iOS 短信验证码倒计时按钮

本文参考(http://www.jianshu.com/p/2104865e7dba)
解决了App进入后台暂停倒计时的问题

- (void)openCountdown {
    
    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); // 每秒执行一次
    
    NSTimeInterval seconds = 60.f;
    CFTimeInterval endTime = CACurrentMediaTime() + seconds; // 最后期限
    
    dispatch_source_set_event_handler(_timer, ^{
        CFTimeInterval interval = endTime - CACurrentMediaTime();
        if (interval > 0) { // 更新倒计时
            NSString *timeStr = [NSString stringWithFormat:@"%.0f秒后重发", interval];
            dispatch_async(dispatch_get_main_queue(), ^{
                self.enabled = NO;
                [self setTitle:timeStr forState:UIControlStateNormal];
            });
        } else { // 倒计时结束，关闭
            dispatch_source_cancel(_timer);
            dispatch_async(dispatch_get_main_queue(), ^{
                self.enabled = YES;
                [self setTitle:@"发送验证码" forState:UIControlStateNormal];
            });
        }
    });
    dispatch_resume(_timer);
}