@implementation UIView (Positioning)

// Add this to a category on UIView. (Remember to add as a subview on another view BEFORE calling this method!)
- (void)centerOnSuperviewHorizontally:(BOOL)horizontally vertically:(BOOL)vertically {
    NSAssert(self.superview, @"Cannot center a view on its superview if the view has no superview.");
    CGSize selfSize = self.frame.size;
    CGSize superviewSize = self.superview.frame.size;
    CGPoint origin = self.frame.origin;
    if (horizontally) origin.x = (superviewSize.width - selfSize.width)/2;
    if (vertically) origin.y = (superviewSize.height - selfSize.height)/2;
    CGRect frame = self.frame;
    frame.origin = origin;
    self.frame = frame;
}

@end