iOS UI布局-定时器

/**
 *  倒计时GCD通用方法
 *  通常用的计时器都是用NSTimer，但是NSTimer在线程很吃紧的时候效果不佳，使用GCD计时相对更好
 *
 *  @param seconds   倒计时间 单位：秒
 *  @param showLable 需要显示的文本框
 *  @param endBlock  倒计时结束后，回调的Block
 */
- (void)startTimerWithSeconds:(long)seconds showLable:(UILabel *)showLable endBlock:(void (^)())endBlock
{
    __block long timeout = seconds; // 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，回调block
            dispatch_source_cancel(_timer);
            dispatch_async(dispatch_get_main_queue(), ^{
                if (endBlock) {
                    endBlock();
                }
            });
        } else{

            NSString *strTime = [NSString stringWithFormat:@"%02ld分%02ld秒",(long)(timeout % 3600 / 60), (long)(timeout  % 60)];

            // 回到主界面,显示倒计时
            dispatch_async(dispatch_get_main_queue(), ^{
                showLable.text = strTime;
            });
            
            timeout--;
        }
    });
    
    dispatch_resume(_timer);
}

[self startTimerWithSeconds:1800 showLable:self.timerLabel endBlock:nil];