一，效果图。
二，代码。
RootViewController.h
#import <UIKit/UIKit.h>@interface RootViewController : UIViewController
{UILabel *label;NSDateFormatter *dateFormatter ;
}
@end
RootViewController.m
#import "RootViewController.h"@interface RootViewController ()@end@implementation RootViewController- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];if (self) {// Custom initialization}return self;
}- (void)viewDidLoad
{[super viewDidLoad];// Do any additional setup after loading the view.//labellabel=[[UILabel alloc]initWithFrame:CGRectMake(50, 100, 200, 200)];label.backgroundColor=[UIColor redColor];[self.view addSubview:label];//定时器[NSTimer scheduledTimerWithTimeInterval:1.0f target:self selector:@selector(updateTime) userInfo:nil repeats:YES];dateFormatter = [[NSDateFormatter alloc] init];[dateFormatter setDateFormat:@"HH:mm:ss"];}
- (void)updateTime
{label.text = [dateFormatter stringFromDate:[NSDate date]];
}
