Here is a method that does it:
private void drawOnTouch(object data)
{
UITouch touch = data as UITouch;
if (null != touch)
{
UIGraphics.BeginImageContext(this.Image == null ? this.Frame.Size : this.Image.Size);
using (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,this.Image.Size.Width, this.Image.Size.Height), this.Image.CGImage);
cont.ScaleCTM(1.0f, -1.0f);
cont.TranslateCTM(0f, -this.Image.Size.Height);
} //end if
PointF lastLocation = touch.PreviousLocationInView(this);
PointF pt = touch.LocationInView(this);
using (CGPath path = new CGPath())
{
cont.SetLineCap(CGLineCap.Round);
cont.SetLineWidth(3);
cont.SetRGBStrokeColor(0, 2, 3, 1);
path.MoveToPoint(lastLocation.X, lastLocation.Y);
path.AddLines(new PointF[] { new PointF(lastLocation.X,
lastLocation.Y),
new PointF(pt.X, pt.Y) });
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
If you place this method in a subclass of UIImageView and call it from TouchesBegan and TouchesMoved, when you touch the screen it will draw on the image.
