middleware
什么是middleware，个人理解就是一个可以让你在dispatch action的时候，可以同时做几个动作，比如想打印日志，异步请求
middleware运行
middleware logger
export const logger = store => next => action = {
console.log(1111)
next(action)
console.log(2222)
}
export const logger1 = store => next => action = {
console.log(333333)
next(action)
console.log(44444)
}
运行
const dispatch_1 = applyMiddleware([logger, logger1])
dispatch_1 = logger(logger1(store.dispatch))
这是执行dispatch(action)的时候
第一步就是执行logger函数
1，打印出1111
2，执行logger1(store.dispatch)(action)
3, 打印出333
4，执行store.dispatch(action)
4，打印出222
5，打印出 4444
