CODEWITHSUNDEEP
iphoneiosobjective-cbackgroundnstimer
EvilAegis
EvilAegis

Reputation: 733

Saving NSTimer's time to disk on entering background and retrieving on foreground

I want my timer to work in the background, and I have figured the only way to actually do that is to save the time on appliationDidEnterBackground and retrieve it with applicationDidLaunchWithOptions. My timer uses Core Data and my timeInterval (testTask.timeInterval) is saved after every decrement as so:

-(IBAction)startTimer:(id)sender{
    if (timer == nil) {
        [startButton setTitle:@"Pause" forState:UIControlStateNormal];
       timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerAction:) userInfo:nil repeats:YES];
    } else {
        [startButton setTitle:@"Resume" forState:UIControlStateNormal];
        [timer invalidate];
        timer = nil;
    }

}
-(void)timerAction:(NSTimer *)t
{
    if(testTask.timeInterval == 0)
    {
        if (self.timer)
        {
            [self timerExpired];
            [self.timer invalidate];
            self.timer = nil;
        }
    }
    else
    {
        testTask.timeInterval--;
        NSError *error;
        if (![self.context save:&error]) {
            NSLog(@"couldn't save: %@", [error localizedDescription]);
        }
    }
    NSUInteger seconds = (NSUInteger)round(testTask.timeInterval);
    NSString *string = [NSString stringWithFormat:@"%02u:%02u:%02u",
                        seconds / 3600, (seconds / 60) % 60, seconds % 60];
    timerLabel.text = string;
    NSLog(@"%f", testTask.timeInterval);
}
-(void)timerExpired{
    UILocalNotification* localNotification = [[UILocalNotification alloc] init];
    localNotification.alertBody = @"Time is up";
    localNotification.alertAction = @"Ok";
    localNotification.timeZone = [NSTimeZone defaultTimeZone];
    [[UIApplication sharedApplication]presentLocalNotificationNow:localNotification];
}

These methods are in a detail view controller, which is initialized by this:

-(void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath{
    DetailViewController *detailVC;
    if (![self.detailViewsDictionary.allKeys containsObject:indexPath]){
        detailVC = [[DetailViewController alloc]initWithNibName:@"DetailViewController" bundle:nil];
        [self.detailViewsDictionary setObject:detailVC forKey:indexPath];
        detailVC.context = self.managedObjectContext;
    }else{
        detailVC = self.detailViewsDictionary[indexPath];
    }
        Tasks *task = [[self fetchedResultsController] objectAtIndexPath:indexPath];
        detailVC.testTask = task;
        [[self navigationController] pushViewController:detailVC animated:YES];
    NSLog(@"%@", self.detailViewsDictionary);
}

What I don't understand is...how would I access the timeInterval (each detailviewcontroller has a different timeinterval...) such that I can put it in appliationDidEnterBackground? Also, I am guessing I should save the time that the application enters the background and then save the time when it enters the foreground, and subtract? and then I would subtract that value from the timeinterval, correct?

Upvotes: 0

Views: 386

Answers (1)

jscs
jscs

Reputation: 64002

First, you shouldn't be using a direct countdown like this if you care at all about accuracy. The NSTimer will not fire on exact one-second intervals, and delays will accumulate. The better way to do this is to create an NSDate when the timer starts, and get the NSTimeInterval since then at each tick, then calculate the minutes and seconds.

For each view controller to store its own start time, you can register any object to receive the UIApplicationDidEnterBackgroundNotification. This is posted at the same time that the app delegate gets applicationDidEnterBackground:.

Also, I am guessing I should save the time that the application enters the background and then save the time when it enters the foreground, and subtract? and then I would subtract that value from the timeinterval, correct?

Yes:

NSTimeInterval idleTime = [dateReturnedToForeground timeIntervalSinceDate:dateEnteredBackground];
NSTimeInterval elapsedTime = [[NSDate date] timeIntervalSinceDate:startDate];
elapsedTime -= idleTime;

will give you the active time that has elapsed for your timer.

Upvotes: 2

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