How do you handle touch/gesture input on a GraphicsView for interactive drawing?

Attach gesture recognizers (PanGestureRecognizer, TapGestureRecognizer) or handle pointer events, translate the touch points into your drawable's coordinate space, update model state, and call Invalidate. For freehand drawing, accumulate points on pan and render them as a path.

var pan = new PanGestureRecognizer();
pan.PanUpdated += (s, e) =>
{
    _points.Add(new PointF((float)e.TotalX, (float)e.TotalY));
    graphicsView.Invalidate();
};
graphicsView.GestureRecognizers.Add(pan);