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Below is the image of my UITabBarController structure in storyboard.

Right now in storyboard, AboutUsViewController(UIViewController) is bound with my tabBar button click event i.e. If I click on tabBar button, AboutUsViewController is opening but now my functionality is based on certain conditions. I want to call ContactRequstViewController instead of AboutUsViewController on same tabBar button click.

Following is my code to open ContactRequstViewController.

-(void)tabBarController:(UITabBarController *)tabBarController didSelectViewController:(UIViewController *)viewController
 if (tabBarController.selectedIndex == 2)
 UIStoryboard *story = [UIStoryboard storyboardWithName:@"iPhone" bundle:nil];
 ContactRequstViewController *contactVC = [story instantiateViewControllerWithIdentifier:@"ContactUsView"];
 [self.navigationController pushViewController:contactVC animated:YES];
 

After writing above code I am not able to load ContactRequestViewController.

If you want to replace the root view controller at that selected tab based on custom logic, try using setViewControllers:animated: method of the UITabBarController.

You could do something like this:

- (void)tabBarController:(UITabBarController *)tabBarController didSelectViewController:(UIViewController *)viewController 
 if (tabBarController.selectedIndex == 1 && tabBarController.viewControllers.count > tabBarController.selectedIndex) 
 BOOL shouldShowContactVC = (BOOL)(rand() % 2);
 NSMutableArray *viewControllers = [[tabBarController viewControllers] mutableCopy];
 UIStoryboard *main = [UIStoryboard storyboardWithName:@"Main" bundle:nil];
 UIViewController *newVC = nil;
 if (shouldShowContactVC) 
 newVC = [main instantiateViewControllerWithIdentifier:@"ContactUsVC"];
 else 
 newVC = [main instantiateViewControllerWithIdentifier:@"AboutUsVC"];
 
 if (newVC) 
 [viewControllers replaceObjectAtIndex:tabBarController.selectedIndex withObject:newVC];
 newVC.tabBarItem = viewController.tabBarItem;
 [tabBarController setViewControllers:viewControllers animated:YES];
 
 

In my experience , sometimes we don't use tabbarVC, because it's not flexible, such as hide, show, or change items number. So we set a navigationVC as rootVC, u can create a View, just make it's look like a tabbarVC.
This view is more easy to used.And u don't need to care any delegate method or something else. u can do anything u want.
That's all.