使用GCD实现获取验证码的倒计时

先创建一个Button

- (void)viewDidLoad {
    [super viewDidLoad];
    
    _l_timeButton = [[UIButton alloc] initWithFrame:CGRectMake(100, 100, 150, 30)];
    [_l_timeButton setTitle:@"获取验证码" forState:UIControlStateNormal];
    [_l_timeButton setTitleColor:[UIColor redColor] forState:UIControlStateNormal];
    [_l_timeButton addTarget:self action:@selector(startTime) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:_l_timeButton];
}

在button的点击事件中实现

-(void)startTime{
    __block int timeout=30; //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
                [_l_timeButton setTitle:@"发送验证码" forState:UIControlStateNormal];
                _l_timeButton.userInteractionEnabled = YES;
            });
        }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);
                [UIView beginAnimations:nil context:nil];
                [UIView setAnimationDuration:1];
                [_l_timeButton setTitle:[NSString stringWithFormat:@"%@秒后重新发送",strTime] forState:UIControlStateNormal];
                [UIView commitAnimations];
                _l_timeButton.userInteractionEnabled = NO;
            });
            timeout--;
        }
    });
    dispatch_resume(_timer);
}