根据页面宽度计算宽度

  // 根据宽度计算位置
        protected override void OnSizeAllocated(double pageWidth, double pageHeight)
        {
            base.OnSizeAllocated(pageWidth, pageHeight);
            this.imgBannerHome.Margin = new Thickness((pageWidth-1161/3)/2, 0, 0, 0);
        }