Wechat applet obtains the information of an element (height, width, etc.) and converts PX to rpx.

<!--  Customize the bottom navigation bar        Here we have to define class Below heel .select() Agreement -->
<tab-bar class="tabbar" :currentTabBar="0" :themeColors="themeColors"></tab-bar>

//  Get the height of the bottom navigation bar 
    getTabBarHeight() {
      let query = uni.createSelectorQuery().in(this);
      query
      .select(".tabbar")               //  there .tabbar Is the class name of the element 
      .boundingClientRect()
      .exec((res) => {
        let system = uni.getSystemInfoSync();
        let tabBarHeightPx = res[0].height;
        let rpx = system.windowWidth / 750;
        this.tabBarHeightRpx = tabBarHeightPx / rpx
        console.log('tabBarHeightRpx', this.tabBarHeightRpx);
      })
    },


//  You need to wait while calling dom Call after rendering 
this.$nextTick(() => {
   this.getFixedItemInfo()
   this.getTabBarHeight()
})

//   Here is to convert the obtained height into rpx, Because in different models   px turn rpx It's inconsistent 
let system = uni.getSystemInfoSync();
let tabBarHeightPx = res[0].height;
let rpx = system.windowWidth / 750;
this.tabBarHeightRpx = tabBarHeightPx / rpx