Custom Renderers - Xamarin (https://developer.xamarin.com/guides/xamarin-forms/custom-renderer/) CustomView.cs goes in the PCL project Views folder. MainPage.cs/MainPage.xaml goes in the PCL project Pages folder. CustomViewRenderer.cs goes in the iOS/Android project

CustomView.cs

using Xamarin.Forms;

namespace Myproject.Views
{
    public class CustomView : View
    {
        public CustomView()
        {
        }
    }
}

CustomViewRenderer.cs

using Myproject.iOS.Renderers;
using Myproject.Views;
using UIKit;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;


[assembly: ExportRenderer(typeof(CustomView), typeof(CustomViewRenderer))]
namespace Myproject.iOS.Renderers
{
    public class CustomViewRenderer : ViewRenderer<CustomView, UIControl>
    {
        /// <summary>
        /// Method used to instantiate the native view.
        /// </summary>
        protected override void OnElementChanged(ElementChangedEventArgs<CustomView> e)
        {
            base.OnElementChanged(e);

            if (Control == null)
            {
                UIButton btn = new UIButton();
                //Customize the button here.
                SetNativeControl(btn);
            }
        }
    }
}

MainPage.cs

//Use of the view in the xamarin forms project using code.
public class MainPage : ContentPage
{
  public MainPage ()
  {
    Content = new StackLayout {
      Children = {
        new Label {
          Text = "This is my custom renderer",
        },
        new CustomView {
          VerticalOptions="StartAndExpand",
        }
      },
      VerticalOptions = LayoutOptions.CenterAndExpand,
      HorizontalOptions = LayoutOptions.CenterAndExpand,
    };
  }
}

MainPage.xaml

<ContentPage ...
    xmlns:local="clr-namespace:Myproject.Views;assembly=Myproject"
    ...>
    ...
    <Label Text="This is my custom renderer." />
    <local:CustomView VerticalOptions="StartAndExpand" /> <!-- Use anything that a Xamarin Forms View can normall use -->
    ...
</ContentPage>