如何为RelativeLayout ConstraintExpression编写OnPlatform代码

using Xamarin.Forms;

RelativeLayout relativeLayout = new RelativeLayout();

// 添加视图
var view1 = new BoxView { Color = Color.Red };
relativeLayout.Children.Add(view1,
    xConstraint: Constraint.RelativeToParent((parent) => {
        return parent.Width / 2 - 50; // 设置视图的X坐标
    }),
    yConstraint: Constraint.RelativeToParent((parent) => {
        return parent.Height / 2 - 50; // 设置视图的Y坐标
    }),
    widthConstraint: Constraint.Constant(100), // 设置视图的宽度
    heightConstraint: Constraint.Constant(100) // 设置视图的高度
);

// 根据不同的平台设置不同的ConstraintExpression
view1.SetBinding(RelativeLayout.WidthConstraintProperty, new Binding(
    propertyName: "OnPlatform",
    source: Device.OnPlatform<double>(iOS: 150, Android: 200, WinPhone: 250) // 不同平台上的宽度值
));

view1.SetBinding(RelativeLayout.HeightConstraintProperty, new Binding(
    propertyName: "OnPlatform",
    source: Device.OnPlatform<double>(iOS: 150, Android: 200, WinPhone: 250) // 不同平台上的高度值
));