Redux源码分析之combineReducers

Redux源码分析之基本概念

Redux源码分析之createStore

Redux源码分析之bindActionCreators

Redux源码分析之combineReducers

Redux源码分析之compose

Redux源码分析之applyMiddleware

combineReducers：把recuder函数们，合并成一个新的reducer函数，dispatch的时候，挨个执行每个reducer

我们依旧先看一下combineReduers的使用效果

let { createStore, bindActionCreators, combineReducers } = self.Redux

//默认state
let todoList = [], couter = 0
// reducer
let todoReducer = function (state = todoList, action) {
    switch (action.type) {
        case 'add':
            return [...state, action.todo]
        case 'delete':
            return state.filter(todo => todo.id !== action.id)
        default:
            return state
    }
},
    couterReducer = function (state = couter, action) {
        switch (action.type) {
            case 'add':
                return ++state
            case 'decrease':
                return --state
            default:
                return state
        }
    }

var reducer = combineReducers({ todoReducer, couterReducer })

//创建store
let store = createStore(reducer)

//订阅
function subscribe1Fn() {
    // 输出state
    console.log(store.getState())
}
store.subscribe(subscribe1Fn)

// action creater
let actionCreaters = {
    add: function (todo) { //添加
        return {
            type: 'add',
            todo
        }
    }, delete: function (id) {
        return {
            type: 'delete',
            id
        }
    }
}

let boundActions = bindActionCreators(actionCreaters, store.dispatch)
console.log('todo add')
boundActions.add({
    id: 12,
    content: '睡觉觉'
})

let boundAdd = bindActionCreators(actionCreaters.add, store.dispatch)
console.log('todo add')
boundAdd({
    id: 13,
    content: '陪媳妇'
})


let counterActionCreater = {
    add: function () {
        return {
            type: 'add'
        }
    },
    decrease: function () {
        return {
            type: 'decrease'
        }
    }
}

let boundCouterActions = bindActionCreators(counterActionCreater, store.dispatch)

console.log('counter add:')
boundCouterActions.add()
console.log('counter decrease:')
boundCouterActions.decrease()

下面是执行结果

   我们一起分析一下：

• 执行todo add的时候，看到counterReducer和 todoReducer的数据都有更新，说明两个reducer都执行了。
• 执行counter add的时候，同样两个recuder都执行，但是因为没有参数，加入的是无效数据，这里就提示我们，是不是该进行一些必要的参数判断呢
• 执行counter decrease的时候，同样两个reducer都执行，但是 todoReducer没有tyepe为decrease的action处理函数，当然没有任何产出

我们再回归源码，删除一些判断的代码逻辑，简化后如下：

• 过滤一下reducer，把reducer和key都存起来
• 返回一个新的reducer函数，新的reducer函数执行的时候，便利存起来的reducer，挨个执行

export default function combineReducers(reducers) {
  const reducerKeys = Object.keys(reducers)
  const finalReducers = {}
  for (let i = 0; i < reducerKeys.length; i++) {
    const key = reducerKeys[i]
    if (typeof reducers[key] === 'function') {
      finalReducers[key] = reducers[key]
    }
  }
  const finalReducerKeys = Object.keys(finalReducers)

  return function combination(state = {}, action) {
    let hasChanged = false
    const nextState = {}
    for (let i = 0; i < finalReducerKeys.length; i++) {
      const key = finalReducerKeys[i]
      const reducer = finalReducers[key]
      const previousStateForKey = state[key]
      const nextStateForKey = reducer(previousStateForKey, action)   
      nextState[key] = nextStateForKey
      hasChanged = hasChanged || nextStateForKey !== previousStateForKey
    }
    return hasChanged ? nextState : state
  }
}

这里额外的分析一下，当store的recuder是复合型的时候，怎么初始化state的

createStore传入的第一个参数recuder，是调用 combineReducers 新生成的reducer(依旧是一个函数)

createStore方法返回之前，会这样一下dispatch({ type: ActionTypes.INIT })，disptach的里面我们就关心下面几句

  try {
      isDispatching = true
      currentState = currentReducer(currentState, action)
    } finally {
      isDispatching = false
    }

