倒计时功能的实现

@implementation SYViewController
{
    int totalSeconds;
    NSTimer *downTimer;
    UILabel *labelText;
}

- (void)viewDidLoad {
    
    [super viewDidLoad];
    
    totalSeconds = 60;
    
    labelText = [[UILabel alloc] init];
    labelText.frame = CGRectMake(100, 100, 300, 30);
    [self.view addSubview:labelText];
    downTimer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerChange) userInfo:nil repeats:YES];
    labelText.text = [NSString stringWithFormat:@"%d",totalSeconds];

}

- (void)timerChange
{
    totalSeconds--;
    labelText.text = [NSString stringWithFormat:@"%d",totalSeconds];

    if (totalSeconds==0) {
        [downTimer invalidate];
        downTimer = nil;
    }
}

}

   __block int timeout=60; //倒计时时间
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_source_t _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0,queue);
    dispatch_source_set_timer(_timer,dispatch_walltime(NULL, 0),1.0*NSEC_PER_SEC, 0); //每秒执行
    dispatch_source_set_event_handler(_timer, ^{
        if(timeout<=0){ //倒计时结束，关闭
            dispatch_source_cancel(_timer);
            dispatch_async(dispatch_get_main_queue(), ^{
                //设置界面的按钮显示 根据自己需求设置
                labelText.text = @"倒计时结束";
            });
        }else{
            int minutes = timeout / 60;
            int seconds = timeout % 60;
            NSString *strTime = [NSString stringWithFormat:@"%d分%.2d秒后重新获取验证码",minutes, seconds];
            dispatch_async(dispatch_get_main_queue(), ^{
                //设置界面的按钮显示 根据自己需求设置
                labelText.text = [NSString stringWithFormat:@"%@",strTime];
                
            });
            timeout--;
            
        }
    });
    dispatch_resume(_timer);