@class ViewController;      

@interface AppDelegate : UIResponder <UIApplicationDelegate>     

@property (strong, nonatomic) UIWindow *window;     

@property (strong, nonatomic) ViewController *viewController;     

@property (strong, nonatomic) UINavigationController *navController;   

@end  

//设置导航按钮

    UIBarButtonItem *backButton = [[UIBarButtonItem alloc] initWithTitle:@"Back" style:UIBarButtonItemStyleBordered target:self action:@selector(backButtonPressd)];

    //第一个参数为按钮标题

    //第二个参数是按钮样式

    //第三个参数接受消息的对象

    //第四个参数是按钮的回调函数