angular4响应式表单与校验
http://blog.csdn.net/xiagh/article/details/78360845?locationNum=10&fps=1
How to implement Custom Async Validator in Angular4
https://stackoverflow.com/questions/43366514/how-to-implement-custom-async-validator-in-angular4
How to add debounce time to an async validator in angular 2?
https://stackoverflow.com/questions/36919011/how-to-add-debounce-time-to-an-async-validator-in-angular-2
原文：http://www.cnblogs.com/Ceri/p/7989646.html