iPhone: Rotating a drawing without rotating the view within which it is drawn

Question

Anyone who has gone through the Stanford CS193P class on iTunes will recognize this from Assignment 3 (HelloPoly). For those not familiar:

I have a custom view called polyView, an instance of PolygonView, a subclass of UIView. On this view, I use CGContextDrawLinearGradient to paint a color gradient over the entire rectangular view. Then I use CGContextDrawPath to stroke and fill a polygon within the bounds of polyView. And I have a UILabel called nameLabel in the center of the view (and polygon) that displays the name of the polygon (triangle, quadrilateral, pentagon, etc.). All of this works fine, and the code to do all this is in the -(void)drawRect method of the PolygonView class.

Where I ran into trouble is with an additional requirement to rotate the polygon within the view in response to user gestures. I used CGAffineTransformRotate() in response to touchesBegan() and touchesMoved() events within the PolygonView class, and this basically works, too. But I can only rotate the entire polyView, not just the polygon drawn on it. I'm sure I could go back and recalculate the path of the polygon and redraw/fill the path in response to each touchesMoved() event, but that would be expensive and can't be the best method. How can I use CGAffineTransformRotate to rotate just the polygon, without rotating the gradient-filled view or the label in the center?

Or is there some way to create the polygon on a layer that I can place over the background polyView at the desired rotation angle?

Thanks for any help you can give a beginner here!

Duane

Answer 1

You can do a CGContextSaveGState(...) just before doing the rotation transformation, then drawing the polygon and restoring the drawing state with a CGContextRestoreGState(...) afterwards, so as not to affect any other drawing in the view later.