Implementing Navigation with UINavigationController

#import "AppDelegate.h"
#import "FirstViewController.h"
@interface AppDelegate ()
@property (nonatomic, strong) UINavigationController *navigationController; 
@end
@implementation AppDelegate ...

- (BOOL) application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{
        FirstViewController *viewController = [[FirstViewController alloc]
                                               initWithNibName:nil
                                               bundle:nil];
        self.navigationController = [[UINavigationController alloc]
                                     initWithRootViewController:viewController];
        self.window = [[UIWindow alloc]
                       initWithFrame:[[UIScreen mainScreen] bounds]];
        self.window.rootViewController = self.navigationController;
self.window.backgroundColor = [UIColor whiteColor]; [self.window makeKeyAndVisible];
return YES;
}