vue教程2-05 v-for循环重复数据无法添加问题加track-by='索引'

解决问题的代码示例：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style>
    </style>
    <script src="vue.js"></script>
    <script>
    </script>
</head>
<body>
    <div >
        <input type="button" value="添加" @click="add">
        <ul>
            <li v-for="val in arr" track-by="$index">
                {{val}}
            </li>
        </ul>
    </div>
    <script>

        var vm=new Vue({
            data:{
                arr:['apple','pear','orange']
            },
            methods:{
                add:function(){
                    this.arr.push('tomato');
                }
            }
        }).$mount('#box');

    </script>
</body>
</html>