iOS中倒计时程序，考虑线程暂停场景。
iOS App进入后台时，GCD线程也会跟着暂停。当程序进入前台后，GCD线程恢复。因而倒计时程序需要考虑这一点，通过加入时间的比对来实现。
(void)countDownWithLapseTime:(int)lapseTime andBlock:(void(^)(int timeLapse)) countDownBlock{
__block dispatch_source_t timer;
NSTimeInterval timeInterval=lapseTime;
NSDate *startTime = [NSDate date];
if (timer==nil) {
__block int timeout = timeInterval; //倒计时时间
if (timeout!=0) {
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
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, ^{
大专栏 iOS中如何实现准确的倒计时程序 · 九十里 if(timeout<=0){ //倒计时结束，关闭
dispatch_source_cancel(timer);
timer = nil;
dispatch_async(dispatch_get_main_queue(), ^{
countDownBlock(timeout);
});
}else{
timeout--;
NSDate *currentDate = [NSDate date];
int leftTime = timeInterval - ((int)(currentDate.timeIntervalSince1970 - startTime.timeIntervalSince1970));
if((timeout - leftTime) > 1) //如果真实计时和倒计数计时差别大于1秒，则同步。
timeout = leftTime;
//回主线程执行countDownBlock
dispatch_async(dispatch_get_main_queue(), ^{
countDownBlock(timeout);
});
}
});
dispatch_resume(timer);
}
}
}