nsdocument subclass variable is somehow reset to nil after a proper initialization: why?

Question

I have a disappearing pointer db; the value is properly set during creation of an NSDocument but at the moment I want to open a sub window, the value has changed to nil! I have the following in an NSDocument subclass:

 @interface MW_Document : NSDocument
 {
     MW_WorkerWindowController *workerController;
     __strong MW_db *db;
 }

 - (IBAction)showWorkerManagementPanel:(id)sender;
 //- (IBAction)showSkillManagementPanel:(id)sender;

The implementation contains this:

 - (void)windowControllerDidLoadNib:(NSWindowController *)aController
 {
     [super windowControllerDidLoadNib:aController];
     if (![self db]) {
         db = [[MW_db alloc] init];
         NSLog ( @"Debug - Init of db: [%ld]", db ); // never mind the casting problem
     }
 }

db points at something other than nil, a true address.

Later on, I want to open a window and have this in the implementation of the same NSDocument subclass:

 - (IBAction)showWorkerManagementWindow:(id)sender
 {
     if ( !workerController) {
         workerController = [[MW_WorkerWindowController alloc] initWithDb:db];
     }
     [workerController showWindow:self];
 }

I put a break point at the first line, and look at the value of db. It is nil, but I have no idea why. Can anyone explain this to me?

Answer 1

You can implement a lazy accessor:

- (MW_db *)db
{
    if (db == nil) {
        db = [[MW_db alloc] init];
    }
    return db;
}

And then use it instead of the ivar:

workerController = [[MW_WorkerWindowController alloc] initWithDb:[self db]];