初入redux -01

createStore用来生成Store，fn形式为(state, action) => state的纯函数，返回新的state而不是修改的

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

Store对象包含所有数据，state就是Store在某个时刻的数据快照，当前时刻的state可以用store.getState()获取

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

const state = store.getState()

改变内部 state 的唯一方法是 dispatch 一个 action（store.dispatch()）,action只是一个用来描述已发生事件的对象

// action 可以被序列化，用日记记录和储存下来，后期还可以以回放的方式执行
store.dispatch({ type: 'INCREMENT' });
// 1
store.dispatch({ type: 'INCREMENT' });
// 2
store.dispatch({ type: 'DECREMENT' });
// 1
store.dispatch({ type: 'ADD_TODO', payload: 'Learn Redux' });

引用官方原话-- 为了描述action如何改变state tree 需要编者reducers，Reducer只是一些纯函数(固定输入=>固定输出) 随着应用变大，可以把reducers拆成多个小reducers，分别独立操作state tree的不同部分（reducers感觉就是根据action的type加工state后返回新的state. 个人觉得reducers就是state，为何这么说？ reducers=> new state）

// 如何理解reducers的拆分？比如主reducers的结构为
Store.state = {
　　visibilityFilter: 'SHOW_ALL',
　　todos: []
}
// 子reducers分别为 visibilityFilter和todus

function visibilityFilter(state = 'SHOW_ALL', action) {
  switch (action.type) {
    case 'SET_VISIBILITY_FILTER':
      return action.filter
    default:
      return state
  }
}

function todos(state = [], action) {
  switch (action.type) {
    case 'ADD_TODO':
      return [
        ...state,
        {
          text: action.text,
          completed: false
        }
      ]
    case 'COMPLETE_TODO':
      return state.map((todo, index) => {
        if (index === action.index) {
          return Object.assign({}, todo, {
            completed: true
          })
        }
        return todo
      })
    default:
      return state
  }
}

import { combineReducers, createStore } from 'redux'
//然后借助redux的combineReducers函数来合并子reducers
let reducer = combineReducers({ visibilityFilter, todos })
let store = createStore(reducer)