XF相对布局

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

[assembly: XamlCompilation (XamlCompilationOptions.Compile)]
namespace App6
{
    public partial class App : Application
    {
        public App ()
        {
            InitializeComponent();

            RelativeLayout rl = new RelativeLayout();
            rl.Children.Add(new BoxView { Color = Color.Blue, WidthRequest = 150, HeightRequest = 150 }, Constraint.Constant(0), Constraint.Constant(0));
            rl.Children.Add(new BoxView { Color = Color.Red, WidthRequest = 150, HeightRequest = 150 }, Constraint.RelativeToParent((Parent)=>Parent.Width-150), Constraint.Constant(0));
            rl.Children.Add(new BoxView { Color = Color.Purple, WidthRequest = 150, HeightRequest = 150 }, Constraint.Constant(0), Constraint.RelativeToParent((Parent) => Parent.Height - 150));
            rl.Children.Add(new BoxView { Color = Color.Yellow, WidthRequest = 150, HeightRequest = 150 }, Constraint.RelativeToParent((Parent) => Parent.Width - 150), Constraint.RelativeToParent((Parent) => Parent.Height - 150));

            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
        }
    }
}