正式学习React(四) ----Redux源码分析

今天看了下Redux的源码，竟然出奇的简单，好吧。简单翻译做下笔记：

喜欢的同学自己可以去github上看：点这里

createStore.js

 import isPlainObject from 'lodash/isPlainObject'
 import $$observable from 'symbol-observable'

 /**
  * These are private action types reserved by Redux.
  * For any unknown actions, you must return the current state.
  * If the current state is undefined, you must return the initial state.
  * Do not reference these action types directly in your code.
  */
 export var ActionTypes = {
   INIT: '@@redux/INIT'
 }

 /**
  * Creates a Redux store that holds the state tree.
  * The only way to change the data in the store is to call `dispatch()` on it.
  *
  * There should only be a single store in your app. To specify how different
  * parts of the state tree respond to actions, you may combine several reducers
  * into a single reducer function by using `combineReducers`.
  *
  * @param {Function} reducer A function that returns the next state tree, given
  * the current state tree and the action to handle.
  *
  * @param {any} [preloadedState] The initial state. You may optionally specify it
  * to hydrate the state from the server in universal apps, or to restore a
  * previously serialized user session.
  * If you use `combineReducers` to produce the root reducer function, this must be
  * an object with the same shape as `combineReducers` keys.
  *
  * @param {Function} [enhancer] The store enhancer. You may optionally specify it
  * to enhance the store with third-party capabilities such as middleware,
  * time travel, persistence, etc. The only store enhancer that ships with Redux
  * is `applyMiddleware()`.
  *
  * @returns {Store} A Redux store that lets you read the state, dispatch actions
  * and subscribe to changes.
  */
 export default function createStore(reducer, preloadedState, enhancer) {

     //如果没有提供初始的state,提供了enhancer,就将初试state置为undefined

   if (typeof preloadedState === 'function' && typeof enhancer === 'undefined') {
     enhancer = preloadedState
     preloadedState = undefined
   }

     //确保enhancer一定是函数
   if (typeof enhancer !== 'undefined') {
     if (typeof enhancer !== 'function') {
       throw new Error('Expected the enhancer to be a function.')
     }
      //这个会返回一个store,只不过是增强版的。
     return enhancer(createStore)(reducer, preloadedState)
   }
 
 
   if (typeof reducer !== 'function') {
     throw new Error('Expected the reducer to be a function.')
   }

      //初始化一些变量，用作闭包。
   var currentReducer = reducer
   var currentState = preloadedState    //undefined || 传进来的初始值
   var currentListeners = []
   var nextListeners = currentListeners
   var isDispatching = false 

      //这个辅助函数是这样的，如果你没有调用dispacth，那么每次调用subscribe来添加监听器的都会被push到nextListenrs,他是currentListerns的一个副本。

      //总的来说这个函数的意义我个人觉得就是保护了currentListeners不被随意污染.保证每次dispacth前状态不变。
   function ensureCanMutateNextListeners() {
     if (nextListeners === currentListeners) {
       nextListeners = currentListeners.slice()
     }
   }

   /**
    * Reads the state tree managed by the store.
    *
    * @returns {any} The current state tree of your application.
    */
       //这个就是返回当前的state
   function getState() {
     return currentState
   }

   /**
    * Adds a change listener. It will be called any time an action is dispatched,
    * and some part of the state tree may potentially have changed. You may then
    * call `getState()` to read the current state tree inside the callback.
    *
    * You may call `dispatch()` from a change listener, with the following
    * caveats:
    *
    * 1. The subscriptions are snapshotted just before every `dispatch()` call.
    * If you subscribe or unsubscribe while the listeners are being invoked, this
    * will not have any effect on the `dispatch()` that is currently in progress.
    * However, the next `dispatch()` call, whether nested or not, will use a more
    * recent snapshot of the subscription list.
    *
    * 2. The listener should not expect to see all state changes, as the state
    * might have been updated multiple times during a nested `dispatch()` before
    * the listener is called. It is, however, guaranteed that all subscribers
    * registered before the `dispatch()` started will be called with the latest
    * state by the time it exits.
    *
    * @param {Function} listener A callback to be invoked on every dispatch.
    * @returns {Function} A function to remove this change listener.
    */

