CountDownTimer

简单的封装GCD的倒计时，实现了实现了APP在后台倒计时（APP进入后台再返回前台时对比当前的系统时间）

 __block MKCoutDownTimer *t = [MKCoutDownTimer timerWithCountdownTime:60.f rate:1.f executeBlock:^(int second){
NSLog(@"倒计时%d",second);
if (second == 50) {
[t invalidate];
}
} endBlock:^{
NSLog(@"结束");
}];