Question

i have a DetailViewController and a Messages class(this is a TableViewController class). I parse some web information in the first class and want to use some values amongst them in the second class. As i looked arround in here and google for a few haurs and by now i think i need to define some extern variables in my second class and initialize them with the objects of the first class.. I tried a few ways but all failed.

In my first clas i have an NSMutableArray variable called messID, in the second class i do this:

#import DetailViewController
.
.
 extern NSMutableArray *myArray;
 DetailViewController *myObject;
myArray=myObject.messID;

But i got the error below:

Undefined symbols for architecture i386:
  "_myArray", referenced from:
      -[messages tableView:cellForRowAtIndexPath:] in messages.o

What am i doing wrong and what can i do please can any one help?..

EDIT
I’ve imported needed class,
in the .h file of second class i use this:

@interface messages : UITableViewController{

       BNT_1DetailViewController *myObject;
}
@property(retain, nonatomic)BNT_1DetailViewController *myObject;

And its .m file is:

…

@sysnthesize myObject;


     viewDidLoad{
     myObject=[[BNT_1DetailViewController alloc]init];}
    - (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
    {
    #warning Incomplete method implementation.
        // Return the number of rows in the section.
        return myObject.mesID.count;
        }

    - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
    {
        static NSString *CellIdentifier = @"Cell";
        //extern NSMutableArray *messID;


        UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
        if (cell == nil) {
            cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
        }NSLog(@" -> %@",myObject.mesID );
           cell.textLabel.text= [myObject.mesID objectAtIndex:indexPath.row];    
            // Configure the cell...
       // [tableView reloadData]; 
        return cell;

        }

Answer 1

I would not use extern for what you are trying to do, which means you are also defining some global variables (and trying to access them from the files where you use the extern keyword). For the records, anyway, the error you are getting depends on the fact that extern is just a directive that does not define your objects — it simply declares them as defined elsewhere. So you should add in some .m file the proper definition for your objects, without forgetting to also initialize them at some point in time so that they point to meaningful objects.

A better way to do what you are trying to do is through public properties declared in your first class allowing access to the NSMutableArray from the second class.

An example of this could be:

@interface DetailViewController : ... {

    NSMutableArray* array;
}
@property (...) NSMutableArray* array;
@end

@interface Messages : ... {
    DetailViewController* detailViewController;
}
...
@end

@implementation Messages;
...
... detailViewController.array...
...
@end

This solution is better because it does not use global variables and has a better encapsulation.

Actually, you could try and define a Model (like in model-view-controller) which contains all of your data and have them shared among all the controllers that need it. This would be an better approach.

EDIT: on how to connect one controller to another…

Say that at some point you create a new controller:

 messageDetail *detailViewController = [[messageDetail alloc] initWithNibName:@"messageDetail" bundle:nil];

Now, detailViewController needs retrieving its data from messages, which is by the way the controller which is creating it. You have several options for doing that. An easy one is having a public property in messageDetail that links to messages:

    @interface messageDetail : ... {
        ....
    }
    @property (nonatomic, retain) messages* messagesRef;
    ...
    @end

(don’t forget to synthesize this property in messageDetail implementation).

Then, when you instantiate messageDetail, you simply do this:

 messageDetail *detailViewController = [[messageDetail alloc] initWithNibName:@"messageDetail" bundle:nil];
 detailViewController.messagesRef = self;

Once you do this, your detailViewController will have a pointer, correctly initialized, to point to the other controller.

If your other controller exposes (like I explained above) a property with the NSArray, your are done.