vue判斷input值為空時(shí)，返回錯(cuò)誤信息？

如題，input為空的時(shí)候，不再dom上顯示錯(cuò)誤信息，這個(gè)是怎么回事？

<div class="am-form-group err-name" id="form">
                                    <label for="doc-ipt-3" class="am-u-sm-2 am-form-label"><strong>*</strong>資源名:</label>
                                    <div class="am-u-sm-10">
                                        <input type="text" @change="checkname" v-model="resourcesname" id="resourcesName" name="resourcesname">
                                        <span class="err-plchod">{{errname}}</span>
                                    </div>
                                </div>

 var vm = new Vue({
               el:"#form",
               data:{
                   url:'',
                   resourcesname:'',
                   sort:'',
                   errname:'',
                   masgurl:'',
                   masgsort:'',
                   isShow:false,
                   resourcesModular:{
                       "mk":"模塊",
                       "gn":"功能",
                       "zy":"資源",
                   },
                   mydatas:[]
               },
               methods:{
                 checkname:function(){
                       if(this.resourcesname !=""){    
                           this.errname ="";
                       }else{
                          this.errname = "用戶名不能為空";
                       }
                   },
               }
               });