promise待看文档备份

http://swift.gg/2017/03/27/promises-in-swift/

http://www.cnblogs.com/feng9exe/p/9043715.html

https://zhuanlan.zhihu.com/p/20531896

http://swiftcn.io/topics/105

http://www.cocoachina.com/swift/20160525/16437.html

https://blog.dianqk.org/2016/08/22/rxswift-vs-promisekit/

http://www.cnblogs.com/niithub/p/7487940.html

https://www.cnblogs.com/cbscan/articles/4143573.html

https://www.cnblogs.com/cbscan/articles/4143573.html

https://www.zhihu.com/question/39390484

http://www.cnblogs.com/gaoxing/p/4401794.html

https://www.jianshu.com/p/ee604d3a3e80

http://ifeve.com/promise-future-callback/

编程结构：Promise和Future