redux源码浅入浅出

　　运用redux有一段时间了，包括redux-thunk和redux-saga处理异步action都有一定的涉及，现在技术栈转向阿里的dva+antd，好用得不要不要的，但是需要知己知彼要对react家族有一点源码上的深入了，就从redux开始吧。

　　redux源码是那么简洁、清晰、干净，让我忍不住一口气全部看完了还意犹未尽的写一篇随笔，mark一下，过段时间回头再重新细细评味学习一波。原谅我把整片代码贴出来，因为我懒啊，我会尽量把代码注解写详细一点。

　index

　  redux对外暴露出的api，这里可以看出文件结构和功能块相关分得很清晰。

import createStore from './createStore'
import combineReducers from './combineReducers'
import bindActionCreators from './bindActionCreators'
import applyMiddleware from './applyMiddleware'
import compose from './compose'
import warning from './utils/warning'
import __DO_NOT_USE__ActionTypes from './utils/actionTypes'

/*
 * This is a dummy function to check if the function name has been altered by minification.
 * If the function has been minified and NODE_ENV !== 'production', warn the user.
 */
// isCrushed 函数仅仅用于判断代码是否处于压缩并再压缩时抛出警告
function isCrushed() {}

if (
  process.env.NODE_ENV !== 'production' &&
  typeof isCrushed.name === 'string' &&
  isCrushed.name !== 'isCrushed'
) {
  warning(
    'You are currently using minified code outside of NODE_ENV === "production". ' +
      'This means that you are running a slower development build of Redux. ' +
      'You can use loose-envify (https://github.com/zertosh/loose-envify) for browserify ' +
      'or setting mode to production in webpack (https://webpack.js.org/concepts/mode/) ' +
      'to ensure you have the correct code for your production build.'
  )
}

export {
  createStore,
  combineReducers,
  bindActionCreators,
  applyMiddleware,
  compose,
  __DO_NOT_USE__ActionTypes
}

　　createStore

　　　redux最重要的api，用于构建Store，并在创建以后还有自己的api，结构十分的清晰

　　

import $$observable from 'symbol-observable'
import ActionTypes from './utils/actionTypes'
import isPlainObject from './utils/isPlainObject'

/**
 * 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 {Function} reducer：返回一个完整独立全新的state tree，接受参数（当前state，需要触发actions集合）
 *
 * @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 {any} [preloadedState] 初始化state，不是必需，可以与服务端渲染水合初始状态，
 * 如果使用combineReduers必需与其中key值一一对应，查看combineReduers实现
 *
 * @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()`.
 * @param {Function} [enhancer] store的外挂，常用middleware中间件，其他暂时不去深入
 *
 * @returns {Store} A Redux store that lets you read the state, dispatch actions
 * and subscribe to changes.
 */
export default function createStore(reducer, preloadedState, enhancer) {

  // 判断参数个数，类似jq===on参数处理方式
  if (typeof preloadedState === 'function' && typeof enhancer === 'undefined') {
    enhancer = preloadedState
    preloadedState = undefined
  }

  // 首先判断enhancer（常见的便是middlewares中间件），循环回调将跳过此处
  if (typeof enhancer !== 'undefined') {
    if (typeof enhancer !== 'function') {
      throw new Error('Expected the enhancer to be a function.')
    }
    // middlewares详细解释返回值，
    return enhancer(createStore)(reducer, preloadedState)
  }
  // redux为了方便开发者做了很多友好的提示，只有深入源码才知道的良苦用心，reducer只接受是一个函数
  if (typeof reducer !== 'function') {
    throw new Error('Expected the reducer to be a function.')
  }

  // 保存当前的传入值，后边会涉及到这些值的来回更迭
  let currentReducer = reducer
  let currentState = preloadedState
  let currentListeners = []
  // 监听函数事件队列 为什么不写成let nextListeners = currentListeners = [] 风格吗?
  // 还有为什么需要两个listener数组来存放呢？答案再订阅和dispatch里面
  let nextListeners = currentListeners
  // 是否处于dispatch过程中，我也好奇异步dispatch的时候将怎么变化
  let isDispatching = false

  // 当前监听队列与接下来的监听队列指向同一个数组时，slice出新的数组
  function ensureCanMutateNextListeners() {
    if (nextListeners === currentListeners) {
      // 还是为了去除引用，完成next和current的交替，可以将next看作是current的快照
      nextListeners = currentListeners.slice()
    }
  }

