title: Checkbox click invalid data inside v-for loop
body: <p>I would like to validate at least one need to check the checkbox.
  I'm trying to validate when I clicked on checkbox through the loop.
  When I checked one, the output is invalid return false in the first click. When I unchecked return true. But when I checked my sizes array data with vue-dev tools data is valid. I'm trying to find that bugs but I didn't find.
  I don't know why.I would like to know that's why? Am I wrong?</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>new Vue({
    el: '#app',
    data: {
       sizes:[
         {id:1,name:'small',ischeck:false,price:0},
          {id:2,name:'medium',ischeck:false,price:0},
          {id:3,name:'large',ischeck:false,price:0}        
        ],
        newval:[]
    },
    methods: {
      checkchanged(){
       for(var i=0;i&lt;this.sizes.length;i++){           
         console.log(this.sizes[i].ischeck);
        }     
        
      }
    }
  })</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;script src="https://unpkg.com/vue@2.5.17/dist/vue.js"&gt;&lt;/script&gt;
  
  &lt;div id="app"&gt;
  &lt;span v-for="size in sizes"&gt;
    &lt;input type="checkbox" v-model="size.ischeck" :value="size.ischeck" 
      @click="checkchanged"&gt;
      &lt;label&gt;{{size.name}}&lt;/label&gt;
      &lt;input type="text" :disabled="!size.ischeck" v-model="size.price"&gt;
  &lt;/span&gt;
  
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>