开头几张图，剩下全靠编
Redux形象图

Redux的在react-redux的架构

代码片段转自Redux 入门教程（一）：基本用法--阮一峰的网络日志

Store

import { createStore } from 'redux';
const store = createStore(fn);

State

import { createStore } from 'redux';
const store = createStore(fn);

const state = store.getState();

Action

const action = {
  type: 'ADD_TODO',
  payload: 'Learn Redux'
};

Action Creator

import { createStore } from 'redux';
const store = createStore(fn);

store.dispatch()

import { createStore } from 'redux';
const store = createStore(fn);

store.dispatch({
  type: 'ADD_TODO',
  payload: 'Learn Redux'
});

//结合 Action Creator
store.dispatch(addTodo('Learn Redux'));

Reducer

// 基本形式
const reducer = function (state, action) {
  // ...
  return new_state;
};

// 创建、使用
const defaultState = 0;
const reducer = (state = defaultState, action) => {
  switch (action.type) {
    case 'ADD':
      return state + action.payload;
    default: 
      return state;
  }
};

const state = reducer(1, {
  type: 'ADD',
  payload: 2
});

// store.dispatch方法会触发 Reducer 的自动执行，需要将 Reducer 传入createStore方法
import { createStore } from 'redux';
const store = createStore(reducer);

// 实际调用
const actions = [
  { type: 'ADD', payload: 0 },
  { type: 'ADD', payload: 1 },
  { type: 'ADD', payload: 2 }
];

const total = actions.reduce(reducer, 0); // 3

store.subscribe()

// 对于 React 项目，就是组件的render方法或setState方法）放入listen，就会实现 View 的自动渲染。
import { createStore } from 'redux';
const store = createStore(reducer);

store.subscribe(listener);

// unsubscribe
let unsubscribe = store.subscribe(() =>
  console.log(store.getState())
);

unsubscribe();

Store 实现方式

import { createStore } from 'redux';
let { subscribe, dispatch, getState } = createStore(reducer);

###### Reducer 的拆分合并——combineReducers
```javascript
import { combineReducers } from 'redux';

const chatReducer = combineReducers({
  chatLog,
  statusMessage,
  userName
})

export default todoApp

// 
const reducer = combineReducers({
  a: doSomethingWithA,
  b: processB,
  c: c
})

// 分拆到单个文件
import { combineReducers } from 'redux'
import * as reducers from './reducers'

const reducer = combineReducers(reducers)

主要使用方式

store.dispatch(action);
let nextState = todoApp(previousState, action);
// 设置监听函数
store.subscribe(listener);

//其中
function listerner() {
  let newState = store.getState();
  component.setState(newState);   // 更新component局部的state
}

代码片段转自Redux 入门教程（二）：基本用法--阮一峰的网络日志

中间件

在store.dispatch改造：Action

方法: applyMiddlewares[作用是将所有中间件组成一个数组，依次执行], applyMiddleware

同步、异步操作

同步操作只要发出一种 Action 即可，异步操作的差别是它要发出三种 Action。

• 操作发起时的 Action
• 操作成功时的 Action
• 操作失败时的 Action

useReducer

代码片段(用于函数组件),参考用 useReducer 代替 Redux

使用 useReducer 创建状态机

const [state, dispatch] = useReducer(reducer, {
    filter: filterOptions.SHOW_ALL,
    todoList: []
});

使用 createContext 和 useContext 暴露状态机接口

// Context
import {createContext} from 'react';

const Context = createContext(null);

export default Context

// App
function App() {
    const [state, dispatch] = useReducer(reducer, {
        filter: filterOptions.SHOW_ALL,
        todoList: []
    });
    return (
        <Context.Provider value={{ state, dispatch }}>
            <div className="App">
                我是 APP，要点：useReducer 的初始值不要传 null，要初始化,否则使用 ajax fetch 不成功
                <AddTodo/>
                <TodoList/>
                <Filter/>
            </div>
        </Context.Provider>
    );
}

// TodoList Component
const TodoList = () => {
    const {state, dispatch} = useContext(Context);
    useEffect(()=> {
        fetchTodoList(dispatch)
    },[])
    const getVisibleTodoList = (state, filter)=>{
        switch (filter) {
            case filterOptions.SHOW_ALL:
                return state.todoList
            case filterOptions.SHOW_COMPLETE:
                return state.todoList.filter(todo => todo.isComplete)
            case filterOptions.SHOW_UNCOMPLETE:
                return state.todoList.filter(todo => !todo.isComplete)
        }
    }
    return state.todoList.length > 0 ? (
        <ul>
            {getVisibleTodoList(state, state.filter).map((todo, index) => (
                <li key={index} onClick={() => dispatch(toggleTodo(index))}
                    style={{textDecoration: todo.isComplete ? 'line-through' : 'none'}}>{todo.text}</li>
            ))}
        </ul>
    ) : (<div>加载中...</div>);
};

后面的看文章用 useReducer 代替 Redux