angular4響應式表單與校驗
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

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