FrameCornerRadius.cs

using Xamarin.Forms;

namespace YourProject.Core.Effects
{
  public class FrameCornerRadius : RoutingEffect
  {
    public FrameCornerRadius() 
      : base("YourProject.FrameCornerRadiusEffect")
    {
    }

    public float CornerRadius { get; set; }
  }
}

FrameCornerRadiusEffectAndroid.cs

using Android.Graphics;
using Android.Views;
using System.Linq;
using YourProject.Core.Effects;
using YourProject.Droid.Effects;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;

[assembly: ResolutionGroupName(nameof(YourProject))]
[assembly: ExportEffect(typeof(FrameCornerRadiusEffect), nameof(FrameCornerRadiusEffect))]
namespace YourProject.Droid.Effects
{
  class FrameCornerRadiusEffect : PlatformEffect
  {
    protected override void OnAttached()
    {
      try
      {
        var effect = (FrameCornerRadius)Element.Effects.FirstOrDefault(e => e is FrameCornerRadius);
        if (effect != null)
        {
          Control.ClipToOutline = true;
          Control.OutlineProvider = new RoundedOutlineProvider(effect.CornerRadius);
        }
      }
      catch (System.Exception ex)
      {
        throw new System.Exception($"Cannot set property on attached control. Error: {ex.Message}");
      }
    }

    protected override void OnDetached()
    {
    }
    
    private  class RoundedOutlineProvider : ViewOutlineProvider
    {
      private readonly float radius;
      public RoundedOutlineProvider(float radius)
      {
        this.radius = radius;
      }
      public override void GetOutline(Android.Views.View view, Outline outline)
      {
        outline?.SetRoundRect(0, 0, view.Width, view.Height, radius);
      }
    }
  }
}

FrameCornerRadiusEffectIos.cs

using Foundation;
using System.Diagnostics;
using System.Linq;
using YourProject.Core.Effects;
using YourProject.iOS.Effects;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;

[assembly: ResolutionGroupName(nameof(YourProject))]
[assembly: ExportEffect(typeof(FrameCornerRadiusEffect), nameof(FrameCornerRadiusEffect))]
namespace YourProject.iOS.Effects
{
  [Preserve(AllMembers = true)]
  public class FrameCornerRadiusEffect : PlatformEffect
  {
    protected override void OnAttached()
    {
      try
      {
        var effect = (FrameCornerRadius)Element.Effects.FirstOrDefault(e => e is FrameCornerRadius);
        if (effect != null)
        {
          Container.Layer.CornerRadius = effect.CornerRadius;
          Container.Layer.MasksToBounds = true;
        }
      }
      catch (System.Exception ex)
      {
        throw new System.Exception($"Cannot set property on attached control. Error: {ex.Message}");
      }
    }

    protected override void OnDetached()
    {
      
    }
  }
}

Thank you very much for this snippet. I'm looking at implementing rounded view using the outline provider too. I have gotten it to work, although I find that when I rotate on Android the effect is removed. Have you solved that issue, or encountered it yourself?