CrossView.cs

#if __IOS__
    using PlatformView = UIKit.UIView;
    using PlatformViewGroup = UIKit.UIView;
    using PlatformTextArea = UIKit.UITextView;
#endif
#if __ANDROID__
    using PlatformView = Android.Views.View;
    using PlatformViewGroup = Android.Views.ViewGroup;
    using PlatformTextArea = Android.Widgets.EditText;
#endif

    //Note struct here for extra efficiency
    /// <summary>
    /// View builder provides methods to build view hierarchy which can be used in Shared code
    /// </summary>
    public struct ViewBuilder
    {
        public PlatformViewGroup Parent;
        public List<PlatformView> Views;
#if __ANDROID__
        public Android.Content.Context Context;
        public ViewBuilder(ViewGroup parent, Android.Content.Context context)
        {
            Parent = parent;
            Context = context;
            Views = new List<Android.Views.View>();
        }
#endif
#if __IOS__
        public ViewBuilder(PlatformViewGroup parent)
        {
            Parent = parent;
            Views = new List<PlatformView>();
        }
#endif

        public T AddSubview<T>(T view) where T : PlatformView
        {
            Parent.AddSubview(view);
            Views.Add(view);
            return view;
        }

        public void Dispose()
        {
            foreach (var v in Views)
            {
                v.Dispose();
            }
        }
    }

    public struct View
    {
        public static implicit operator PlatformView(View area) { return area.PlatformView; }
        public PlatformView PlatformView { get; set; }
    }

    public static class ViewUtil
    {
        public static View View(this ViewBuilder b)
        {
#if __IOS__
            return new View() { PlatformView = b.AddSubview(new PlatformView()) };
#endif
#if __ANDROID__
            return new View() { PlatformView = b.AddSubview(new PlatformView(b.Context)) };
#endif
        }

        //You can also use View and other wrapper structs here since all of them 
        //implicitly cnverted to PlatformView subclasses
        public static View AddSubview(this View view, PlatformView child)
        {
#if __IOS__
            view.PlatformView.AddSubview(child);
#endif
#if __ANDROID__
            view.PlatformView.AddView(child);
#endif
            return view;
        }

        public static View RemoveFromSuperview(this View view)
        {
#if __IOS__
            view.PlatformView.RemoveFromSuperview();
#endif
#if __ANDROID__
            view.PlatformView.Parent?.RemoveView(view);
#endif
            return view;
        }

        public static View SetBackgroundColor(this View view, RGB color)
        {
            view.PlatformView.BackgroundColor = color.ToColor();
            return view;
        }

        //Since View is just a struct it is absolutely ok to create it 
        //as many times as needed. Don't box it and you will be ok.
        public static View AsView(this PlatformView view)
        {
            return new View() { PlatformView = view };
        }
    }

    //Note that struct is used here for extra efficiency.
    //Note how simple it is to define own cross platform views!
    /// <summary>
    /// Text area wrapper compatible with Shared code.
    /// </summary>
    public struct TextArea
    {
        public static implicit operator PlatformTextArea(TextArea area) { return area.PlatformView; }
        public View AsView() { return new View() { PlatformView = PlatformView }; }
        public PlatformTextArea PlatformView { get; set; }
    }

    /// <summary>
    /// View builder extension to build text area control (UITextView on iOS and Android.Widgets.TextArea on Android)
    /// </summary>
    public static class TextAreaUtil
    {
        public static TextArea TextArea(this ViewBuilder b)
        {
#if __IOS__
            return new TextArea() { PlatformView = b.AddSubview(new PlatformTextArea()) };
#endif
#if __ANDROID__
            return new TextArea() { PlatformView = b.AddSubview(new PlatformTextArea(b.Context)) };
#endif
        }

        public static TextArea SetText(this TextArea area, string text)
        {
#if __IOS__
            area.PlatformView.Text = text;
#endif
#if __ANDROID__
            area.PlatformView.SetText(text);
#endif
            return area;
        }
    }

    /// <summary>
    /// Base view using above infrastructure.
    /// </summary>
    public class QodenView : PlatformViewGroup
    {
#if __IOS__
        public QodenView()
        {
            Builder = new ViewBuilder(this);
        }
#endif
#if __ANDROID__
        public NewQodenView(Android.Content.Context context)
        {
            Builder = new ViewBuilder(this, context);
        }
#endif
        public ViewBuilder Builder;

        protected override void Dispose(bool disposing)
        {
            base.Dispose(disposing);
            if (disposing)
            {
                Builder.Dispose();
            }
        }
    }

    /// <summary>
    /// Common functions to style controls
    /// </summary>
    public static class NewTheme
    {
        public static TextArea StyleTextArea(this TextArea area)
        {
            area.AsView().SetBackgroundColor(RGB.Clear);
            return area;
        }
    }

    /// <summary>
    /// Sample view demonstrating how to use above infrastructure to create cross paltform views.
    /// If below code is put into Shared lib then it compiles fine on iOS and Android.
    /// On iOS you get TextArea with UITextView inside
    /// On Android you get TextArea with Android.Widgets.TextArea inside
    /// </summary>
    public class SampleView : QodenView
    {
        public TextArea TextArea;

        public SampleView()
        {
            TextArea = Builder.TextArea().StyleTextArea();
            //You can set properties on PlatformView
            TextArea.PlatformView.Text = "Hello World";
            TextArea.PlatformView.BackgroundColor = RGB.Clear.ToColor();
            //or on wrapper
            TextArea
                .SetText("Hello World").AsView()
                .SetBackgroundColor(RGB.Clear);

            TextArea.AsView().RemoveFromSuperview();
            this.AsView().AddSubview(TextArea);
        }
    }