Coordinate Rotation at a Point

FROM : Gordon Apple
DATE : Tue Nov 06 06:33:50 2007

Although I found references to rotating a Bezier path, searching for
rotating coordinates at a point wasn't fruitful.  In my case, I wanted to
rotate a shape in place about its center, including shading and/or content.
I believe the following is about as simple as you can get and can be used to
rotate about any point.

@implementation  NSAffineTransform (RTPTransformAdditions)

- (void)rotate:(float)angle atPoint:(NSPoint)point
{
    [self rotateByDegrees:angle];
  NSAffineTransform* pointXfrm = [NSAffineTransform transform];
    [pointXfrm rotateByDegrees:-angle];    //    Needed to get original
point in rotated coordinates.
    NSPoint oldPoint = [pointXfrm transformPoint:point];
    float x = oldPoint.x - point.x;
    float y = oldPoint.y - point.y;
    [self translateXBy:x yBy:y];    //    Done in the rotated coordinates.
}