redux的源码解析

一、 redux出现的动机

1. Javascript 需要管理比任何时候都要多的state
2. state 在什么时候，由于什么原因，如何变化已然不受控制。
3. 来自前端开发领域的新需求
4. 我们总是将两个难以理清的概念混淆在一起：变化和异步。
5. Redux 视图让state 的变化变得可预测。

二、 核心概念

 1. 想要更新state中的数据，你需要发起一个action,Action就是一个普通的JavaScript 对象用来描述发生了什么。为了把actin 和state串起来开发一些函数，就是redcer。

三、 三大原则

1. 单一数据源 整个应用的state被存储在一棵objecttree中， 并且这个 object tree 只 存在于一个唯一的store 中。

2. state 是只读的，唯一改变state的方法就是触发action，action 是一个用于描述已发 生事件的普通对象。（确保了视图和网络请求不能直接修改state，只能表达想要修改的意图）

3. 使用纯函数来执行修改为了描述action如何改变state tree ，你需要编写reducers。

四、 源码解析

1. 入口文件index.js

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.
 */

// 判断文件是否被压缩了
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.'
  )
}
/*
  从入口文件可以看出 redux 对外暴露了5个API。
  createStore ,
  combineReducers,
  bindActionCreators,
 applyMiddleware,
 compose,
 */
export {
  createStore,
  combineReducers,
  bindActionCreators,
  applyMiddleware,
  compose,
  __DO_NOT_USE__ActionTypes
}

2. 对外暴露的第一个API createStore => createStore.js

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 {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.
 */
/*
  从源码上可以看出 createStore 是一个函数。接收三个参数 reducer, preloadeState, enhancer
 */
export default function createStore(reducer, preloadedState, enhancer) {
  // 如果 preloadeState 是一个函数 && enhancer未定义preloadeState 和 enhancer交换位置
  if (typeof preloadedState === 'function' && typeof enhancer === 'undefined') {
    enhancer = preloadedState
    preloadedState = undefined
  }
  //
  if (typeof enhancer !== 'undefined') {
    if (typeof enhancer !== 'function') {
      throw new Error('Expected the enhancer to be a function.')
    }
    // 上面两个判断是为了确保 enchancer是个函数
    return enhancer(createStore)(reducer, preloadedState)
  }

  // reducer必须是 个函数，如果不是个函数给出友好的 提示
  if (typeof reducer !== 'function') {
    throw new Error('Expected the reducer to be a function.')
  }

  let currentReducer = reducer    // 把reducer 暂存起来
  let currentState = preloadedState // 把preloadeState暂存起来
  let currentListeners = []
  let nextListeners = currentListeners
  let isDispatching = false   //判断是否正处于dispatch中

  // 如果 nextListeners 和 currrentListeners 都指向一个内存空间的时候， 深复制一份出来。确保两个之间不会相互影响。
  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.
   */
  // 获取目前的 state的值。
  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.'
      )
    }

    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.
   */
  // 经典的订阅函数
  function subscribe(listener) {
    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.'
      )
    }
    // 闭包的经典应用 每次订阅一个事件的时候，都有一个内部状态， 用于后续的取消订阅
    let isSubscribed = true
    // 深复制一份监听对象
    ensureCanMutateNextListeners()
    // 把每个监听对象都放置于一个数组中，保存下来，（精华之处，对闭包的使用登峰造极）
    nextListeners.push(listener)
    // 当注册一个监听事件的返回一个函数，调用这个函数可以取消订阅，具体操作方法就是从监听的数组中移出掉。
    return function unsubscribe() {
      // 防止重复取消订阅
      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.'
        )
      }
      // 对应上面那条，防止重复取消订阅
      isSubscribed = false

      ensureCanMutateNextListeners()
      const index = nextListeners.indexOf(listener)
        // 删除数组中某一项的方法 splice
      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).
   */
  // 派发一个事件
  function dispatch(action) {
    // p、判断action是否是个对象
    if (!isPlainObject(action)) {
      throw new Error(
        'Actions must be plain objects. ' +
          'Use custom middleware for async actions.'
      )
    }
    // 严格控制 action 的书写格式  { type: 'INCREMENT'}
    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.')
    }
    // isDipatching 也是闭包的经典用法
      /*
        触发 dispatch的时候 把 isDispatching 改为 true。 照应全篇中对 dispatching 正在触发的时候的判断
        finally 执行完毕的时候 置为 false
       */
    try {
      isDispatching = true
      // 获取最新 的state 值。 currentState 经典之处闭包
      currentState = currentReducer(currentState, action)
    } finally {
      isDispatching = false
    }

