微信小程序自定义导航栏高度获取出错windowHeight值不一致

    var that = this
    wx.getSystemInfo({
      success: function (res) {
        this.setData({
            tbodyHeight:res.windowHeight
        })
      }
    })

  onLaunch: function () {
    wx.getSystemInfo({
      success: function (res) {
        that.globalData.windowHeight = res.windowHeight
      }
    })
  }
  ,
  globalData: {
    windowHeight:null
  }

const app = getApp()
onReady:function(){
    var tbodyHeight = app.globalData.windowHeight - 90; //90为头部固定高度 
    that.setData({
        tbodyHeight: tbodyHeight.toFixed(0)
    })
 }