NSArray from one view controller become NULL when using at viewWillApper method of another view controller?

Question

i have a problem while calling NSArray from another view controller, solution may be simple but as a beginner i am confused.

here is my problem,

i am tryiin to send an array from my firstViewController to SecondViewController and i need to update the array value in to my UILabel which is on SecondViewController.

secondViewController.m

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil  andStationName:(NSArray *)stationName;
// stationName is from firstViewController
    {

// now the stationName have the array values


   NSArray *stationNames=[[stationName mutableCopy]retain];

 //take a copy to stationNames

 }

now i need to update this value to my UILabel

-(void)viewWillAppear:(BOOL)animated
 {
//stationNameDisplay is a uilabel

 stationNameDisplay.text=[stationNames objectAtIndex:0];
  NSLog(@"station name %@",[stationNames objectAtIndex:0]);

 }

but [stationNames objectAtIndex:0] only shows null value, i don't know why?

Answer 1

Don't try to send values through - (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil method or a viewDidLoad because in a tabbar the users will switch between views without unloading the view or reinitializing the view controller.

Declare a variable in the Application appdelegate.h as follows

@interface AppDelegate : NSObject{
 NSMutableArray *stationnamespassed;
}
@property (nonatomic, retain) NSMutableArray *stationnamespassed;
@end

In appdelegate.m

@implementation AppDelegate
@synthesize stationnamespassed;

 -(void) dealloc{
          [stationnamespassed release];
          [super release];
         }
@end

In firstviewcontroller.h declare this

@interface firstviewcontroller {
  NSMutableArray *stationnamestobepassed;
}
@property (nonatomic, retain) NSMutableArray *stationnamestobepassed;
@end

In firstviewcontroller.m declare this

@implementation firstviewcontroller
@synthesize stationnamestobepassed;

-(void) someaction{
   stationnamestobepassed = [NSMutableArray     
     arrayWithObjects:@"string1",@"string2",@"string3"];
    AppDelegate *appDelegate=[[UIApplication sharedApplicaton] delegate];
    appDelegate.stationnamespassed = self.stationnamestobepassed;
}

-(void) dealloc{
  [stationnamestobepassed release];
  [super release];
 }
@end

In secondviewcontroller.m access app delegate variable as follows

@implementation secondviewcontroller

   -(void)viewWillAppear:(BOOL)animated
 {
//stationNameDisplay is a uilabel
AppDelegate *appDelegate=[[UIApplication sharedApplicaton] delegate];
 stationNameDisplay.text=[appDelegate.stationnamespassed objectAtIndex:0];
  NSLog(@"station name %@",[appDelegate.stationnamespassed objectAtIndex:0]);

 }
-(void) dealloc{
      [super release];
     }
@end

Try this way.For more info on different ways of persisting data across different view controllers, Read this article.