  /**
   * Reads the state tree managed by the store.
   *
   * @returns {any} The current state tree of your application.
   */
  // 只有在非触发状态才能通过api获取当前state的快照
  function getState() {
    if (isDispatching) {
      throw new Error(
        'You may not call store.getState() while the reducer is executing. ' +
          'The reducer has already received the state as an argument. ' +
          'Pass it down from the top reducer instead of reading it from the store.'
      )
    }
    // 注意这里闭包了，直接给里currentState，且他是时常变化的值，需要再稳定的时候取值
    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.
   */
  // dva里面也有监听器，下次去看看源码
  function subscribe(listener) {
    // 老规矩容错
    if (typeof listener !== 'function') {
      throw new Error('Expected the listener to be a function.')
    }

    if (isDispatching) {
      throw new Error(
        'You may not call store.subscribe() while the reducer is executing. ' +
          'If you would like to be notified after the store has been updated, subscribe from a ' +
          'component and invoke store.getState() in the callback to access the latest state. ' +
          'See https://redux.js.org/api-reference/store#subscribe(listener) for more details.'
      )
    }
    // 监听已经完成标志，用于清除监听
    let isSubscribed = true
    // 函数入其名，得到nextListeners
    ensureCanMutateNextListeners()
    // 将监听的事件添加到nextListeners队列中，注意可能添加了队列中已有的事件，不管执行两遍
    nextListeners.push(listener)
    // 返回函数可以移除事件监听
    return function unsubscribe() {
      // 只移除一次
      if (!isSubscribed) {
        return
      }

      if (isDispatching) {
        throw new Error(
          'You may not unsubscribe from a store listener while the reducer is executing. ' +
            'See https://redux.js.org/api-reference/store#subscribe(listener) for more details.'
        )
      }
      // 控制标志位，不多余移除
      isSubscribed = false
      // 再次得到新的nextListeners
      ensureCanMutateNextListeners()
      // 感觉这里如果注册两个相同的事件，会移除前面那个，不知道会不会有问题
      const 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).
   */
  // 相当重要的方法，纯粹的dispatch的参数只接受Object类型的，thunk就是对它进行处理进而能传入
  // function用回调的形式重新dispatch，下次再详细thunk和saga
  function dispatch(action) {
    // isPlainObject用于判断是否是对象
    if (!isPlainObject(action)) {
      throw new Error(
        'Actions must be plain objects. ' +
          'Use custom middleware for async actions.'
      )
    }
    // action关键字限制为 type，为了不造成命名上的困惑一般type前缀我会设置与文件夹同名
    if (typeof action.type === 'undefined') {
      throw new Error(
        'Actions may not have an undefined "type" property. ' +
          'Have you misspelled a constant?'
      )
    }

    // 正在dispatch，这里什么情况会出现这个警告呢！！！
    // 在dispatch中嵌套的调用dispatch会触发这类警告，可能是担心dispatchA(dispatchB(dispatchA))的嵌套循环问题把
    if (isDispatching) {
      throw new Error('Reducers may not dispatch actions.')
    }

    try {
      isDispatching = true
      // 进行reduce操作，记得参数是当前state和action对象，返回全新的State对象，这一手操作是react就高兴了
      currentState = currentReducer(currentState, action)
    } finally {
      //完成一波reducer记得复位标志，表示我的完成dispatch。 
      isDispatching = false
    }

    // 执行事件队列前才拿到最新的listenters，在此之前可能会出现订阅与退订的嵌套等问题，暂存的nextlisteners可以保证dispatch的正常执行
    // 假如出现listenerA(){store.subscribe(listenerA);}的嵌套情况，listeners的长度将再每一次执行延长一直至无限长
    // 当然如果采用len = listeners.length；直接固定循环次数可以解决现在的情况，但是退订等事件的发生也会出现问题，所以暂存是最安全的做法
    const listeners = (currentListeners = nextListeners)
    // 为什么要用for循环不用foreach，想想forEach对空元素的处理的性能问题把
    for (let i = 0; i < listeners.length; i++) {
      const listener = listeners[i]
      // 为什么不直接listeners[i]（）执行呢？而是负值单独调用呢？
      // 赋值之后this的指向不再是listens而是window
      listener()
    }

    // 返回了整个action对象
    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}
   */

  // 替换reducer函数
  function replaceReducer(nextReducer) {
    if (typeof nextReducer !== 'function') {
      throw new Error('Expected the nextReducer to be a function.')
    }

    currentReducer = nextReducer
    // 触发私有的replace action
    dispatch({ type: ActionTypes.REPLACE })
  }

  /**
   * Interoperability point for observable/reactive libraries.
   * @returns {observable} A minimal observable of state changes.
   * For more information, see the observable proposal:
   * https://github.com/tc39/proposal-observable
   */
  // 可以看作是对redux观察者的一个扩展，可作为全局的每次dispatch都执行方法入口
  function observable() {
    const 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.
       */
      // 需要传入一个带next方法的对象，将返回退订钩子
      subscribe(observer) {
        if (typeof observer !== 'object' || observer === null) {
          throw new TypeError('Expected the observer to be an object.')
        }

        function observeState() {
          if (observer.next) {
            // next方法将获得当时的store
            observer.next(getState())
          }
        }

        observeState()
        const unsubscribe = outerSubscribe(observeState)
        // 返回包含退订对象
        return { unsubscribe }
      },
      // 用于获取observeable，这名字取的。。。
      [$$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.
  // 初始化store对象
  dispatch({ type: ActionTypes.INIT })

