XF相对控件布局

using System;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;

[assembly: XamlCompilation (XamlCompilationOptions.Compile)]
namespace App6
{
    public partial class App : Application
    {
        public App ()
        {
            InitializeComponent();
            BoxView bv = new BoxView { Color = Color.Red, WidthRequest = 150, HeightRequest = 150 };
            Label lbl = new Label { Text = "这是一个红色的方块", TextColor = Color.Red };
            RelativeLayout rl = new RelativeLayout();
            rl.Children.Add(bv,Constraint.RelativeToParent((parent)=>parent.Width/2),Constraint.RelativeToParent((parent)=>parent.Height/2));
            rl.Children.Add(lbl, Constraint.RelativeToView(bv, (parent, sibling) => sibling.X+bv.Width), Constraint.RelativeToView(bv, (parent, sibling) => sibling.Y));
            

            MainPage = new ContentPage { Content=rl};
        }

        protected override void OnStart ()
        {
            // Handle when your app starts
        }

        protected override void OnSleep ()
        {
            // Handle when your app sleeps
        }

        protected override void OnResume ()
        {
            // Handle when your app resumes
        }
    }
}