在AngularJS自定义验证指令中调用异步服务

我有自定义验证的指令(验证用户名尚不存在).验证使用$http服务询问服务器是否存在用户名,因此返回是一个promise对象.这对验证非常有用.表单无效并包含myform.$error.usernameVerify已经使用用户名.但是,user.username始终未定义,因此它违反了我的ng-model指令.我想这可能是因为.success中的函数正在创建它自己的作用域,并且控制器$scope上没有使用返回值.我如何解决这个问题,以便ng-model绑定仍然有效？

commonModule.directive("usernameVerify",[
    'userSvc',function(userSvc) {
        return {
            require: 'ngModel',scope: false,link: function(scope,element,attrs,ctrl) {
                ctrl.$parsers.unshift(checkForAvailability);
                ctrl.$formatters.unshift(checkForAvailability);

                function checkForAvailability(value) {
                    if (value.length < 5) {
                        return value;
                    }
                    // the userSvc.userExists function is just a call to a rest api using $http
                    userSvc.userExists(value)
                        .success(function(alreadyUsed) {
                            var valid = alreadyUsed === 'false';
                            if (valid) {
                                ctrl.$setValidity('usernameVerify',true);
                                return value;
                            } 
                            ctrl.$setValidity('usernameVerify',false);
                            return undefined;
                        });
                }
            }
        }
    }
]);

这是我的模板：

<div class="form-group" ng-class="{'has-error': accountForm.username.$dirty && accountForm.username.$invalid}">
    <label class=" col-md-3 control-label">Username:</label>
    <div class="col-md-9">
        <input name="username"
               type="text"
               class="form-control"
               ng-model="user.username"
               ng-disabled="user.id"
               ng-minlength=5
               username-verify
               required />
        <span class="field-validation-error" ng-show="accountForm.username.$dirty && accountForm.username.$error.required">Username is required.</span>
        <span class="field-validation-error" ng-show="accountForm.username.$dirty && accountForm.username.$error.minlength">Username must be at least 5 characters.</span>
        <span class="field-validation-error" ng-show="accountForm.username.$dirty && accountForm.username.$error.usernameVerify">Username already taken.</span>
    </div>
</div>

ngModel.$asyncValidators.uniqueUsername = function(modelValue,viewValue) {
var value = modelValue || viewValue;

// Lookup user by username
return $http.get({url:'/api/users/' + value}).
   then(function resolved() {
     //username exists,this means validation fails
     return $q.reject('exists');
   },function rejected() {
     //username does not exist,therefore this validation passes
     return true;
   });
};