阅读redux源码

redux/index.js中export出的函数:(为了简洁省略了部分代码)

import createStore from './createStore'
import combineReducers from './combineReducers'
import bindActionCreators from './bindActionCreators'
import applyMiddleware from './applyMiddleware'
import compose from './compose'

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

先看其中几个简单的函数

bindActionCreators

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.
 */
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)
    }
  }
  return boundActionCreators
}

这个函数很好理解。bind一组action creators。

applyMiddleware

/**
 * 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) {
  // 返回一个enhancer
  return createStore => (...args) => {
    const store = createStore(...args)
    //改写dispatch函数。在compose middleware的过程中禁止调用dispatch
    let dispatch = () => {
      throw new Error(
        `Dispatching while constructing your middleware is not allowed. ` +
          `Other middleware would not be applied to this dispatch.`
      )
    }
    //假的store,改写了dispatch函数
    const middlewareAPI = {
      getState: store.getState,
      dispatch: (...args) => dispatch(...args)
    }
    // 将middleware数组映射为对middleware的调用,并且在调用时传入假的store(middlewareAPI)
    const chain = middlewares.map(middleware => middleware(middlewareAPI))
    dispatch = compose(...chain)(store.dispatch)

    // 返回一个store。该store的dispatch函数是所有middleware组合而来的。
    return {
      ...store,
      dispatch
    }
  }
}

applyMiddleware函数将middlewares塞进原有dispatch的过程中。然后返回一个包含这个新dispatch函数的store。

applyMiddleware是redux提供的唯一一个enhancer。(从下面createStore的注释中可以知道)

compose

/**
 * 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]
  }

  return funcs.reduce((a, b) => (...args) => a(b(...args)))
}

了解过函数式编程的小伙伴应该见过compose。它多个函数按从右往左的次序组合成一个函数。最右边的函数接受任意个参数。其余函数都只接受一个参数。

这个compose是用来组合enhancer的:

const composeEnhancers = window.__REDUX_DEVTOOLS_EXTENSION_COMPOSE__ || compose
const enhancer = composeEnhancers(
  applyMiddleware(...middlewares),
)

compose是用reduce函数实现的:

combineReducers

/**
 * 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) {
  const reducerKeys = Object.keys(reducers)
  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}"`)
      }
    }
    // 摘出值函数的放到finalReducers里
    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
  }

  // 返回一个函数,将finalReducers里的reducers都调以便
  // 然后加他们的返回值付给state对应key的属性上
  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]
      // 调用reducer
      const nextStateForKey = reducer(previousStateForKey, action)
      if (typeof nextStateForKey === 'undefined') {
        const errorMessage = getUndefinedStateErrorMessage(key, action)
        throw new Error(errorMessage)
      }
      // 将reducer返回的结果保存到key对应的属性里
      nextState[key] = nextStateForKey
      // 记录state是否发生变化
      hasChanged = hasChanged || nextStateForKey !== previousStateForKey
    }
    // 如果state没有变化则返回原始值
    return hasChanged ? nextState : state
  }
}

React的reducer本质上就是一个巨大的switch-case。然后被拆分到不同的文件里,再用这个combineReducers组合起来。

其实store的dispatch就是调用这个“具大的switch-case”(reducer)。

createStore

这是最终我们用来创建store的函数。

/**
 * 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) {
  if (
    (typeof preloadedState === 'function' && typeof enhancer === 'function') ||
    (typeof enhancer === 'function' && typeof arguments[3] === 'function')
  ) {
    throw new Error(
      'It looks like you are passing several store enhancers to ' +
        'createStore(). This is not supported. Instead, compose them ' +
        'together to a single function'
    )
  }

  // 处理 createStore(reducer, enhancer)的情况
  if (typeof preloadedState === 'function' && typeof enhancer === 'undefined') {
    enhancer = preloadedState
    preloadedState = undefined
  }

  // 如果传了enhancer,则调用之,并将createStore传入,在其中调用createStore并对返回的store做“增强”
  if (typeof enhancer !== 'undefined') {
    if (typeof enhancer !== 'function') {
      throw new Error('Expected the enhancer to be a function.')
    }

    return enhancer(createStore)(reducer, preloadedState)
  }

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

  // 私有变量,见下面OOP改写
  let currentReducer = reducer
  let currentState = preloadedState
  let currentListeners = []
  let nextListeners = currentListeners
  let isDispatching = false

  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.
   */
  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.
   */
  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.'
      )
    }

    let isSubscribed = true

    ensureCanMutateNextListeners()
    // 加到nextListener数组中
    nextListeners.push(listener)

    // 返回一个可以将listener从数组中移除的函数
    return function unsubscribe() {
      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.'
        )
      }

      isSubscribed = false

      ensureCanMutateNextListeners()
      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).
   */
  function dispatch(action) {
    // 各种检查
    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
      // 调用reducer(想象成一个大大的switch-case)
      currentState = currentReducer(currentState, action)
    } finally {
      isDispatching = false
    }
    // 调用通过subscribe注册的所有listener
    // listener只知道store发生变化了,但不知道其他细节
    const listeners = (currentListeners = nextListeners)
    for (let i = 0; i < listeners.length; i++) {
      const listener = listeners[i]
      // 没有传任何参数
      listener()
    }

    return action
  }

  /**
   * 动态替换reducer
   * 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}
   */
  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
   */
  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.
  dispatch({ type: ActionTypes.INIT })

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

粗略改写成OOP的形式

public class Store {
  private Function currentReducer;
  private Object currentState;
  private Array currentListener;
  private Array nextListener;
  private Boolean isDispatching;

  public Store (reducer, preloadState, enhancer) {
    this.currentReducer = reducer;
    this.currentState = preloadState;
    this.currentListener = [];
    this.nextListener = this.currentListener;
    this.isDispatching = false;
    this.dispatch({ type: ActionTypes.INIT })
  }

  public Store (reducer, enhancer) {
    // ommited
  }

  public Object getState () {
    return this.currentState;
  }

  public subscrible(Function listener) {
    // omitted
  }

  public dispatch (Action action) {
    // omitted
  }

  public replaceReducer (Function nextReducer) {
    this.currentReducer = nextReducer;
  }
}

 

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