How should I interact with the geometry of a UIView, disregarding any transforms applied?

Question

I have a view that I would like the user to rotate around its center, by tapping and holding somewhere and just move their finger round and round.

I have all the geometry worked out; What I do is store the initial touch angle relative to the center as offsetAngle, then my touchesMoved method looks like this:

- (void) touchesMoved: (NSSet *)touches withEvent:(UIEvent *)event {
    self.transform = CGAffineTransformMakeRotation(0);
    CGPoint location = [[touches anyObject] locationInView: self];    
    CGPoint actualCenter = [self convertPoint: self.center fromView: self.superview];    
    CGPoint relativeTouch = [MathHelper translatePoint:location relativeTo: actualCenter];
    CGPoint zero = CGPointMake(0, 0);
    float angle = [MathHelper getAngleForPoint:zero andPoint:relativeTouch];
    self.transform = CGAffineTransformMakeRotation(offsetAngle-angle);
}

The ugly bit is the first line, where I have to restore the rotation in order to get the correct event positions in the view. If I don't that, then the locations jump all over the place as there's no continuity, since the view is rotating...

Another issue is when you want to manipulate a view's frame (eg. moving it down), when the view has a transform applied:

- (IBAction)toggleSettingsView {
    BOOL state = [settingsSwitch isOn];
    float height = optionsView.bounds.size.height;
    CGRect polygonFrame = polygonView.frame; 

    [UIView beginAnimations:@"advancedAnimations" context:nil]; 
    [UIView setAnimationDuration:0.3]; 
    if (state) {
        optionsView.alpha = 1.0; 
        polygonFrame.origin.y += height; 
    } else {
        optionsView.alpha = 0.0; 
        polygonFrame.origin.y -= height; 
    }
    polygonView.frame = polygonFrame;
    [UIView commitAnimations]; 

}

This distorts the view heavily.

I must mention that both the CGTransform and the CALayer transform have the same effect.

This smells like I'm doing something wrong, but I don't know what I should be doing.

Answer 1

I would use the coordinate system of the superview, since it is unaffected by the rotation:

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    CGPoint location = [[touches anyObject] locationInView:self.superview];
    CGPoint relativeTouch = [MathHelper translatePoint:location relativeTo:self.center];
    CGPoint zero = CGPointMake(0, 0);
    float angle = [MathHelper getAngleForPoint:zero andPoint:relativeTouch];
    self.transform = CGAffineTransformMakeRotation(offsetAngle-angle);
}

Answer 2

Just save the original transform in an ivar and call it a day.