angularjs下拉框空白的解决办法

<select class="form-control" ng-model="UserState"  ng-init="UserState=0"> <option value="-1">选择状态</option> <option value="0">在职</option> <option value="1">离职</option> </select>

app.directive("dModel",function () {  return {   restrict:'A',   compile:function(element, attrs, transclude){    console.log(transclude);    var dModel=attrs["dModel"];    return{     pre:function(scope, iElement, iAttrs){      var selectValue=scope[dModel]+"";      if(selectValue)      {       var options=element.children();       for(var i=0;i<options.length;i++)       {        if(selectValue==options[i].value)        {         options[i].setAttribute("selected",true);         break;        }       }      }     },     post:function(scope, iElement, iAttrs){      element.on("change",function () {       var selectValue=this.value;       scope.$apply(function () {        scope[dModel]= selectValue;       });      })     }    }   }  } });