How does anchorPoint influence center in a UIView? - ios

Question

I'm working on an app that lets the user resize and rotate a photo using UIGestureRecognizers. I have this code which adjusts the anchorPoint based on where the user is applying touches (to make it look like they're scaling the image at the point where their fingers actually are):
- (void)adjustAnchorPointForGestureRecognizer:(UIGestureRecognizer *)gestureRecognizer
{
UIView *gestureRecognizerView = gestureRecognizer.view;
CGPoint locationInView = [gestureRecognizer locationInView:gestureRecognizerView];
CGPoint locationInSuperview = [gestureRecognizer locationInView:gestureRecognizerView.superview];
gestureRecognizerView.layer.anchorPoint = CGPointMake(locationInView.x / gestureRecognizerView.bounds.size.width, locationInView.y / gestureRecognizerView.bounds.size.height);
gestureRecognizerView.center = locationInSuperview;
}
Later on, I'm simply wanting to calculate the origin based on the center and bounds with this code:
CGRect transformedBounds = CGRectApplyAffineTransform(view.bounds, view.transform);
CGPoint origin = CGPointMake(view.center.x - (transformedBounds.size.width * view.layer.anchorPoint.x), view.center.y - (transformedBounds.size.height * view.layer.anchorPoint.y));
And it's coming out incorrectly (I'm comparing against the frame value which ironically is supposed to be invalidated but actually does have the correct value).
So all in all I'm wondering, what am I not taking into account here? How is the anchorPoint influencing the center in a way I'm not able to determine?

Answer 1

I think the problem is that the origin you are calculating is not really an origin, but rather an offset of the origin of your transformedBounds rect.
I haven't fully tested it, but if you do something like this you should get the correct frame:
CGRect transformedBounds =
CGRectApplyAffineTransform(view.bounds, view.transform);
CGSize originOffset = CGSizeMake(
view.center.x - (transformedBounds.size.width * view.layer.anchorPoint.x),
view.center.y -
(transformedBounds.size.height * view.layer.anchorPoint.y));
transformedBounds.origin.x += originOffset.width;
transformedBounds.origin.y += originOffset.height;