      //简单点说就是 设置监听函数，每次dispatch(action)的时候，这些传进去的listenr们就会全部被调用
   function subscribe(listener) {
     if (typeof listener !== 'function') {
       throw new Error('Expected listener to be a function.')
     }

     var isSubscribed = true

     ensureCanMutateNextListeners()
     nextListeners.push(listener)

     return function unsubscribe() {
       if (!isSubscribed) {
         return
       }

       isSubscribed = false

       ensureCanMutateNextListeners()
       var index = nextListeners.indexOf(listener)
       nextListeners.splice(index, 1)
     }
   }

   /**
    * Dispatches an action. It is the only way to trigger a state change.
    *
    * The `reducer` function, used to create the store, will be called with the
    * current state tree and the given `action`. Its return value will
    * be considered the **next** state of the tree, and the change listeners
    * will be notified.
    *
    * The base implementation only supports plain object actions. If you want to
    * dispatch a Promise, an Observable, a thunk, or something else, you need to
    * wrap your store creating function into the corresponding middleware. For
    * example, see the documentation for the `redux-thunk` package. Even the
    * middleware will eventually dispatch plain object actions using this method.
    *
    * @param {Object} action A plain object representing “what changed”. It is
    * a good idea to keep actions serializable so you can record and replay user
    * sessions, or use the time travelling `redux-devtools`. An action must have
    * a `type` property which may not be `undefined`. It is a good idea to use
    * string constants for action types.
    *
    * @returns {Object} For convenience, the same action object you dispatched.
    *
    * Note that, if you use a custom middleware, it may wrap `dispatch()` to
    * return something else (for example, a Promise you can await).
    */

      //通知store,我要更新state了。
   function dispatch(action) {
     if (!isPlainObject(action)) {
       throw new Error(
         'Actions must be plain objects. ' +
         'Use custom middleware for async actions.'
       )
     }

     if (typeof action.type === 'undefined') {
       throw new Error(
         'Actions may not have an undefined "type" property. ' +
         'Have you misspelled a constant?'
       )
     }

     if (isDispatching) {
       throw new Error('Reducers may not dispatch actions.')
     }

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

     var listeners = currentListeners = nextListeners
     for (var i = 0; i < listeners.length; i++) {
       var listener = listeners[i]
       listener()
     }

     return action
   }

   /**
    * Replaces the reducer currently used by the store to calculate the state.
    *
    * You might need this if your app implements code splitting and you want to
    * load some of the reducers dynamically. You might also need this if you
    * implement a hot reloading mechanism for Redux.
    *
    * @param {Function} nextReducer The reducer for the store to use instead.
    * @returns {void}
    */

      //重置reducer,然后初始化state
   function replaceReducer(nextReducer) {
     if (typeof nextReducer !== 'function') {
       throw new Error('Expected the nextReducer to be a function.')
     }

     currentReducer = nextReducer
     dispatch({ type: ActionTypes.INIT })
   }

   /**
    * Interoperability point for observable/reactive libraries.
    * @returns {observable} A minimal observable of state changes.
    * For more information, see the observable proposal:
    * https://github.com/zenparsing/es-observable
    */

      //暂时我还不知道这个有什么吊用，先不管好了。
   function observable() {
     var outerSubscribe = subscribe
     return {
       /**
        * The minimal observable subscription method.
        * @param {Object} observer Any object that can be used as an observer.
        * The observer object should have a `next` method.
        * @returns {subscription} An object with an `unsubscribe` method that can
        * be used to unsubscribe the observable from the store, and prevent further
        * emission of values from the observable.
        */
       subscribe(observer) {
         if (typeof observer !== 'object') {
           throw new TypeError('Expected the observer to be an object.')
         }

         function observeState() {
           if (observer.next) {
             observer.next(getState())
           }
         }

         observeState()
         var unsubscribe = outerSubscribe(observeState)
         return { unsubscribe }
       },

       [$$observable]() {
         return this
       }
     }
   }

   // When a store is created, an "INIT" action is dispatched so that every
   // reducer returns their initial state. This effectively populates
   // the initial state tree.

