一小时倒计时59：59

self.timeLabel = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, SCREEN_WIDTH, 200)];
self.timeLabel.text = [NSString stringWithFormat:@"倒计时   %@",self.format_time];
self.timeLabel.textAlignment = NSTextAlignmentCenter;
self.timeLabel.font = [UIFont systemFontOfSize:19];
self.timeLabel.textColor = [UIColor  redColor];
[self.view addSubview:self.timeLabel];


__block NSInteger time = 60; //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(time <= 0){ //倒计时结束，关闭
        dispatch_source_cancel(_timer);
        dispatch_async(dispatch_get_main_queue(), ^{
            
        });
    }else{
        dispatch_async(dispatch_get_main_queue(), ^{
            //设置按钮显示读秒效果
            NSString *str_minute = [NSString stringWithFormat:@"%02ld",(time%3600)/60];
            NSString *str_second = [NSString stringWithFormat:@"%02ld",time%60];
            self.format_time = [NSString stringWithFormat:@"%@:%@",str_minute,str_second];
            self.timeLabel.text = [NSString stringWithFormat:@"倒计时   %@",self.format_time];
        });
        time--;
    }
});
dispatch_resume(_timer);