一、简介

        react中的状态只属于某个组件。而Redux是一个全局管理js状态的架构，让组件通信更加容易。

之前是状态在所有组件间传递，而redux通过store来实现这个功能。

Redux特性：

1.Single source Of truth，通过store唯一维护状态。

2.可预测性，state + action -> new state

3.纯函数更新store (纯函数 -> 输入决定输出)

举个例子: 待办事项表

function todos(state = [], action){
    switch(action.type){
        case 'ADD_TODO':
            return state.concat([{text: action.text, completed: false}]);
        case 'TOGGLE_TODO':
            return state.map(
                (todo, idx) => {
                    action.index === idx ? {text: todo.text, completed: !toto.completed}
                    : todo
                }
            )
        default:
            return state
        
    }

}

二、核心概览

拿上图举个例子，前端ui产生了deposit和withdraw2个事件，store通过dispatcher分发对应的事件给reducer处理，reducer处理后更新state。前端ui根据state重新渲染。

1.store

Redux的状态机实现，store 通过state存储状态，通过dispatch传播事件，通过Reducer处理事件。

2.action

        action表示变更事件。一般包含event type 和 context。

3.Reducer

        reducer是状态事件处理。

举个例子: 待办事项表

function todoApp(state = initialState, action){
    switch (action.type){
        case ADD_TODO:
            return Object.assign({}, state, {
                todos:[
                    ...state.todos,
                    {
                        text: action.text,
                        completed: false
                    }
                ]
            });
        default:
            return state;
        
    }
}

三、使用示例

用redux实现一个简单计数器，包括增加、减少功能。

import React from 'react';
import {createStore} from 'redux';
function run(){
    // store initial state
    const initialState = { count: 0};
    // reducer
    const counter = (state = initialState, action) => {
        switch (action.type){
            case "PLUS_ONE":
                return {count: state.count + 1};
            case "MiNUS_ONE":
                return {count: state.count - 1};
            case "CUSTOM_COUNT":
                return {
                    count: state.count + action.payload.count
                };
            default:
              return state;
        }  
    }
    // create store
    const store = createStore(counter);
    // Action creator
    function plusOne(){
        return {type: "PLUS_ONE"}
    }
    
    function minusOne(){
        return {type: "MINUS_ONE"};
    }
    function customCount(count){
        return {type: "CUSTOM_COUNT", payload:{count}};
    }
    // 订阅redux 状态变更
    store.subscribe(() => console.log(store.getState()));
    store.dispatch(plusOne());
    store.dispatch(minusOne());
    store.dispatch(customCount(5));
}

export default ()  => {
    <div>
        <button onClick={run}>Run</button>
        <p>* 请打开控制台查看运行结果</p>
    </div>
}

bindActionCreators使用,工具类，可以减少显示dispatch操作

import React from 'react';
import {createStore, bindActionCreators} from 'redux';
function run(){
    // store initial state
    const initialState = { count: 0};
    // reducer
    const counter = (state = initialState, action) => {
        switch (action.type){
            case "PLUS_ONE":
                return {count: state.count + 1};
            case "MiNUS_ONE":
                return {count: state.count - 1};
            case "CUSTOM_COUNT":
                return {
                    count: state.count + action.payload.count
                };
            default:
              return state;
        }  
    }
    // create store
    const store = createStore(counter);
    // Action creator
    function plusOne(){
        return {type: "PLUS_ONE"}
    }
    
    function minusOne(){
        return {type: "MINUS_ONE"};
    }
    function customCount(count){
        return {type: "CUSTOM_COUNT", payload:{count}};
    }

    plusOne = bindActionCreators(plusOne, store.dispatch);
    minusOne = bindActionCreators(minusOne, store.dispatch);
    customCount = bindActionCreators(customCount, store.dispatch);
    // 订阅redux 状态变更
    store.subscribe(() => console.log(store.getState()));
    plusOne();
    minusOne();
    customCount(5);
}

export default ()  => {
    <div>
        <button onClick={run}>Run</button>
        <p>* 请打开控制台查看运行结果</p>
    </div>
}

四、结合React使用

使用Connect功能，将redux的action和state作为props传递给组件。

计数器组件使用样例

import React from 'react';
import {createStore, bindActionCreators} from 'redux';
import {Provider, connect} fomr 'react-redux';

// store initial state
const initialState = {count: 0};

// reducer
const counter = (state = initialState, action) => {
        switch (action.type){
            case "PLUS_ONE":
                return {count: state.count + 1};
            case "MiNUS_ONE":
                return {count: state.count - 1};
            case "CUSTOM_COUNT":
                return {
                    count: state.count + action.payload.count
                };
            default:
              return state;
        }  
}
// create store
const store = createStore(counter);
// Action creator
function plusOne(){
   return {type: "PLUS_ONE"}
}
function minusOne(){
        return {type: "MINUS_ONE"};
}

export class Counter extends React.Component{
    render(){
        const {count, plusOne, minusOne} = this.props;
        return (
            <div className = "counter">
                <button onClick = {minusOne}>-</button>
                <span>{count}</span>
                <button onClick = {plusOne}>+</button>
             </div>
        );
    }
}


// 将store的state与组件props绑定
function mapStateToProps(state){
    return {
        count: state.count
    };
}

// 将store的action与组件props绑定
function mapDispatchToProps(dispatch){
    return bindActionCreators({plusOne, minusOne}, dispatch)
}


// 通过connect组件将store与counter组件关联
const ConnectedCounter = connect(mapStateToProps, mapDispatchToProps)(Counter) ;

// 返回使用样例
export default class CounterSample extends React.Component{
    render(){
        return (
            // prodiver 基于react context实现，可以让里面的子组件都能获取到store
            <Provider store = {store}>
                <ConnectedCounter/>
            </Provider>
        );
    }
}