position view relative to tab indicator - ios

Question

I've been given a UI that uses a UITabBarController, and in one of the tabbed ViewControllers, a View (an arrow as a UIImageView) needs to be positioned relative to the tab indicator (the arrow needs to point down at the indicator, centered along the x-axis). I used basic math to emulate the position based on known variables, and it works properly for iPhone/Pod in both landscape and portrait orientations, but fails on iPad (it's a little too far right in both orientations - seems worse in landscape but that might just be my perception).
Here's what I used:
int visibleWidth = self.view.frame.size.width;
int tabBarSize = self.tabBarController.tabBar.frame.size.width;
int tabSize = tabBarSize / [self.tabBarController.viewControllers count];
int tabPosition = 2; // hint should point at the 3rd tab
int arrowIconWidth = hintIcon.image.size.width;
int arrowIconHeight = hintIcon.image.size.height;
int tabEmulationPosition = visibleWidth / 2 - tabBarSize / 2;
int tabOffsetPosition = tabEmulationPosition + ( tabSize * tabPosition );
int iconPosition = tabOffsetPosition + ( tabSize / 2 - arrowIconWidth / 2 );
Is there a better way than trying to "fake" the math (maybe a getBoundingClientRect type method?) Or is the math approach the best bet, assuming there's a correct-able flaw in what I've got?
TYIA

Answer 1

Your math seems alright except one caveat: the Core Graphics distances are all of type CGFloat not int. Remember that integer division works differently in C than what you might expect:
int x = 5 / 2; // x == 2
Thus, replace your ints with CGFloats and divide by 2.0 rather than 2.
Also, in the iPad the tab button are not necessarily spaced evenly across the whole bar but have a certain padding to the left and right. You would have to fiddle for finding a suitable constant, or experimentally find a reliable way to calculate it based on the number of view controllers.
Assuming a fixed tab bar button width, you could calculate an offset to add to the x of your image:
CGFloat tabSize =
FIXED_TAB_BUTTON_WIDTH * self.tabBarController.viewControllers.count;
CGFloat offset = (tabBarSize - tabSize) / 2.0;