Add UITabbarController after Login Screen.

Often we are stuck when we are creating a default UITabbar project 

and we need to add a login screen before it.

Quick Fix:

Declare and initialize two VC  in AppDelegate.h

@property (strong, nonatomic) LoginVC* _loginVCObj;

@property (strong, nonatomic) UITabBarController *_tabBarController;

one for loginVC and  one for otherVC (for default tabbar) in appDelegate and 

On launch make LoginVC as RootViewController

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{// declare LoginVC and make it rootViewController
 self.window.rootViewController = self._loginVCObj;
    [self.window makeKeyAndVisible];
}

After login is successful call the function declared in AppDelegate class in LoginVC as

       AppDelegate* appDelegateObj = (AppDelegate*)[[UIApplication sharedApplication] delegate];

[appDelegateObj continueToNextView];

In AppDelegate define the function as:

#pragma mark- Continue to next screen after successful Login

-(void) continueToNextView
{   // Handle UI after Login like.
    [_loginVCObj._indicator stopAnimating];
    [_loginVCObj._loginButton setEnabled:YES];

    self._tabBarController = [[UITabBarController alloc]init];

//adding the VCs to the tabbar // set the viewControllers declared in the array, we can also use navigationController

self._tabBarController.viewControllers = [NSArray arrayWithObjects:myNavigationControllerList,_VC1Obj, _VC2Obj, _settingNavObj, nil]; 

// make tabbar as rootViewController
     self.window.rootViewController = self._tabBarController;
}

Note In the above logic we have changed the rootview controller of the app Window.