iOS 数字时钟（时间显示并持续更新）

思路分析：

代码示例：

- (void)viewDidLoad {
[super viewDidLoad];

//定时器 反复执行
NSTimer *timer = [NSTimer timerWithTimeInterval:1.0 target:self selector:@selector(updateTime) userInfo:nil repeats:YES];

[[NSRunLoop currentRunLoop] addTimer:timer forMode:NSDefaultRunLoopMode];

//设置停止按钮
UIButton *button = [[UIButton alloc]initWithFrame:CGRectMake(100, 200, 100, 40)];

[button setTitle:@"STOP" forState:UIControlStateNormal];

}

-(void)updateTime{
UILabel *timeLable = [[UILabel alloc]initWithFrame:CGRectMake(100, 100, 300, 60)];

timeLable.backgroundColor = [UIColor orangeColor];

[self.view addSubview:timeLable];

NSDate *currentDate = [NSDate date];

NSDateFormatter *dataFormatter = [[NSDateFormatter alloc]init];

[dataFormatter setDateFormat:@"YYYY - MM - dd   HH : mm : ss "];

NSString *dateString = [dataFormatter stringFromDate:currentDate];
NSLog(@"%@",dateString);

timeLable.text = dateString;

}