I use AngularJS UI-Mask (1.8.7) and show warning about field validation like this:

<div ng-message="mask">Warning</div>

This box is shown when user entered less symbols then needed, and blur event happened (controller.$setValidity('mask', isValid) in mask.js).

I need to show some warning right when user press an invalid key — this is an additional hint to notice that an invalid character is being entered into the field. There is no such option in UI-Mask. How I can do it?