这两种方法分别进行说明。