NSTimer *timer = [NSTimer timerWithTimeInterval:1 repeats:YES block:^(NSTimer * _Nonnull timer) {
}];
[[NSRunLoop currentRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];
[[NSRunLoop currentRunLoop] run];
timer加入runloop,自动进入倒计时.
[timer setFireDate:[NSDate distantFuture]]; //倒计时暂停
[timer setFireDate:[NSDate distantPast]]; //继续倒计时
[timer invalidate];//倒计时永久停止,但不会释放内存为空.
