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