微信小程序 Now you can provide attr `wx:key` for a `wx:for` to improve performance.

微信小程序用到"wx:for"时可能会在控制台报"Now you can provide attr “wx:key” for a “wx:for” to improve performance"警告

不要慌张这只是警告不妨碍运行，消除警告只需要在"wx:for"后面添加wx:key=“xxx” 就行了

eg：

<view wx:if="{{ifshow}}" class='ranklist' wx:for="{{rankingdata}}" wx:key=""><view class='ritem'><span><text>第{{index+1}}名</text></span><span><text>{{item.score}}</text></span><span class="user"><image src="{{item.headimg}}" ></image><text class='name'>{{item.nickname}}</text></span></view></view>