Xamarin.iOS version of the code for this amazing tutorial about Making Programmatic Auto Layout Easy through Extensions

ViewController.cs

using System;
using CoreGraphics;
using UIKit;

namespace testlayout
{
    public partial class ViewController : UIViewController
    {
        public ViewController(IntPtr handle) : base(handle)
        {
        }

        public override void ViewDidLoad()
        {
            base.ViewDidLoad();
            // Perform any additional setup after loading the view, typically from a nib.

            var redView = new UIView();
            redView.BackgroundColor = UIColor.Red;

            var blueView = new UIView();
            blueView.BackgroundColor = UIColor.Blue;

            var greenView = new UIView();
            greenView.BackgroundColor = UIColor.Green;

            UIView[] viewArray = new UIView[] { redView, blueView, greenView };
            View.AddSubviews(viewArray);

            //// enable auto layout
            //redView.TranslatesAutoresizingMaskIntoConstraints = false;

            //redView.TopAnchor.ConstraintEqualTo(View.TopAnchor).Active = true;
            //redView.LeadingAnchor.ConstraintEqualTo(View.LeadingAnchor).Active = true;
            //redView.TrailingAnchor.ConstraintEqualTo(View.TrailingAnchor).Active = true;
            //redView.BottomAnchor.ConstraintEqualTo(View.BottomAnchor).Active = true;

            //redView.Anchor(top: View.SafeAreaLayoutGuide.TopAnchor, leading: View.LeadingAnchor, bottom: View.SafeAreaLayoutGuide.BottomAnchor, trailing: View.TrailingAnchor, padding: new UIEdgeInsets(0, 16, 0, 16), size: new CGSize(100, 100));
            redView.Anchor(top: View.SafeAreaLayoutGuide.TopAnchor, trailing: View.TrailingAnchor, padding: new UIEdgeInsets(0, 16, 0, 16), size: new CGSize(125, 0));
            redView.HeightAnchor.ConstraintEqualTo(redView.WidthAnchor).Active = true;

            blueView.Anchor(top: redView.BottomAnchor, trailing: redView.TrailingAnchor, padding: new UIEdgeInsets(12, 0, 0, 0));
            blueView.AnchorSize(to: redView);
            //blueView.WidthAnchor.ConstraintEqualTo(redView.WidthAnchor).Active = true;
            //blueView.HeightAnchor.ConstraintEqualTo(blueView.WidthAnchor).Active = true;

            greenView.Anchor(top: redView.TopAnchor, leading: View.SafeAreaLayoutGuide.LeadingAnchor, bottom: blueView.BottomAnchor, trailing: redView.LeadingAnchor, padding: new UIEdgeInsets(0, 12, 0, 12));

            var purpleView = new UIView();
            purpleView.BackgroundColor = UIColor.Purple;
            View.AddSubview(purpleView);
            purpleView.FillParentView();

        }

        public override void DidReceiveMemoryWarning()
        {
            base.DidReceiveMemoryWarning();
            // Release any cached data, images, etc that aren't in use.
        }
    }

    internal static class extensions
    {

        internal static void FillParentView(this UIView uIView)
        {
            uIView.Anchor(top: uIView.Superview?.TopAnchor, leading: uIView.Superview?.LeadingAnchor, bottom: uIView.Superview?.BottomAnchor, trailing: uIView.Superview.TrailingAnchor);
        }

        internal static void AnchorSize(this UIView uIView, UIView to)
        {
            uIView.WidthAnchor.ConstraintEqualTo(to.WidthAnchor).Active = true;
            uIView.HeightAnchor.ConstraintEqualTo(to.HeightAnchor).Active = true;
        }

        internal static void Anchor(this UIView uIView, NSLayoutYAxisAnchor top = null, NSLayoutXAxisAnchor leading = null, NSLayoutYAxisAnchor bottom = null, NSLayoutXAxisAnchor trailing = null, UIEdgeInsets padding = default, CGSize size= default)
        {
            uIView.TranslatesAutoresizingMaskIntoConstraints = false;
            if (top != null)
            {
                uIView.TopAnchor.ConstraintEqualTo(top, padding.Top).Active = true;
            }

            if (leading != null)
            {
                uIView.LeadingAnchor.ConstraintEqualTo(leading, padding.Left).Active = true;
            }

            if (bottom != null)
            {
                uIView.BottomAnchor.ConstraintEqualTo(bottom, -padding.Bottom).Active = true;
            }

            if (trailing != null)
            {
                uIView.TrailingAnchor.ConstraintEqualTo(trailing, -padding.Right).Active = true;
            }

            if ( size.Width != 0)
            {
                uIView.WidthAnchor.ConstraintEqualTo(size.Width).Active = true;
            }

            if ( size.Height != 0)
            {
                uIView.HeightAnchor.ConstraintEqualTo(size.Height).Active = true;
            }
        }
    }
}

The YouTube video is here.