dart - Icon is not changing -

i trying change icon when specific radio-button selected.

however, switch statement in .dart code never changes icon though switch triggered.

thanks help.

.dart

class gender extends object observable {     @observable   string sex = '';  }  @observable string ikon = 'alarm';    void onchangeradiogrouphandler(dom.customevent e) {     gender.sex = (e.target paperradiobutton).label;      switch(gender.sex)     {       case 'male':         ikon = 'done';         break;        case 'female':         ikon = 'cached';         break;   } 

.html

<div layout vertical center>     <div layout horizontal self-center>       <paper-icon-button icon='{{ikon}}' on-click='{{toggle}}'></paper-icon-button>     </div>   </div>    <core-collapse id='core--collapse'>     <div layout vertical'>       <label id='genderlbl'></label>       <paper-radio-group id='gender-group' on-change='{{onchangeradiogrouphandler}}'>         <paper-radio-button name='male' label='male'></paper-radio-button><br>         <paper-radio-button name='female' label='female'></paper-radio-button>       </paper-radio-group>     </div>   </core-collapse> </div>