【微信小程序】【Demo】获取页面元素宽高，按相对百分比计算页面其他元素应设置宽度的像素数值。

   /**
   * 生命周期函数--监听页面初次渲染完成
   */
  onReady: function () {
    var that = this;
    //获取元素
    var query = wx.createSelectorQuery();
    query.select("#navbar").boundingClientRect((rect) => {
      var wW = wx.getSystemInfoSync().windowWidth;
      var wH = wx.getSystemInfoSync().windowHeight;
      //原始主体宽高
      var oldW = 551;
      var oldH = 694;

      //显示宽
      var bgW = wW * 0.8;
      //主体元素宽高比例，则高为
      var bgH = bgW * oldH / oldW ;
      var bgLeft = wW * 0.1; 
      var bgTop = (wH-bgH)/2;
 
      //券和按钮占高度
      var btnH =bgH * 130/oldH;
      var ruleH =bgH * 50/oldH;
      
      that.setData({
        bgStyle:"width:"+bgW+"px;"+"height:"+bgH+"px;"+"top:"+bgTop+"px;"+"left:"+bgLeft+"px;",
        bgTop: bgTop,
        bgLeft: bgLeft,
        btnH:btnH,
        ruleH:ruleH,
      })
    }).exec();
  },