Detecting Day Change in iOS App

Question

So I've been doing a bit of research on how to tell if it's a new day in my iOS app. Basically, I want to reset a bunch of values to 0 as soon as a new day starts. I've seen that I can use "UIApplicationSignificantTimeChangeNotification" which sends a notification at midnight. However, will this still fire if the user killed the app at 9:00AM and opens it up again at 9:00AM the next day?

I also saw that I could use "NSCalendarDayChangedNotification" which to my understanding has the same behavior. My only concern is that if either of these will accomplish detecting a day change even after the user kills the app.

Ideally I'd like to put add the notification observer at the top of my .m file in the viewDidLoad like so:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(resetValues) name:UIApplicationSignificantTimeChangeNotification object:nil];

and then somewhere else in the file...

- (void)resetValues {

... // reset values to 0

}

The goal would be for this notification to trigger at the beginning of every day change. Would the code above achieve the behavior I'm looking for?

Answer 1

Solution which saves an integer value (the day) in NSUserDefaults

• In AppDelegate create a method checkDayChange which compares the day component of the current date with a saved value in NSUserDefaults (default is 0). If the values are not equal, call resetValues and save the current day.

  - (void)checkDayChange
  {
     NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
     NSInteger currentDay = [[NSCalendar currentCalendar] component:NSCalendarUnitDay fromDate:[NSDate date]];
     NSInteger savedDay = [defaults integerForKey:@"day"]; // default is 0
     if (currentDay != savedDay) {
         [self resetValues];
         [defaults setInteger:currentDay forKey:@"day"];
     }
  }
  

• Observe NSCalendarDayChangedNotification with selector checkDayChange

• Call checkDayChange also in applicationDidFinishLaunching and applicationDidBecomeActive