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

　　

　　compose

　　　一个关于reduce的函数设计，需要特别拿出来说说

　　

/**
 * 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) {
  if (funcs.length === 0) {
    return arg => arg
  }

  if (funcs.length === 1) {
    return funcs[0]
  }
  // reduce用在这里太巧妙了，奇妙的洋葱函数，好吧也没那么奇妙
  // compose(f, g, h)(...args) 等同于 f(g(h(...args)))在后面会形成一个currying函数
  return funcs.reduce((a, b) => (...args) => a(b(...args)))
}

　　

　　applyMiddleware

　　　重点，废话不多说直接上代码

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.
 */
// 非常精髓的一段代码
// createStore中以enhancer(createStore)(reducer, preloadedState)调用
export default function applyMiddleware(...middlewares) {
    // 二阶函数参数...args对应reducer, preloadedState
    return createStore => (...args) => {
        const store = createStore(...args)
        // 这里不应该是 const dispatch = store.dispatch？？有些版本出现这样
        // 猜测：这里避免使用到没有中间件处理过的disptch，后面将传入完整的store.dispatch作为根参数，
        // 求解如果这里只是个警告函数，每个中间件接受到的（{ dispatch, getState }）又是什么呢？
        // 好吧，我又又想到答案了，再下面
        let dispatch = () => {
            throw new Error(
                `Dispatching while constructing your middleware is not allowed. ` +
                `Other middleware would not be applied to this dispatch.`
            )
        }
        // 中间件获取到的能力，获取store快照（isDispatching？？？怎么判断的），触发reducer
        const middlewareAPI = {
            getState: store.getState,
            // 我就是上面的答案：这里dispatch用闭包并不是直接的引用，dispatch会根据dispatch = compose(...chain)(store.dispatch)
            // 而变化，在此之前调用dispatch会爆出警告！！！
            dispatch: (...args) => dispatch(...args)
        }
        // middleware应该是高阶函数，return 了一个function在chain数组
        // 对应thunk的createThunkMiddleware({dispatch, getStat})，这里只要注意传入了什么，thunk内详细分析怎么运行中间件
        const chain = middlewares.map(middleware => middleware(middlewareAPI))
        // 将store.dispatch作为二阶参数传入，最终将对应中间件最内层的action，
        // 注意下面这个例子：
        // applyMiddleware(log1, log2, log3)，在这里通过洋葱函数的处理dispatch变成log11(log22(log33(store.dispatch)))这样一个函数
        // log11是log1({dispatch, getState})的返回函数，以此类推，这种结构也限定里中间件函数的基本结构是
        // ({ dispatch, getState }) => next => action => {} ，最开始可能对这个结构很迷糊，why，看下面
　　　　 //  这里就形成一个第一个参数为store.dispatch的currying函数，之后传入的action，dispatch(action)都将一并视为compose(...chain)(store.dispatch)(action)

        dispatch = compose(...chain)(store.dispatch)

        // 对应返回在了createStore里即Store，全新的dispatch诞生
        return {
            ...store,
            dispatch
        }
    }
}

// 觉得把redux-thunk的代码一起贴出来才有参照性
function createThunkMiddleware(extraArgument) {
    // 其实thunk内容实在是简洁，判断类型将dispatch放入到函数里面，这里的dispatch是层层包装过的
    // 那么我们来分析针对整个箭头函数和中间件结构进行分析一下
    // log11的next(action)对应log22(log33(action))，
    // log22的next(action)对应log33(action)，
    // log33的next对应store.dispatch,最后返回一个需要传参为action的函数，
    // action对应{type: 'TO_DO',text: '1'}一直传递无变化，只有next变化，形成一个层层执行
    // 
    // 而执行顺序有点像冒泡，从外到里再从里到外，如果上面的log每个都有before和after的话，顺序将是
    // log11.before > log22.before > log33.before > store.dispatch > log33.after > log22.after > log11.after > end
    // 每一个中间件将对dispatch之前和之后作些动作
    return ({ dispatch, getState }) => next => action => {
      if (typeof action === 'function') {
        return action(dispatch, getState, extraArgument);
      }
  
      return next(action);
    };
}
  
const thunk = createThunkMiddleware();
thunk.withExtraArgument = createThunkMiddleware;
  
//  export default thunk;

// 调用方式 createStore(reducer, applyMiddleware(thunk))

　　其他还有几个文件就不贴出来了，只有深入到源码才能感受代码之美，redux简直称为精粹。redux就是典型的百行代码千行文档，也只有看了源码才能略微理解其用途和技巧。我很清楚上面的注解十分的混乱，有的地方描述肯定有误，也没能力三言两语把一个经典框架描述得清楚，敬请谅解，与君共勉。