1.注意要将定时器加入RunLoop中去(避免其他事件暂停了定时器事件)
_timer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(timerCountDown) userInfo:nil repeats:YES]; [[NSRunLoop mainRunLoop] addTimer:_timer forMode:UITrackingRunLoopMode];
2.在界面小时,记得销毁定时器,且置为nil
[_timer invalidate];
_timer = nil;