小程序 Now you can provide attr `

Now you can provide attr `wx:key` for a `wx:for` to improve performance.

  6 | </view>

  7 | <view class="todos">

>  8 |      <view class="item" wx:for="{{ todos }} wx:key='i'">

    |      ^

  9 |            <icon type="{{item1.completed ? 'success' : 'circle'}}"></icon>

  10 |            <text>{{item1.name}}</text>

  11 |            <icon type="clear" size="16" />

消除警告 ：在wx:for后面添加  wx:key="key"