利用vue.js实现被选中状态的改变方法

<div data-role="page " class="page "> 
 <div class="center " id="app"> 
 <div class="group "> 
 <ul> 
 <li v-for = "todo in todos "> 
  <div class="groupheader "> 
  <div class="Gheadertext ">{{todo.groupheader}}</div> 
  </div> 
  <div class = "groupbody "> 
  <ul class="list "> 
  <li v-for="cell in todo.groupbody" v-on:click="exchange($event)" class="groupcell"> 
  <div class="celltext"> 
   {{ cell.text }} 
  </div> 
  <img class="selectimg" src="img/select.png "> 
  </li> 
  </ul> 
  </div> 
  </li> 
 </ul> 
 </div> 
 </div> 
</div> 

var datas = { 
 todos :[ 
 { 
 groupheader : 'MB3101', 
 groupbody:[ 
  { text: '调整不当'}, 
  { text: '光电开关损坏' }, 
  { text: '镜面积灰' }, 
  { text: '调整不当' }, 
  { text: '光电开关损坏' }, 
  { text: '镜面积灰' }, 
  { text: '调整不当' }, 
  { text: '光电开关损坏' }, 
  { text: '镜面积灰' }, 
 ] 
 }, 
 { 
 groupheader : 'MB3102', 
 groupbody:[ 
  { text: '调整不当' }, 
  { text: '光电开关损坏' }, 
  { text: '镜面积灰' }, 
  { text: '调整不当' }, 
  { text: '光电开关损坏' }, 
  { text: '镜面积灰' }, 
  { text: '调整不当' }, 
  { text: '光电开关损坏' }, 
  { text: '镜面积灰' }, 
 ] 
 }, 
 { 
 groupheader : 'MB3103', 
 groupbody:[ 
  { text: '调整不当' }, 
  { text: '光电开关损坏' }, 
  { text: '镜面积灰' }, 
  { text: '调整不当' }, 
  { text: '光电开关损坏' }, 
  { text: '镜面积灰' }, 
  { text: '调整不当' }, 
  { text: '光电开关损坏' }, 
  { text: '镜面积灰' }, 
 ] 
 } 
 ] 
} 

new Vue({ 
 el: '#app', 
 data:datas, 
 methods:{ 
 exchange:function(event){ 
  //获取被点击的元素对象 
  var a = event.target; 
  //获取被点击元素中的子元素<img> 
  var cellimg = a.getElementsByTagName("img")[0]; 
  if(a.className == "groupcell") { 
  a.className = "selectcell"; 
  cellimg.style.display = "block"; 
 } 
 else if(a.className == "selectcell") { 
  a.className = "groupcell"; 
  cellimg.style.display = "none"; 
 } 
 } 
 } 
})