react-redux-reducer

reducer是对dispatch(action)的响应,是一个纯函数，接受旧的state和action，返回新的state.

//纯函数要注意的点，下面的例子myname不变

<script>
  var myname = {name1:"wood"};
  A(myname);
  document.write(myname.name1);

  function A(n) {
    //n.name1 = "Yao";
    n = {name1:"Yao"};
  }
</script>

//一般，对象类型的参数传入时，函数内部的更改会反映到原变量；
//当在函数体内使用赋值操作时，系统就创建了一个变量名为p的变量。这个p是函数内部的变量，对它进行赋值当然只在函数体内起作用

纯函数是指不依赖于且不改变它作用域之外的变量状态的函数，纯函数的返回值只由它调用时的参数决定，它的执行不依赖于系统的状态（比如：何时、何处调用它）。

//不要在纯函数内做以下操作

修改传入参数；
执行有副作用的操作，如 API 请求和路由跳转；
调用非纯函数，如 Date.now() 或 Math.random()。

import { VisibilityFilters } from './actions'

const initialState = {
  visibilityFilter: VisibilityFilters.SHOW_ALL,
  todos: []
};

//ES参数默认值
function todoApp(state = initialState, action) {
　

switch (action.type) {
    case SET_VISIBILITY_FILTER:
      return Object.assign({}, state, {
        visibilityFilter: action.filter
      })
    default:
      return state
  　}
}

state设计：尽可能地把 state 范式化，不存在嵌套，把state 想像成数据库

//不要修改 state。 

//使用 Object.assign() 新建了一个副本。
//也可以开启对ES7提案对象展开运算符的支持, 从而使用 { ...state, ...newState } 达到相同的目的。
//有些时候可以简单使用：（深拷贝）
const obj1 = JSON.parse(JSON.stringify(obj));

处理多个action

import {
  ADD_TODO,
  TOGGLE_TODO,
  SET_VISIBILITY_FILTER,
  VisibilityFilters
} from './actions'

...

function todoApp(state = initialState, action) {
  switch (action.type) {
    case SET_VISIBILITY_FILTER:
      return Object.assign({}, state, {
        visibilityFilter: action.filter
      })
    case ADD_TODO:
      return Object.assign({}, state, {
        todos: [
          ...state.todos,
          {
            text: action.text,
            completed: false
          }
        ]
      })
    case TOGGLE_TODO:
      return Object.assign({}, state, {
        todos: state.todos.map((todo, index) => {
          if (index === action.index) {
            return Object.assign({}, todo, {
              completed: !todo.completed
            })
          }
          return todo
        })
     })
    default:
      return state
  }
}

拆分 Reducer

reducer 合成与拆分

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

function todoApp(state = initialState, action) {
  switch (action.type) {
    case SET_VISIBILITY_FILTER:
      return Object.assign({}, state, {
        visibilityFilter: action.filter
      })
    case ADD_TODO:
      return Object.assign({}, state, {
        todos: todos(state.todos, action)
      })
    case TOGGLE_TODO:
      return Object.assign({}, state, {
        todos: todos(state.todos, action)
      })
    default:
      return state
  }
}

//全部拆分，去掉
function todos(state = [], action) {
  switch (action.type) {
    case ADD_TODO:
      return [
        ...state,
        {
          text: action.text,
          completed: false
        }
      ]
    case TOGGLE_TODO:
      return state.map((todo, index) => {
        if (index === action.index) {
          return Object.assign({}, todo, {
            completed: !todo.completed
          })
        }
        return todo
      })
    default:
      return state
  }
}

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

function todoApp(state = {}, action) {
  return {
    visibilityFilter: visibilityFilter(state.visibilityFilter, action),
    todos: todos(state.todos, action)
  }
}

Redux 提供了 combineReducers() 工具类

import { combineReducers } from 'redux'

const todoApp = combineReducers({
  visibilityFilter,
  todos
})

export default todoApp

//==
export default function todoApp(state = {}, action) {
  return {
    visibilityFilter: visibilityFilter(state.visibilityFilter, action),
    todos: todos(state.todos, action)
  }
}

//也可以设置不同的key 

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

function reducer(state = {}, action) {
  return {
    a: doSomethingWithA(state.a, action),
    b: processB(state.b, action),
    c: c(state.c, action)
  }
}

//combineReducers() 生成一个函数，这个函数来调用一系列 reducer，每个 reducer 根据它们的 key 来筛选出 state 中的一部分数据并处理，然后这个生成的函数再将所有 reducer 的结果合并成一个大的对象。

import { combineReducers } from 'redux'
import * as reducers from './reducers'

const todoApp = combineReducers(reducers)