如何在angularjs中对Ui-selelct元素进行验证？

Question

<div class="form-group ">
   <ui-select ng-model="selectpickerVm.withSearchItem.selected"
              choice-key="name" 
              model-key="selectedVehiclename" 
              class="btn-group bootstrap-select form-control"
              ng-disabled="false" append-to-body="true"
              search-enabled="true" required>
       <ui-select-match placeholder="Select With Search"  id="" required>
         {{$select.selected.name}}
       </ui-select-match>
       <ui-select-choices repeat="withSearchItem in selectpickerVm.selectWithSearchItems">
            <span ng-bind-html="withSearchItem.name"></span>
       </ui-select-choices>
   </ui-select>
</div>
</div>

这是我的代码，请帮助我