也就是执行一下新的reducer，我们继续切换到新的reducer代码里面，同样只关心下面的代码

  return function combination(state = {}, action) { 

    let hasChanged = false
    const nextState = {}
    for (let i = 0; i < finalReducerKeys.length; i++) {
      const key = finalReducerKeys[i]
      const reducer = finalReducers[key]
      const previousStateForKey = state[key]
      const nextStateForKey = reducer(previousStateForKey, action)
           nextState[key] = nextStateForKey
      hasChanged = hasChanged || nextStateForKey !== previousStateForKey
    }
    return hasChanged ? nextState : state
  }

我们就看我们这个例子 combineReducers({ todoReducer, couterReducer })， 那么上面的key就会是  todoReducer, couterReducer, 那么初始化完毕的state得数据结构就是这样的

{todoReducer：....,couterReducer：......}，

有人会想，初始化state，你上次不是用了两种方式，我这里只能说对不起，当你用的是复合型的reducer初始化state的时候，你用第二个参数来初始化state行不通的，

因为为了方便解析代码，上面我是省略了一部分的 ，下面再看看更完整一点的代码（我还是省略了一下）

export default function combineReducers(reducers) {
  const reducerKeys = Object.keys(reducers)
  const finalReducers = {}
  for (let i = 0; i < reducerKeys.length; i++) {
    const key = reducerKeys[i]
    if (typeof reducers[key] === 'function') {
      finalReducers[key] = reducers[key]
    }
  }
  const finalReducerKeys = Object.keys(finalReducers) 

  let shapeAssertionError
  try {
    assertReducerShape(finalReducers)
  } catch (e) {
    shapeAssertionError = e
  }

  return function combination(state = {}, action) {
    if (shapeAssertionError) {
      throw shapeAssertionError
    }
     .......
  }

这里如果你没通过 aessertRecucerShape检查，是没法进行下去的，我们那看看aessertRecucerShape是个啥玩意,看备注。

function assertReducerShape(reducers) {
  Object.keys(reducers).forEach(key => {
    const reducer = reducers[key]
    const initialState = reducer(undefined, { type: ActionTypes.INIT })  // 传入 undefined，让recuder默认值生效，

    if (typeof initialState === 'undefined') {   // 如果没有默认值，返回的state就是undefined，然后抛出异常
      throw new Error(
        `Reducer "${key}" returned undefined during initialization. ` +
        `If the state passed to the reducer is undefined, you must ` +
        `explicitly return the initial state. The initial state may ` +
        `not be undefined. If you don't want to set a value for this reducer, ` +
        `you can use null instead of undefined.`
      )
    }

    const type = '@@redux/PROBE_UNKNOWN_ACTION_' + Math.random().toString(36).substring(7).split('').join('.')
    if (typeof reducer(undefined, { type }) === 'undefined') {  // 这个主要是防止在recuder你真的自己定义了对type为ActionTypes.INIT处理，创建一个随机的type，测试一下，你应该返回的是有效的state

throw new Error( `Reducer "${key}" returned undefined when probed with a random type. ` + `Don't try to handle ${ActionTypes.INIT} or other actions in "redux/*" ` + `namespace. They are considered private. Instead, you must return the ` + `current state for any unknown actions, unless it is undefined, ` + `in which case you must return the initial state, regardless of the ` + `action type. The initial state may not be undefined, but can be null.` ) } }) }

这就说明了上述的问题，（-_-)

回顾

1.  combineReducers 的参数是一个对象

2. 执行结果返回的依旧是一个reducer

3. 通过combineReducers 返回的reducer创建的store, 再派发某个action的时候，实际上每个内在的reducer都会执行

4. createStrore使用合成的reducer创建的store， 他再派发action返回的是总的大的state