I have used font awesome CSS on a dropdown button to show plus and minus icon, I wanted to change the CSS from plus to minus when the dropdown is expanded and vice versa.
<a class="btn btn-primary" ng-model="expand" btn-checkbox btn-checkbox-true="true" btn-checkbox-false="false"><i class="fa fa-plus-circle"></i> {{expression}}</a>
Need to change the fa fa-plus-circle class to fa fa-minus-circle class when dropdown expanded and vice versa.