Can i get a notification, when a new day begins?

Question

In Cocoa, is there a notification i can register for, that informs me, when a new day begins - at 00h:00min:01s in the morning?

Answer 1

There's no notification that I know of. You can get a timer to fire whenever a new day begins like so:

[[NSTimer alloc] initWithFireDate:midnight
                         interval:60 * 60 * 24 //one day, in seconds
                           target:someObj
                         selector:@selector(someSelector)
                         userInfo:nil
                          repeats:YES];

The trick is getting an NSDate set to midnight. Check the Date and Time Programming Guide for how to do that with date components and the like.

EDIT: see this question for how to get the midnight NSDate.

Answer 2

As mentioned by others, in iOS 8 you can use NSCalendarDayChangedNotification. In terms of how to do that, this post gives you more info.

Essentially NSCalendarDayChangedNotification requires that you go to your appDelegate file and insert the below code in app (adapted from guide for Swift 3):

NotificationCenter.default.addObserver(self, selector: #selector(ViewController.dayChangedOperations(notif:)), name:NSNotification.Name.NSCalendarDayChanged, object:nil)

Where "ViewController" is the classname of one of my classes and "dayChangedOperations" is the name of the function that I want to run whenever the day changes.

Count Days Since Last Change

Note that all you get from this is an alert that the day changed. You are not given the number of days since the app last ran. So remember to save the date to a userDefault each time this function runs, so you can use it for comparison later.

Answer 3

If it is for iPhone development, you can also listen for a UIApplicationSignificantTimeChangeNotification. It gets posted on more occasions than the arrival of midnight, but when you receive one, you can simply check if you are on or near midnight.

For Mac OS X, you would have to do what Tom Dalling suggests but you should also keep track of changes to the system clock yourself (in order to update your timer) as well as changes to the current time zone.

Answer 4

As of iOS8, you can also directly listen to NSCalendarDayChangedNotification.