沙马林。UI上的iOS手指绘制

我想用一根手指在Xamarin iOS的UIImageView控件上绘制。 我发现: First Second

这两个示例无法正常工作,因为第一个链接存在以下问题:

1. 我必须用两根手指画画,但我只想用一根手指(不要与图片混淆,我必须用两根手指来激活触摸功能)
2. 定位错误,我想问题在于缩放错误(见附图)

这是我目前拥有的代码:

public partial class DetailImageView : UIImageView
{
    DetailImageViewController ctrl;
    public DetailImageView (IntPtr handle) : base (handle)
    {

    }

    public void Initialize(DetailImageViewController rootController)
    {
        ctrl = rootController;
    }

    public override void TouchesBegan(NSSet touches, UIEvent evt)
    {
        base.TouchesBegan(touches, evt);

        UITouch touch = touches.AnyObject as UITouch;

        drawOnTouch(touch);

        //DrawLineOnImage();
    }

    public override void TouchesMoved(NSSet touches, UIEvent evt)
    {
        base.TouchesMoved(touches, evt);
        UITouch touch = touches.AnyObject as UITouch;

        drawOnTouch(touch);

        //DrawLineOnImage();
    }


    private void drawOnTouch(object data)
    {

        UITouch touch = data as UITouch;

        if (null != touch)
        {

            UIGraphics.BeginImageContext(this.Image == null ? this.Frame.Size : this.Image.Size);

            //UIGraphics.BeginImageContext(this.Frame.Size);

            using (CoreGraphics.CGContext cont = UIGraphics.GetCurrentContext())
            {

                if (this.Image != null)
                {

                    cont.TranslateCTM(0f, this.Image.Size.Height);
                    cont.ScaleCTM(1.0f, -1.0f);
                    cont.DrawImage(new RectangleF(0f, 0f, Convert.ToSingle(this.Image.Size.Width)
                                                  , Convert.ToSingle(this.Image.Size.Height)), this.Image.CGImage);
                    cont.ScaleCTM(1.0f, -1.0f);
                    cont.TranslateCTM(0f, -this.Image.Size.Height);


                } //end if

                CGPoint lastLocation = touch.PreviousLocationInView(this);
                CGPoint pt = touch.LocationInView(this);
                using (CGPath path = new CGPath())
                {

                    cont.SetLineCap(CGLineCap.Round);
                    cont.SetLineWidth(10);
                    cont.SetStrokeColor(0, 2, 3, 1);

                    var xMultiplier = Image.Size.Height / Frame.Height;
                    var yMultiplier = Image.Size.Width / Frame.Width;

                    var xLoc = lastLocation.X * xMultiplier;
                    var yLoc = lastLocation.Y * yMultiplier;

                    path.MoveToPoint(xLoc, yLoc);
                    path.AddLines(new CGPoint[] { new CGPoint(xLoc,
                                                              yLoc),
                        new CGPoint(pt.X * xMultiplier, pt.Y * yMultiplier) });
                    path.CloseSubpath();

                    cont.AddPath(path);
                    cont.DrawPath(CGPathDrawingMode.FillStroke);
                    this.Image = UIGraphics.GetImageFromCurrentImageContext();

                }//end using path


            }//end using cont
            UIGraphics.EndImageContext();
            this.SetNeedsDisplay();

        }//end if
    }//end void drawOnTouch
}