Vue.directive('example', {
       twoWay: true,
       bind: function(){
         this.handler = function(){
         //將數據寫回vm
         //如果指令這樣綁定v-example="a.b.c"，它將用給定值設置 'vm.a.b.c'
         this.set(this.el.value)
         }.bind(this)
         this.el.addEventListener('input', this.handler)
       },
       unbind: function(){
         this.el.removeEventListener('input', this.handler)
       }
    })