多层循环点击子元素怎么获得父元素的下标值?

<block wx:for=“{{a}}” wx:for-item=“item” >

    <view data-index=“{{index}}”>   

     <block wx:for=“{{item.b}}” >

                <input type=“text” bindfocus=“index” data-index=“{{index}}”>

        </block>

    <view>

</block>

input获得焦点的时候怎么获取上一层循环的index呢?