此篇文章可作为redux源码导读使用，只说明了其中部分核心代码，并进行了一些简化处理

用法回顾

1. 用createStore来创建store
2. 创建Action
3. 在Reducer中编写纯函数来处理Action
4. 用Store.dispatch()通过Action和Reducer来操作Store里的数据
5. 用Store.subscribe()来监听Store中的数据是否发生了变化
6. 用store.getState()来获取Store中的state，并更新视图

核心代码

createStore(reducer, preloadedState, enhancer)

1.初始化数据

  let currentReducer = reducer
  let currentState = preloadedState //存放state，preloadedState是传入的默认值
  let currentListeners = [] //存放subscribe传入的listener
  let nextListeners = currentListeners

2.创建dispatch, subscribe, getState等方法

dispatch(action)

1.执行reducer

    currentReducer(currentState, action)

2.执行listener

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

subscribe(listener)

    nextListeners.push(listener)

getState()

    return currentState

applyMiddleware(…middlewares)

    const applyMiddleware = (...middlewares) => createStore => (...args) => {
        const 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)))
        }
        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.`
          )
        }
        let chain = []
        const middlewareAPI = {
          getState: store.getState,
          dispatch: (...args) => dispatch(...args)
        }
        chain = middlewares.map(middleware => middleware(middlewareAPI)) //*1 middlewareAPI是store
        dispatch = compose(...chain)(store.dispatch) //*2 ...chain和store.dispatch是next
        return { ...store, dispatch } //*3 执行dispatch传入action
    }

compose是函数式编程的用法用于简化函数嵌套执行

我们来看看middleware是怎么写的

    const logger = store => next => action => {
      console.log('dispatching', action)
      let result = next(action)
      console.log('next state', store.getState())
      return result
    }

为什么会变成 store => next => action => ?

1. 第一个store是传入的middlewareAPI
2. 第二个next是通过compose合成…middleware和store.dispatch实现中间件级联
3. 第三个action是执行dispatch传入的action

(见注释)

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