    // 对监听对象从新赋值 其实里面每个listener都是一个函数。 于subscribe相对应
    // 当每发生一个dispatch 事件的时候， 都循环调用,触发监听事件
    const listeners = (currentListeners = nextListeners)
    for (let i = 0; i < listeners.length; i++) {
      const 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}
   */
  // 替换 reducer 用 新的 reducer替换以前的reducer 参数同样必须是函数
  function replaceReducer(nextReducer) {
    if (typeof nextReducer !== 'function') {
      throw new Error('Expected the nextReducer to be a function.')
    }

    currentReducer = nextReducer
    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
   */
  // 观察模式
  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.
       */
      subscribe(observer) {
        if (typeof observer !== 'object' || observer === null) {
          throw new TypeError('Expected the observer to be an object.')
        }

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

        observeState()
        const 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.
    // 触发一state 树
  dispatch({ type: ActionTypes.INIT })
  /*
     由此可以看出 调用 createStore(); 后。对外暴露的方法
     1. dispatch
     2. subscribe
     3. getState
     4. replaceReducer
     5.观察模式
     const store = createStore(reducer, preloadedState, enchancer);
     store.dispatch(action);
     store.getState(); // 为什么这个方法能够获得 state的值。因为 currentState 的闭包实现。
     store.subscribe(() => console.log(store.getState()));
     store.replaceReducer(reducer);
     总结：纵观createStore方法的实现，其实都是建立在闭包的基础之上。可谓是把闭包用到了极致。
   */
  return {
    dispatch,
    subscribe,
    getState,
    replaceReducer,
    [$$observable]: observable
  }
}

3. combineReducers.js

import ActionTypes from './utils/actionTypes'
import warning from './utils/warning'
import isPlainObject from './utils/isPlainObject'

function getUndefinedStateErrorMessage(key, action) {
  const actionType = action && action.type
  const actionDescription =
    (actionType && `action "${String(actionType)}"`) || 'an action'

  return (
    `Given ${actionDescription}, reducer "${key}" returned undefined. ` +
    `To ignore an action, you must explicitly return the previous state. ` +
    `If you want this reducer to hold no value, you can return null instead of undefined.`
  )
}

function getUnexpectedStateShapeWarningMessage(
  inputState,
  reducers,
  action,
  unexpectedKeyCache
) {
  const reducerKeys = Object.keys(reducers)
  const 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('", "')}"`
    )
  }

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

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

  if (action && action.type === ActionTypes.REPLACE) return

  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 assertReducerShape(reducers) {
  Object.keys(reducers).forEach(key => {
    const reducer = reducers[key]
    const 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. If you don't want to set a value for this reducer, ` +
          `you can use null instead of undefined.`
      )
    }

    if (
      typeof reducer(undefined, {
        type: ActionTypes.PROBE_UNKNOWN_ACTION()
      }) === '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, but can be null.`
      )
    }
  })
}

/**
 * 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.
 */

/*
  combineReducers 顾名思义就是合并reduces的一个方法。
  1. 为了项目便于维护与管理我们就需要拆按模块拆分reducers。
  2. 而combineReducers就是为了解决这个的问题的。

 */
export default function combineReducers(reducers) { // 参数reducers 是一个对象
  const reducerKeys = Object.keys(reducers) // 获取reducers的k
  const finalReducers = {}
  for (let i = 0; i < reducerKeys.length; i++) {
    const key = reducerKeys[i]

    if (process.env.NODE_ENV !== 'production') {
      if (typeof reducers[key] === 'undefined') {
        warning(`No reducer provided for key "${key}"`)
      }
    }

    // 深复制一份reducers出来， 防止后续操作出现不可控因素
    if (typeof reducers[key] === 'function') {
      finalReducers[key] = reducers[key]
    }
  }
  const finalReducerKeys = Object.keys(finalReducers)

  let unexpectedKeyCache
  if (process.env.NODE_ENV !== 'production') {
    unexpectedKeyCache = {}
  }

  let shapeAssertionError
  try {
    assertReducerShape(finalReducers)
  } catch (e) {
    shapeAssertionError = e
  }
  // 闭包的运用， 把合并的 reducer保存下来。
  return function combination(state = {}, action) {
    if (shapeAssertionError) {
      throw shapeAssertionError
    }

    if (process.env.NODE_ENV !== 'production') {
      const warningMessage = getUnexpectedStateShapeWarningMessage(
        state,
        finalReducers,
        action,
        unexpectedKeyCache
      )
      if (warningMessage) {
        warning(warningMessage)
      }
    }

