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using Android.Graphics;
[assembly: ExportRenderer(typeof(Xamarin.Forms.Image), typeof(CustomImageCircleRenderer))]
public class CustomImageCircleRenderer : ImageRenderer
{
    protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        //baseを呼ぶとOutOfMemmoryが発生しますのでコメントアウト
        //base.OnElementPropertyChanged(sender, e);

        ImageSource source = Element.Source;

        if ((Element.Width <= 0 ||
                Element.Height <= 0 ||
                _isDecoded) &&
            (e.PropertyName != "Source" || source == null))
        {
            return;
        }

        try
        {
            var width = (int)Element.Width;
            var height = (int)Element.Height;

            //非同期だとListViewに間違った画像が表示される。
            Bitmap bitmap = ImageService.GetShrinkedBitmap(source, width, height);

            //画像を丸型に変更
            Bitmap circleBitmap = CustomImageCircleRenderer.GetCircularBitmap(bitmap);

            //オリジナルのイメージコントロールに修正した画像ファイルをセットします。
            Control.SetImageBitmap(circleBitmap));

        }
        catch (Exception ex)
        {
            System.Diagnostics.Debug.WriteLine(ex.Message + System.Environment.NewLine +
                                    ex.StackTrace);
        }
    }
    //画像を丸型に編集する
    private static Bitmap GetCircularBitmap(Bitmap bitmap)
    {
        if (bitmap == null)
        {
            return null;
        }
        float radius = bitmap.Width > bitmap.Height ? 
                                        ((float)bitmap.Height) / 2f : 
                                        ((float)bitmap.Width) / 2f;
        if (radius <= 0)
        {
            radius = 0;
        }
        else
        {
            radius -= 2;
        }

        //ベースとなる画像の丸型設定
        Bitmap canvasBitmap = Bitmap.CreateBitmap(bitmap.Width,
                                                    bitmap.Height,
                                                    Bitmap.Config.Argb8888);
        BitmapShader shader = new BitmapShader(bitmap,
                                                Shader.TileMode.Clamp,
                                                Shader.TileMode.Clamp);
        Paint basePaint = new Paint();
        basePaint.AntiAlias = true;
        basePaint.SetShader(shader);

        //白い枠の設定
        var path = new Android.Graphics.Path();
        path.AddCircle(bitmap.Width / 2, bitmap.Height / 2, radius, Android.Graphics.Path.Direction.Ccw);
        var whitePaint = new Paint();
        whitePaint.AntiAlias = true;
        whitePaint.StrokeWidth = 2;
        whitePaint.SetStyle(Paint.Style.Stroke);
        whitePaint.Color = global::Android.Graphics.Color.White;

        //画像を加工する
        Canvas canvas = new Canvas(canvasBitmap);
        canvas.DrawPath(path, whitePaint);  //白枠表示する
        canvas.DrawCircle(bitmap.Width / 2, bitmap.Height / 2,
                radius - 1, basePaint);　//丸型の画像に加工する

        return canvasBitmap;
    }
}

[assembly: ExportRenderer(typeof(Xamarin.Forms.Image), typeof(CustomImageCircleRenderer))]
public class CustomImageCircleRenderer : ImageRenderer
{
    protected override void OnElementChanged(ElementChangedEventArgs<Image> e)
    {
        base.OnElementChanged(e);

        try
        {
            if (e.OldElement != null || Element == null)
            return;

            CreateCircle();
        }
        catch (Exception ex)
        {
            System.Diagnostics.Debug.WriteLine(ex.Message + System.Environment.NewLine +
                                    ex.StackTrace);
        }
    }

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

        try
        {
            if (e.PropertyName == VisualElement.HeightProperty.PropertyName ||
                e.PropertyName == VisualElement.WidthProperty.PropertyName)
            {
                CreateCircle();
            }
        }
        catch (Exception ex)
        {
            System.Diagnostics.Debug.WriteLine(ex.Message + System.Environment.NewLine +
                                    ex.StackTrace);
        }
    }

    private void CreateCircle()
    {
        try
        {
            double min = Math.Min(Element.Width, Element.Height);
            Control.Layer.CornerRadius = (float)(min / 2.0);
            Control.Layer.MasksToBounds = false;
            Control.Layer.BorderColor = Color.White.ToCGColor();
            Control.Layer.BorderWidth = 2;
            Control.ClipsToBounds = true;
        }
        catch (Exception ex)
        {
            System.Diagnostics.Debug.WriteLine(ex.Message + System.Environment.NewLine +
                                    ex.StackTrace);
        }
    }
}