NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:1.0f target:self selector:@selector(updateUI:) userInfo:nil repeats:YES];
// 将timer加入到当前的run loop中,并将run loop mode设置为UITrackingRunLoopMode或NSRunLoopCommonModes,这样,即使用户触摸了屏幕,也不会导致timer暂停界面刷新.
[[NSRunLoop currentRunLoop] addTimer:timer forMode:UITrackingRunLoopMode];