    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]
      // 把合并的时候的key值作为Key值为标准。 在循环遍历的时候取出对应的 reducers 触发 reducer函数。
      /*
        其实对应的createStore.js中的
        try {
          isDispatching = true
          // 获取最新 的state 值。 currentState 经典之处闭包
          currentState = currentReducer(currentState, action)
        } finally {
          isDispatching = false
        }  
       */
      const nextStateForKey = reducer(previousStateForKey, action)
      if (typeof nextStateForKey === 'undefined') {
        const errorMessage = getUndefinedStateErrorMessage(key, action)
        throw new Error(errorMessage)
      }
      nextState[key] = nextStateForKey
      hasChanged = hasChanged || nextStateForKey !== previousStateForKey
    }
    return hasChanged ? nextState : state
  }
}
/**
 * 使用方法
 * const reducers = combineReducers({ reducer1, reducer2 });
 * const store = createStore(reducers, preloadedState, enchancer);
 */

4. bindActionCreators.js

// 主要这个函数
function bindActionCreator(actionCreator, dispatch) {
  return function() {
    return dispatch(actionCreator.apply(this, arguments))
  }
}

/**
 * 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.
 */

/*
   接受两个参数，一个action creator， 一个是 value的 action creator的对象。
   dispatch 。 一个由 Store 实列 提供的dispatch的函数。 看createStore.js源码就可以做知道其中原理。
 */
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"?`
    )
  }

  const keys = Object.keys(actionCreators)
  const boundActionCreators = {}
  for (let i = 0; i < keys.length; i++) {
    const key = keys[i]
    const actionCreator = actionCreators[key]
    if (typeof actionCreator === 'function') {
      boundActionCreators[key] = bindActionCreator(actionCreator, dispatch)
    }
  }
  /*
     一个与原对象类似的对象，只不过这个对象的 value 都是会直接 dispatch 原 action creator 返回的结果的函数。
     如果传入一个单独的函数作为 actionCreators，那么返回的结果也是一个单独的函数。
     本来触发 action 的方法是 store.dispatch(action);
     经过这个方法封装后 可以直接调用函数名字
     aa('参数');
   */
  return boundActionCreators
}

5. applyMiddleware.js  在redux 中最难理解的一个函数。

import compose from './compose'
import createStore from "./createStore";

/**
 * 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.
 */
// 通过看源码知道 applyMiddleware返回一个高阶函数。
/*
  applyMiddleware的使用地方
  const store = createStore(reducer,{}, applyMiddleware(...middlewares));
  
  由此可以看出 applyMiddlewares 的使用方法主要是和 createStore.js 中 createStore方法的第三个参数对应。翻开源码
  if (typeof enhancer !== 'undefined') {
    if (typeof enhancer !== 'function') {
      throw new Error('Expected the enhancer to be a function.')
    }
    // 上面两个判断是为了确保 enchancer是个函数
    return enhancer(createStore)(reducer, preloadedState)
  }
 */
export default function applyMiddleware(...middlewares) {
  // createSotre 中 的第三个参数 enhancer
  return createStore => (...args) => {
    // 通过对应的代码可以发现其实 ...aregs 对应的是  reducer, preloadedState
    const store = createStore(...args)
    let dispatch = () => {
      throw new Error(
        `Dispatching while constructing your middleware is not allowed. ` +
          `Other middleware would not be applied to this dispatch.`
      )
    }
      // 定义中间件必须满足的条件。 API   getState, dispatch();
    const middlewareAPI = {
      getState: store.getState,
      dispatch: (...args) => dispatch(...args)  
    }
    const chain = middlewares.map(middleware => middleware(middlewareAPI))

      /**
       * 

      export default function compose(...funcs) {
          if (funcs.length === 0) {
              return arg => arg
          }

          if (funcs.length === 1) {
              return funcs[0]
          }

          return funcs.reduce((a, b) => (...args) => a(b(...args)))
      }
       compose函数 主要是 利用数组 reducer 方法对参数的处理。

  */
      dispatch = compose(...chain)(store.dispatch)
    // 通过这个返回值我们可以知道 在createStore.js中enchancer的返回值。
    return {
      ...store,
      dispatch
    }
  }
}

/**
 *  applyMiddlewares函数比较难理解。 多看几个中间件，比如 logger 和 redux-thunk 等。对该方法能够更深的理解。
 */

五、 redux的总结

通过阅读redux的源码，印象最深的就是如何手动写个订阅模式，数据改变的时候，如何触发所有监听事件。闭包的运用登峰造极。其中最难的两个函数 applyMiddlewares 和compose.js 还需要细细体会。没有真正领悟到其精华之处。

谢谢大家。