使用带有angularjs的自定义指令进行验证

I have a form with input fields and have a custom directive for each input field to validate the data entered by user.Requirement is that when user leaves the input field with invalid data, error message to be displayed.However, few of the fields are optional that if User skips with out entering any data for these fields, no Validation is required.I have a form with input fields and have a cust