      //给currentState设定初始状态
   dispatch({ type: ActionTypes.INIT })

   return {
     dispatch,
     subscribe,
     getState,
     replaceReducer,
     [$$observable]: observable
   }
 }

关于 createStore,我们就关注它返回的对象，subscribe是订阅监听函数的，getState是返回state的，dispacth是发布消息的，更新state的。

剩下那2个就不管他算了。

combineReducers.js

 import { ActionTypes } from './createStore'
 import isPlainObject from 'lodash/isPlainObject'
 import warning from './utils/warning'

 var NODE_ENV = typeof process !== 'undefined' ? process.env.NODE_ENV : 'development'

 function getUndefinedStateErrorMessage(key, action) {
   var actionType = action && action.type
   var actionName = actionType && `"${actionType.toString()}"` || 'an action'

   return (
     `Given action ${actionName}, reducer "${key}" returned undefined. ` +
     `To ignore an action, you must explicitly return the previous state.`
   )
 }

 function getUnexpectedStateShapeWarningMessage(inputState, reducers, action, unexpectedKeyCache) {
   var reducerKeys = Object.keys(reducers)
   var argumentName = action && action.type === ActionTypes.INIT ?
     'preloadedState argument passed to createStore' :
     'previous state received by the reducer'

   if (reducerKeys.length === 0) {
     return (
       'Store does not have a valid reducer. Make sure the argument passed ' +
       'to combineReducers is an object whose values are reducers.'
     )
   }

   if (!isPlainObject(inputState)) {
     return (
       `The ${argumentName} has unexpected type of "` +
       ({}).toString.call(inputState).match(/\s([a-z|A-Z]+)/)[1] +
       `". Expected argument to be an object with the following ` +
       `keys: "${reducerKeys.join('", "')}"`
     )
   }

   var unexpectedKeys = Object.keys(inputState).filter(key =>
     !reducers.hasOwnProperty(key) &&
     !unexpectedKeyCache[key]
   )

   unexpectedKeys.forEach(key => {
     unexpectedKeyCache[key] = true
   })

   if (unexpectedKeys.length > 0) {
     return (
       `Unexpected ${unexpectedKeys.length > 1 ? 'keys' : 'key'} ` +
       `"${unexpectedKeys.join('", "')}" found in ${argumentName}. ` +
       `Expected to find one of the known reducer keys instead: ` +
       `"${reducerKeys.join('", "')}". Unexpected keys will be ignored.`
     )
   }
 }

 function assertReducerSanity(reducers) {
   Object.keys(reducers).forEach(key => {
     var reducer = reducers[key]
     var initialState = reducer(undefined, { type: ActionTypes.INIT })

     if (typeof initialState === 'undefined') {
       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.`
       )
     }

     var type = '@@redux/PROBE_UNKNOWN_ACTION_' + Math.random().toString(36).substring(7).split('').join('.')
     if (typeof reducer(undefined, { type }) === 'undefined') {
       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.`
       )
     }
   })
 }

 /**
  * Turns an object whose values are different reducer functions, into a single
  * reducer function. It will call every child reducer, and gather their results
  * into a single state object, whose keys correspond to the keys of the passed
  * reducer functions.
  *
  * @param {Object} reducers An object whose values correspond to different
  * reducer functions that need to be combined into one. One handy way to obtain
  * it is to use ES6 `import * as reducers` syntax. The reducers may never return
  * undefined for any action. Instead, they should return their initial state
  * if the state passed to them was undefined, and the current state for any
  * unrecognized action.
  *
  * @returns {Function} A reducer function that invokes every reducer inside the
  * passed object, and builds a state object with the same shape.
  */
 export default function combineReducers(reducers) {
   var reducerKeys = Object.keys(reducers)
   var finalReducers = {}

   for (var i = 0; i < reducerKeys.length; i++) {
     var key = reducerKeys[i]

     if (NODE_ENV !== 'production') {

        //排除undefined.
       if (typeof reducers[key] === 'undefined') {
         warning(`No reducer provided for key "${key}"`)
       }
     }

