Rotating UIView with 1 finger iPhone , iPad - cocoa

Question

I implemented a UIPanGestureRecognizer since I wish to use one finger to rotate a UIView along its axis. A button within the uiview begins the gesture at which point the UIView rotates. Problem is that it only rotates correctly if the button is in the 1st quadrant, top left. Any other quadrant and it rotates erratically. Can someone tell me what is wrong with my math. By the way ang calculates the angle using the superview's coordinates since the users finger might be outside the rotating views bounds, but that might not be necessary.
thank you
- (void)rotateItem:(UIPanGestureRecognizer *)recognizer
{
NSLog(#"Rotate Item");
float ang = atan2([recognizer locationInView:self.superview].y - self.center.y, [recognizer locationInView:self.superview].x - self.center.x);
float angleDiff = deltaAngle - ang;
self.transform = CGAffineTransformRotate(startTransform, -angleDiff);
CGFloat radians = atan2f(self.transform.b, self.transform.a);
NSLog(#"rad is %f", radians);
}
#pragma mark - Touch Methods
- (BOOL)gestureRecognizerShouldBegin:(UIPanGestureRecognizer *)recognizer
{
if (recognizer == rotateGesture) {
NSLog(#"rotate gesture started");
deltaAngle = atan2([recognizer locationInView:self].y-self.center.y, [recognizer locationInView:self].x-self.center.x);
startTransform = self.transform;
}
return YES;
}

Answer 1

I did some logging and it seems that the center of my uiview was changing during the touch drag event. Hence I stored the center of the uiview with the touches began method and used it instead.
- (void)rotateItem:(UIPanGestureRecognizer *)recognizer
{
NSLog(#"Rotate Item");
CGPoint superPoint = [self convertPoint:itemCenter toView:self.superview];
float ang = atan2([recognizer locationInView:self.superview].y - superPoint.y, [recognizer locationInView:self.superview].x - superPoint.x);
float angleDiff = deltaAngle - ang;
self.transform = CGAffineTransformRotate(startTransform, -angleDiff);
}
#pragma mark - Touch Methods
- (BOOL)gestureRecognizerShouldBegin:(UIPanGestureRecognizer *)recognizer
{
if (recognizer == rotateGesture) {
NSLog(#"rotate gesture started");
deltaAngle = atan2([recognizer locationInView:self.superview].y-self.center.y, [recognizer locationInView:self.superview].x-self.center.x);
startTransform = self.transform;
}
return YES;
}