Does a view reset all its variables every time it opens? - objective-c

Question

I have a int that resets itself every time the view re-opens/leaves. I have tried every way of declaring the int that i can think of, from public, to instance variable to global variable, but it still seems to reset!
#interface MainGameDisplay : UIViewController
extern int theDay;
#implementation MainGameDisplay
- (void)viewDidLoad {
[super viewDidLoad];
NSLog(#"%i", theDay);
}
- (IBAction)returnToHome:(id)sender {
ViewController *new = [[ViewController alloc] initWithNibName:nil bundle:nil];
[self presentViewController: new animated:YES completion:NULL];
NSLog(#"%i", theDay);
}
- (IBAction)theDayAdder:(id)sender {
theDay++;
}
Okay so theDay is a global integer variable. on View load NSLog returns an output of 0. I can then click theDayAdder as many times as I want, and when I click returnToHome, it will tell me what theDay is. When I come back to MainGameDisplay page however, theDay will be reset back to zero, even though it is a global variable?
Output:
0
N (number of times you clicked 'theDayAdder' button)
0

Answer 1

The problem is that you alloc init'ing a new instance of MainGameDisplay every time you go back to it, so of course your global variable will be reset to 0. You need to create a property (typed strong) in ViewController, use that to go back to the same instance each time.
- (IBAction)returnToGameDisplay:(id)sender {
if (! self.mgd) {
self.mgd = [[MainGameDisplay alloc] initWithNibName:nil bundle:nil];
}
[self presentViewController: self.mgd animated:YES completion:NULL];
NSLog(#"%i", theDay);
}
In this example mgd is the property name created in the .h file.

Answer 2

You should know that viewDidLoad() is called when the view is loaded--not when when the view "opens" as you say. You might have a view opened in a retained value and re-opened time and time again and have vieDidLoad() called only once. However, whenever the view becomes visible, then viewWillAppear() is the delegate that is called. So, try outputting your value in viewWillAppear()--instead of viewDidLoad() and call the view appropriately (i.e., have it stick around and not created every time you need it). This will keep the view from being destroyed between calls. The code for your view should look like the following:
#interface MainGameDisplay : UIViewController
extern int theDay;
#implementation MainGameDisplay
- (void)viewDidLoad {
[super viewDidLoad];
}
-(void) viewWillAppear:(BOOL) animated {
[super viewWillAppear:animated];
NSLog(#"%i", theDay);
}
- (IBAction)returnToHome:(id)sender {
ViewController *new = [[ViewController alloc] initWithNibName:nil bundle:nil];
[self presentViewController: new animated:YES completion:NULL];
NSLog(#"%i", theDay);
}
- (IBAction)theDayAdder:(id)sender {
theDay++;
}
The parent of the view (I assume the appDelegate) should do the following
#property (nonatomic, strong) MainGameDisplay *mainGameDisplay = [[MainGameDisplay alloc] initWithNib:#"MainGameDisplay" …]
ViewDidLoad() is called once--after the view is created and loaded. However, viewWillAppear() and other functions triggered by IBAction etc. are called appropriately.

Answer 3

extern variables are meant to be constant. If you expect your MainGameDisplay class to be long-lived, or if theDay is otherwise only supposed to be tied to that class, why not either declare theDay as a property, or, if you only ever need to set it internally in MainGameDisplay, as an ivar.
The other alternative, if you want that value to continue to exist independently of the class instance where it's declared, is to declare it static. A static var will retain its value, even across the lifetime of different instances of the class where it's declared.