createStore用來生成Store，fn形式為(state, action) => state的純函數，返回新的state而不是修改的

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

Store對象包含所有數據，state就是Store在某個時刻的數據快照，當前時刻的state可以用store.getState()獲取

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

const state = store.getState()

改變內部 state 的唯一方法是 dispatch 一個 action（store.dispatch()）,action只是一個用來描述已發生事件的對象

// action 可以被序列化，用日記記錄和儲存下來，後期還可以以回放的方式執行
store.dispatch({ type: ‘INCREMENT‘ });
// 1
store.dispatch({ type: ‘INCREMENT‘ });
// 2
store.dispatch({ type: ‘DECREMENT‘ });
// 1
store.dispatch({ type: ‘ADD_TODO‘, payload: ‘Learn Redux‘ });
 

引用官方原話-- 為了描述action如何改變state tree 需要編者reducers，Reducer只是一些純函數(固定輸入=>固定輸出) 隨著應用變大，可以把reducers拆成多個小reducers，分別獨立操作state tree的不同部分

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‘
let reducer = combineReducers({ visibilityFilter, todos })
let store = createStore(reducer)

初入redux -01