在Xamarin.forms中自定义边框的边角半径

using Xamarin.Forms;

namespace YourNamespace
{
    public class CustomView : View
    {
        public static readonly BindableProperty BorderColorProperty = BindableProperty.Create(nameof(BorderColor), typeof(Color), typeof(CustomView), Color.Default);
        public static readonly BindableProperty BorderWidthProperty = BindableProperty.Create(nameof(BorderWidth), typeof(double), typeof(CustomView), 0.0);
        public static readonly BindableProperty CornerRadiusProperty = BindableProperty.Create(nameof(CornerRadius), typeof(double), typeof(CustomView), 0.0);

        public Color BorderColor
        {
            get { return (Color)GetValue(BorderColorProperty); }
            set { SetValue(BorderColorProperty, value); }
        }

        public double BorderWidth
        {
            get { return (double)GetValue(BorderWidthProperty); }
            set { SetValue(BorderWidthProperty, value); }
        }

        public double CornerRadius
        {
            get { return (double)GetValue(CornerRadiusProperty); }
            set { SetValue(CornerRadiusProperty, value); }
        }
    }
}

using Android.Content;
using Android.Graphics;
using Android.Graphics.Drawables;
using YourNamespace;
using YourNamespace.Droid;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;

[assembly: ExportRenderer(typeof(CustomView), typeof(CustomViewRenderer))]
namespace YourNamespace.Droid
{
    public class CustomViewRenderer : ViewRenderer<CustomView, Android.Views.View>
    {
        public CustomViewRenderer(Context context) : base(context)
        {
        }

        protected override void OnElementChanged(ElementChangedEventArgs<CustomView> e)
        {
            base.OnElementChanged(e);

            if (Control == null)
            {
                SetNativeControl(new Android.Views.View(Context));
            }

            if (e.NewElement != null)
            {
                UpdateBackground();
            }
        }

        protected override void OnElementPropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
        {
            base.OnElementPropertyChanged(sender, e);

            if (e.PropertyName == CustomView.BorderColorProperty.PropertyName ||
                e.PropertyName == CustomView.BorderWidthProperty.PropertyName ||
                e.PropertyName == CustomView.CornerRadiusProperty.PropertyName)
            {
                UpdateBackground();
            }
        }

        private void UpdateBackground()
        {
            var gradientDrawable = new GradientDrawable();
            gradientDrawable.SetStroke((int)Element.BorderWidth, Element.BorderColor.ToAndroid());
            gradientDrawable.SetCornerRadius((float)Element.CornerRadius);
            Control.Background = gradientDrawable;
        }
    }
}

using CoreGraphics;
using YourNamespace;
using YourNamespace.iOS;
using UIKit;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;

[assembly: ExportRenderer(typeof(CustomView), typeof(CustomViewRenderer))]
namespace YourNamespace.iOS
{
    public class CustomViewRenderer : ViewRenderer<CustomView, UIView>
    {
        protected override void OnElementChanged(ElementChangedEventArgs<CustomView> e)
        {
            base.OnElementChanged(e);

            if (Control == null)
            {
                SetNativeControl(new UIView());
            }

            if (e.NewElement != null)
            {
                UpdateBackground();
            }
        }

        protected override void OnElementPropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
        {
            base.OnElementPropertyChanged(sender, e);

            if (e.PropertyName == CustomView.BorderColorProperty.PropertyName ||
                e.PropertyName == CustomView.BorderWidthProperty.PropertyName ||
                e.PropertyName == CustomView.CornerRadiusProperty.PropertyName)
            {
                UpdateBackground();
            }
        }

        private void UpdateBackground()
        {
            var nativeView = new UIView(new CGRect(0, 0, Element.Width, Element.Height));
            nativeView.Layer.BorderColor = Element.BorderColor.ToCGColor();
            nativeView.Layer.BorderWidth = (nfloat)Element.BorderWidth;
            nativeView.Layer.CornerRadius = (nfloat)Element.CornerRadius;
            nativeView.ClipsToBounds = true;

            SetNativeControl(nativeView);
        }
    }
}

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:YourNamespace"
             x:Class="YourNamespace.MainPage">
    <local:CustomView BorderColor="Black" BorderWidth="2" CornerRadius="10">
        <!-- Add your content here -->
    </local:CustomView>
</ContentPage>