iOS：如何防止计时器在切换屏幕或屏幕休眠时停止？

Question

我正在编写一个基本的 hh:mm:ss 计时器应用程序。我的计时器在开始、暂停和停止选项中都能完美运行。我的问题是当我在应用程序上交换屏幕时，计时器停止并返回到 00:00:00。

我是否需要将它保存在一个无论如何都会更新的数组中？有没有我可以实现的方法或教程可以让计时器连续计数，即使我交换屏幕、转到另一个应用程序或屏幕休眠？

任何想要实现计时器的人的代码：

//==================开始工作计时器=======================//

//Work Timer
- (void)updateWorkTimer {

if(working == false) return;

//calculate elapsed time
NSTimeInterval currentTime = [NSDate timeIntervalSinceReferenceDate];
NSTimeInterval elapsed = workSecondsAlreadyRun + currentTime - startWorkTime;

// extract out the minutes, seconds, and hours of seconds from elapsed time:
int hours = (int)(mins / 60.0);
elapsed -= hours * 60;
int workingMins = (int)(elapsed / 60.0);
elapsed -= workingMins * 60;
int secs = (int) (elapsed);

//update our label using the format of 00:00:00
self.workTimerLabel.text = [NSString stringWithFormat:@"%02u:%02u:%02u", hours, workingMins, secs];

//call updateTime every 1 second
[self performSelector:@selector(updateWorkTimer) withObject:self afterDelay:1];

}

//=============================================== ==================================================== ===================

- (IBAction)startWorkButton:(id)sender {

if(working == false) {

    //start timer
    working = true;
    startWorkDate = [[NSDate alloc] init];
    startWorkTime = [NSDate timeIntervalSinceReferenceDate];

    //Set color and title for the button
    [self.startWorkButton setBackgroundColor:[UIColor blueColor]];
    [sender setTitle:@"Pause Work" forState:UIControlStateNormal];
    [self updateWorkTimer];
}
else {
    //pause timer
    workSecondsAlreadyRun += [[NSDate date] timeIntervalSinceDate:startWorkDate];
    startWorkDate = [[NSDate alloc] init];

    //Set color and title for the button
    UIColor * color = [UIColor colorWithRed:56/255.0f green:171/255.0f blue:10/255.0f alpha:1.0f];
    [self.startWorkButton setBackgroundColor: color ];
    [sender setTitle:@"Resume Work" forState:UIControlStateNormal];
    working = false;
}


}

提前致谢。

Answer 1

您不需要让计时器一直运行。相反，您可以简单地计算应用再次打开后的时间间隔（应用进入后台后经过的时间），然后将其添加到计时器的值中。

为此，您需要在应用程序进入后台时使用 NSUserDefaults 存储计时器的当前值和当前时间 ([NSDate date])，并使用此数据计算应用程序再次启动后计时器应具有的值.要拦截进入后台和前台，您应该订阅UIApplicationDidEnterBackgroundNotification 和UIApplicationDidBecomeActiveNotification，因为不会调用viewDidAppear 和viewDidDisappear。