Check if a View has loaded since midnight - ios

Question

I have a ViewController that performs a random shuffle on an array and spits out text to a Label (in the viewDidLoad method). The problem is that whenever I navigate to the same ViewController it performs the shuffle again and I only need it to shuffle ONCE every day.
So I need to check if this ViewController has been loaded before on the SAME day (i.e. since midnight) and I can then put the shuffle into an if statement. Could I schedule the shuffle at midnight regardless of the app being open or not?
I have looked into setting a boolean to NSUserDefaults: something like hasLoadedSinceMidnight but then can't work out how to RESET the boolean at midnight.

Answer 1

You could implement the AppDelegate's significantTimeChange method:
-(void)applicationSignificantTimeChange:(UIApplication *)application {
//tell your view to shuffle
}
This method is called every midnight and during significant time changes such as a time zone change. If your app is closed when the event is received, this method will be called when your app is next opened.
More information can be viewed here
An additional way to do the same thing inside of your ViewController instead of in the AppDelegate would be to add:
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(performAction) name:UIApplicationSignificantTimeChangeNotification object:nil];
and then you can just perform your shuffle operation in the -(void)performAction; method of that ViewController.

Answer 2

Instead of storing a BOOL, you could store the date/time (NSDate) of the last shuffle.
Then check whether or not it has passed midnight since the last shuffle by comparing the stored date and the current date in the viewDidAppear.
See NSTime Documentation: http://developer.apple.com/library/ios/#documentation/Cocoa/Reference/Foundation/Classes/NSDate_Class/Reference/Reference.html
And NSDateFormatter Documentation:
https://developer.apple.com/library/ios/#documentation/Cocoa/Reference/Foundation/Classes/NSDateFormatter_Class/Reference/Reference.html#//apple_ref/doc/uid/TP40003643
UPDATE:
As requested, here is some sample code. Admittedly, there very well may be better solutions to this, but I believe this code snippet will help you get you past your problem. What this does is check if there is a date saved using NSUserDefaults, then compare with the current date. If the dates do not match, shuffle the array, then save the current date (again using NSUserDefaults). (I've taken the liberty of making the assumption that time will indeed continue to move forward, so it doesn't check to make sure that lastSavedDate is before the currentDate.)
NSDate *currentDate = [[NSDate alloc] init];
NSDate *lastShuffleDate = [[NSUserDefaults standardUserDefaults] objectForKey:#"lastShuffleDate"];
// check to see if there is a prior shuffle date
// if there is not, shuffle the array and save the current date
if (!lastShuffleDate) {
NSLog(#"No object set for 'lastShuffleDate'");
//[self shuffleMyArray];
[[NSUserDefaults standardUserDefaults] setObject:currentDate forKey:#"lastShuffleDate"];
return;
}
// set up the date formatter
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
NSLocale *usLocale = [[NSLocale alloc] initWithLocaleIdentifier:#"en_US"];
[dateFormatter setLocale:usLocale];
[dateFormatter setDateStyle:NSDateFormatterShortStyle];
NSLog(#"Current Date: %#", [dateFormatter stringFromDate:currentDate]);
NSLog(#"Saved Date: %#", [dateFormatter stringFromDate:lastShuffleDate]);
// check to see if the dates are the same by comparing the dates as a string
if (![[dateFormatter stringFromDate:currentDate] isEqualToString:[dateFormatter stringFromDate:lastShuffleDate]]) {
NSLog(#"Dates are different...!");
//[self shuffleMyArray];
} else {
NSLog(#"Dates are the same... (midnight has not passed)");
}
// save the time of the last shuffle
[[NSUserDefaults standardUserDefaults] setObject:currentDate forKey:#"lastShuffleDate"];
At this point, there is no real reason for you to check the times, but I've included it in case you were curious.
// remote dateStyle and set timeStyle to check times
[dateFormatter setDateStyle:NSDateFormatterNoStyle];
[dateFormatter setTimeStyle:NSDateFormatterShortStyle];
NSLog(#"Current Time: %#", [dateFormatter stringFromDate:currentDate]);
NSLog(#"Saved Time: %#", [dateFormatter stringFromDate:lastShuffleDate]);