Validators: For validation error message tooltips

I like how Form validation error messages in Ionic 2 are displayed with the inbuilt validators (maxlength, required etc) using tooltips which does not require messing around with hidden fields to display errors.

How does one achieve such custom error messages with custom validators without using hidden fields?

I have googled around and what I see is stuff like:

>   <small [hidden]="username.valid || (username.pristine && !f.submitted)">
>         Username is required (minimum 5 characters).
>     </small>