Androidview Lollipop中的Imageview帆布黑色背景 - Xamarin

Question

我在Xamarin中有一个带圆角的ImageView。我的代码适用于Android 4.4及以下版本，但Lollipop现在它添加了黑色背景 黑区应该是透明的 - http://i.stack.imgur.com/GRFnl.png

    public class RoundedImageView : ImageView
{
    private int _cornerRadiusPercentage;
    private Path _path = null;

    public RoundedImageView(Context context, IAttributeSet attrs)
        : base(context, attrs)
    {
        this._cornerRadiusPercentage = attrs.GetAttributeIntValue("http://schemas.boomo.android/roundedImageView", "corner_radius_percentage", 10);

        this.SetLayerType(LayerType.Software, null);
    }

    public RoundedImageView(Activity activity, int cornerRadiusPercentage)
        : base(activity)
    {
        this._cornerRadiusPercentage = cornerRadiusPercentage;
        this.SetLayerType(LayerType.Software, null);
    }

    private void SetPath(Canvas canvas)
    {
        int offsetLeft = this.PaddingLeft;
        int offsetRight = this.PaddingRight;
        int offsetTop = this.PaddingTop;
        int offsetBottom = this.PaddingBottom;

        int cw = canvas.Width - offsetLeft - offsetRight;
        int ch = canvas.Height - offsetTop - offsetBottom;

        int cornerRadius = (int)((this._cornerRadiusPercentage / 100f) * cw);

        _path = new Path();
        _path.AddRoundRect(new RectF(offsetLeft, offsetTop, cw, ch), cornerRadius, cornerRadius, Path.Direction.Cw);
        _path.Close();
    }

    protected override void OnDraw(Canvas canvas)
    {
        //Paint paint = new Paint();
        //paint.Color = Color.Transparent;
        //paint.Alpha = 254;
        //paint.AntiAlias = true;

        if (_path == null)
            SetPath(canvas);
        canvas.ClipPath(_path);
        //canvas.DrawPath(_path, paint);
        base.OnDraw(canvas);
    }
}

*注意注释掉的部分是我试图用来解决问题的部分