       //排除不是func的
     if (typeof reducers[key] === 'function') {
       finalReducers[key] = reducers[key]
     }
   }
   var finalReducerKeys = Object.keys(finalReducers)

   if (NODE_ENV !== 'production') {
     var unexpectedKeyCache = {}
   }

   var sanityError
   try {
        //这里会对每个子reducer的state进行检查。返回不能为undefined
     assertReducerSanity(finalReducers)
   } catch (e) {
     sanityError = e
   }

      //我们最最关心的就是这个返回函数，其实我们如果已经堆这个函数有一定的了解，就知道这个函数其实就把子reducers全部在这个函数里执行一边，
      //返回最后的state
   return function combination(state = {}, action) {
     if (sanityError) {
       throw sanityError
     }

     if (NODE_ENV !== 'production') {
       var warningMessage = getUnexpectedStateShapeWarningMessage(state, finalReducers, action, unexpectedKeyCache)
       if (warningMessage) {
         warning(warningMessage)
       }
     }

     var hasChanged = false
     var nextState = {}
     for (var i = 0; i < finalReducerKeys.length; i++) {
       var key = finalReducerKeys[i]
       var reducer = finalReducers[key]
       var previousStateForKey = state[key]
       var nextStateForKey = reducer(previousStateForKey, action)
       if (typeof nextStateForKey === 'undefined') {
         var errorMessage = getUndefinedStateErrorMessage(key, action)
         throw new Error(errorMessage)
       }
       nextState[key] = nextStateForKey
       hasChanged = hasChanged || nextStateForKey !== previousStateForKey
     }
     return hasChanged ? nextState : state
   }
 }

compose.js

 /**
  * Composes single-argument functions from right to left. The rightmost
  * function can take multiple arguments as it provides the signature for
  * the resulting composite function.
  *
  * @param {...Function} funcs The functions to compose.
  * @returns {Function} A function obtained by composing the argument functions
  * from right to left. For example, compose(f, g, h) is identical to doing
  * (...args) => f(g(h(...args))).
  */

 export default function compose(...funcs) {

      //funcs就是我们传入的中间件，fn1,fn2...
      //如果什么都没有，就返回一个function(arg){return arg};当作默认的中间件
   if (funcs.length === 0) {
     return arg => arg
   }

      //排除所有中间件参数中不是function的
   funcs = funcs.filter(func => typeof func === 'function')

       //如果只有一个中间件参数，就把这个唯一的中间件返回。
   if (funcs.length === 1) {
     return funcs[0]
   }

      //如果中间件参数个数超过1个

      //取出最后一个中间件参数
   const last = funcs[funcs.length - 1]

      //将funcs中最开头到倒数第二个的数组复制一份，即排除了最后一个中间件
   const rest = funcs.slice(0, -1)

   //返回(...args) => f(g(h(...args))).
   return (...args) => rest.reduceRight((composed, f) => f(composed), last(...args))
 }

解释完compose.js，我们可以简单的理解为： compose就是用来将函数组合起来使用。

 假如：compose(f, g, h)
 分装后：(...args) => f(g(h(...args))).

applyMiddleware.js

 import compose from './compose'

 /**
  * Creates a store enhancer that applies middleware to the dispatch method
  * of the Redux store. This is handy for a variety of tasks, such as expressing
  * asynchronous actions in a concise manner, or logging every action payload.
  *
  * See `redux-thunk` package as an example of the Redux middleware.
  *
  * Because middleware is potentially asynchronous, this should be the first
  * store enhancer in the composition chain.
  *
  * Note that each middleware will be given the `dispatch` and `getState` functions
  * as named arguments.
  *
  * @param {...Function} middlewares The middleware chain to be applied.
  * @returns {Function} A store enhancer applying the middleware.
  */
 export default function applyMiddleware(...middlewares) {

