mp-loading show设置为true后不显示

https://developers.weixin.qq.com/miniprogram/dev/extended/weui/loading.html

<mp-loading duration="{{900}}" ext-class="rv-loading" type="dot-gray" show="{{isShowMpLoading}}" animated="{{true}}"></mp-loading>

this.setData({

isShowMpLoading: true

});

请问为啥setData isShowMpLoading为true后，一直不显示呢