how to get the event that switch tab menu on iphone

Implement UITabBarControllerDelegate e.g. in your app delegate's applicationDidFinishLaunching

- (void)applicationDidFinishLaunching:(UIApplication *)application
{
    tabBarController.delegate = self;
    [window addSubview:tabBarController.view];
}

Then implement either:

- (BOOL)tabBarController:(UITabBarController *)tabBarController shouldSelectViewController:(UIViewController *)viewController;

- (void)tabBarController:(UITabBarController *)tabBarController didSelectViewController:(UIViewController *)viewController;

The first method is called before the view switch and gives you a chance to 'veto' the view switch by returning NO

The second method is called after the view switch has taken place

Have a look at the following method in UITabBarControllerDelegate:

- (void)tabBarController:(UITabBarController *)tabBarController didSelectViewController:(UIViewController *)viewController

Tells the delegate that the user selected an item in the tab bar.

Better late than never. In case of swift 4 you can do it in the following way.

tabBarViewController.delegate = self

And implement UITabBarDelegate in your class. You will get the callback in

override func tabBar(_ tabBar: UITabBar, didSelect item: UITabBarItem) {
  //Stuff to do
}

Is UITabBarControllerDelegate what you're looking for, particularly -tabBarController:didSelectViewController:?