     //短短几行，很简单哟。
     //返回我们在creteStore里看到的enhancer

   return (createStore) => (reducer, preloadedState, enhancer) => {
     var store = createStore(reducer, preloadedState, enhancer)
     var dispatch = store.dispatch
     var chain = []

     var middlewareAPI = {
       getState: store.getState,
       dispatch: (action) => dispatch(action)
     }

      //将 当前store的getState和dispacth方法挂载到中间件里。
     chain = middlewares.map(middleware => middleware(middlewareAPI))
     dispatch = compose(...chain)(store.dispatch)

     return {
       ...store,
       dispatch
     }
   }
 }

假如我们在代码里看到：

const store =  applyMiddleware(promise, thunk, observable)(createStore)(reducer);
亦或是

const store = createStore(

reducer,state,applyMiddleware(promise, thunk, observable)

);

我们根据上面的代码走一遍，看看是怎么个执行过程。

1： applyMiddleware(promise, thunk, observable) 会返回enhancer.

2：enhancer(createStore) 返回一个将中间件参数都闭包进来的 createStore函数，此时我们叫他加强版！我给他标注是红色；

3：createStore(reducer) 然后就是上面代码的21-34行。 最后我们关心一下这个返回的store里的dispatch,其实是加强版的，因为里面是执行中间件的！！但是中间件里面到底干了什么，
我们目前不得而知，总之记住，dispacth现在牛逼了就行。比如可以处理异步啦等等。

bindActionCreators.js

 function bindActionCreator(actionCreator, dispatch) {
   return (...args) => dispatch(actionCreator(...args))
 }

 /**
  * Turns an object whose values are action creators, into an object with the
  * same keys, but with every function wrapped into a `dispatch` call so they
  * may be invoked directly. This is just a convenience method, as you can call
  * `store.dispatch(MyActionCreators.doSomething())` yourself just fine.
  *
  * For convenience, you can also pass a single function as the first argument,
  * and get a function in return.
  *
  * @param {Function|Object} actionCreators An object whose values are action
  * creator functions. One handy way to obtain it is to use ES6 `import * as`
  * syntax. You may also pass a single function.
  *
  * @param {Function} dispatch The `dispatch` function available on your Redux
  * store.
  *
  * @returns {Function|Object} The object mimicking the original object, but with
  * every action creator wrapped into the `dispatch` call. If you passed a
  * function as `actionCreators`, the return value will also be a single
  * function.
  */
 export default function bindActionCreators(actionCreators, dispatch) {
   if (typeof actionCreators === 'function') {
     return bindActionCreator(actionCreators, dispatch)
   }

   if (typeof actionCreators !== 'object' || actionCreators === null) {
     throw new Error(
       `bindActionCreators expected an object or a function, instead received ${actionCreators === null ? 'null' : typeof actionCreators}. ` +
       `Did you write "import ActionCreators from" instead of "import * as ActionCreators from"?`
     )
   }

   var keys = Object.keys(actionCreators)
   var boundActionCreators = {}
   for (var i = 0; i < keys.length; i++) {
     var key = keys[i]
     var actionCreator = actionCreators[key]
     if (typeof actionCreator === 'function') {
       boundActionCreators[key] = bindActionCreator(actionCreator, dispatch)
     }
   }
   return boundActionCreators
 }

这个东西唯一的好处是往下传actionCreator和dispatch的时候，少些代码，无形中在props里加了内容！！！！

具体应用场景，大家自行谷歌。因为我也才学这个第8天，这函数我自己都没用过····哈哈。不过以后肯定要用了！！！

代码十分简单，我就不分析了。

这里有一篇园内的朋友写的它的应用，凑合看吧：点这里