redux中dispatch只能是object对象，这对于同步的操作是可行的，但如果是异步的操作，那么将行不通，对此，redux提供了applyMiddleware来增强相应的disptch方法，同时将applyMiddleware返回的函数作为参数传入createStore中，替换当前store中的dispatch方法。
下面我们具体结合redux-thunk中间件来分析下dispatch增强的源码：

// applyMiddleware源码

export default function applyMiddleware(...middlewares) {
  return createStore => (...args) => { //传入createStore保证middleware只会在store上应用一次
    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.`
      )
    }

    const middlewareAPI = {   //将getState和dispatch传入middleware中
      getState: store.getState,
      dispatch: (...args) => dispatch(...args) 
      //这时dispatch是上述构造的方法，当dispatch被中间件改造后则dispatch为改造后的
    }
    const chain = middlewares.map(middleware => middleware(middlewareAPI))
    dispatch = compose(...chain)(store.dispatch) 
    // 将原先的store.dispatch作为参数next传入middleware中
    // compose将多个function如a,b,c。compose(a,b,c)将返回(...args) => a(b(c(...args)))，
    // 即dispatch action时， c函数作为b函数的next，b函数作为a函数的next，所以将依次执行函数a，b，c。
    // dispatch被中间件改造了 
    
    return {
      ...store,
      dispatch
    }
  }
}

// redux-thunk源码

function createThunkMiddleware(extraArgument) {
  return ({ dispatch, getState }) => next => action => {
    //next是store上的dispatch方法
    if (typeof action === 'function') { 
      //如果action返回的是function则继续将dispatch方法往下传，直到action非function类型
      return action(dispatch, getState, extraArgument);
    }
    //如果action为非function类型，则调用store.dispatch方法
    return next(action);
  };
}

// createStore中的片段代码

export default function createStore(reducer, preloadedState, enhancer) {
  ...
  if (typeof enhancer !== 'undefined') {
    if (typeof enhancer !== 'function') {
      throw new Error('Expected the enhancer to be a function.')
    }
    //将createStore和reducer，初始state传入applyMiddleware中创建store
    return enhancer(createStore)(reducer, preloadedState)
  }
  ...

通过applyMiddleware传入中间件增强了dispatch方法，并返回增强的dispatch和其他的createStore上的方法。