调用REST API的自定义指令验证程序设置表单太慢。$ invalid以防止表单提交

I have a custom directive on a input text field that calls a REST Web Service Api using $http (Restful Resource actually). This web service call typically takes 500ms which by the time the call returns, the user may have hit on the submit button of the form and proceed.I have a custom directive on a input text field