html

 <div class="form-group" ng-class="{'has-error': myForm.myName.$touched && myForm.myName.$invalid}">
    <label class="col-xs-2 control-label">name</label>
    <div class="col-xs-6">
        <input type="text" class="form-control" name="myName" ng-model="formData.username"
               placeholder="2-10个中英文字符，aaa会重名，必填"
               name-check minlength="2" maxlength="10" required>
    </div>
    <div class="col-xs-4 form-control-static text-danger" ng-messages="myForm.myName.$error"
                     ng-messages-include="demos/form/error.html" ng-show="myForm.myName.$touched">
   </div>
</div>

ng-messages=”myForm.myName.$error” 给出错误类型(required, char, exist, minlength)

error.html

<span ng-message="required">必填</span>
<span ng-message="char">非法字符</span>
<span ng-message="exist">名称已存在</span>
<span ng-message="minlength">太短了</span>

directive name-check 利用ngModel 的$validators,& $asyncValidators 添加自定义验证

app.directive('nameCheck', nameCheck);

nameCheck.$inject = ['HttpService', '$q'];

function nameCheck(HttpService, $q){

    var NAME_REG = /^[a-zA-Z\u4e00-\u9fa5]+$/;

    return {
        restrict: 'A',
        require: 'ngModel',
        link:function($scope,element,attrs,ctrl){

            ctrl.$validators.char = function(modelValue, viewValue) {
                var value = modelValue || viewValue;
                if(!NAME_REG.test(value)){
                    return false;
                }
                return true;
            };
            ctrl.$asyncValidators.exist = function(modelValue, viewValue){
                var value = modelValue || viewValue;
                var deferred = $q.defer();
                HttpService.get('api/users/' + value).then(function(res) {
                    if(res.isExist){
                        deferred.reject(false);

                    }
                    deferred.resolve(true);
                })

                return deferred.promise;
            }
        }
    }
}