如何在RelativeLayout.XConstraint中使用转换器？

Question

我想在RelativeLayout内居中放置图像

X点的等式应为=（父级宽度-视图的宽度）/ 2 ..

我试图用XAML编写它：

 <RelativeLayout x:Name="rel1">
                <Image x:Name="image1" Aspect="AspectFit" 
                       RelativeLayout.XConstraint="{ConstraintExpression Type=Constant,
                    Constant={Binding Source=rel1, Path=Width, Converter={StaticResource centerFromWidth}, 
                    ConverterParameter={Binding Source=image1, Path=Width}}}"
                       RelativeLayout.YConstraint="{ConstraintExpression Type=RelativeToParent,
                    Property=Height, Factor=0.5, Constant=0}"
                           Source="{Binding File1, Converter={StaticResource base64ToImage}}">
                </Image>
                <RelativeLayout.GestureRecognizers>
                    <TapGestureRecognizer Command="{Binding OpenImagePopupCommand}" CommandParameter="1"/>
                </RelativeLayout.GestureRecognizers>

            </RelativeLayout>

这是转换器：

 public class FromDimensionToCenter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            var parentWidth = (double)value;
            var viewWidth = (double)parameter;
            return (parentWidth - viewWidth) / 2;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }

但是我得到这个错误：

  

未找到“常量”的属性，可绑定属性或事件，或者   值和属性之间的类型不匹配。