Redux源码分析之combineReducers
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
