gcd倒计时

@interface ViewController ()
{
    dispatch_source_t _timer;
}
@property (weak, nonatomic) IBOutlet UILabel *timeLab;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

- (IBAction)alertBtnClick:(id)sender {

    UIButton *_codeBtn =(UIButton *)sender;

    __block int timeout=59; //倒计时时间
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    //typedispatch源可处理的事件
    _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
                _codeBtn.hidden = NO;
                _timeLab.hidden = YES;
                [_codeBtn setTitle:@"获取验证码" forState:UIControlStateNormal];
            });
        }else{
            int seconds = timeout % 60;
            NSString *strTime = [NSString stringWithFormat:@"%.2d", seconds];
            dispatch_async(dispatch_get_main_queue(), ^{
                //设置界面的按钮显示 根据自己需求设置
                //NSLog(@"____%@",strTime);
                _timeLab.hidden = NO;
                _codeBtn.hidden = YES;
                _timeLab.text = [NSString stringWithFormat:@"%@秒后重新获取",strTime];
            });
            timeout--;
        }
    });
    //立即执行
    dispatch_resume(_timer);

}
-(void)viewWillDisappear:(BOOL)animated{

    [super viewWillDisappear:animated];
    if(_timer){
        dispatch_source_cancel(_timer);
    }
}