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];