How do get reference to another class properly?

Question

I have some class initialized in Appdelegate, but when I get this class instance form Appdelegate in another class it has "fresh" state.

I have following in AppDelegate:

Interface:

@property (nonatomic, retain) DataController *dataController;

Implementation:

@synthesize dataController;
- (id)init {
    if (self = [super init]) {
        DataController *controller = [[DataController alloc] init];
        self.dataController = controller;
        [controller release];
        NSLog(@"items: %d",[self.dataController numberOfItems]);
    }
    return self;
}

At this point DataControlelr class loads objects form database. Log output show "items: 10".

I have TableViewController where I need to use DataController.

TableViewController header:

@interface TableViewController : UITableViewController {
    DataController *dataController;
}

@property (retain) DataController *dataController;
@end

Implementation:

-(id)init{
    if (self =[super init]) {
        DataController *dc  =[(AppDelegate *)[[UIApplication sharedApplication] delegate] dataController];
        [dc retain];
        dataController = dc;
        NSLog(@"items: %d",[self.dataController numberOfItems]);
    }
    return self;
}

Here it always says that DataController has 0 items. "fresh" state.

The Log output is always

items: 10

items: 0

It seems like assigning that class creates reference to freshly initialised DataController somehow?

How do I reference another class properly?

Thanks.

Answer 1

The first thing to check would be to ensure that the dc variable in the second class isn't nil-- that would cause any method called on it to 'return' 0.

It might also be useful to print out the address of the app delegate from both of those methods-- just in case the -init method is resulting from an incorrectly-allocated second instance of that class somewhere, while the regular version hasn't been initialized in the same way (or was using -initWithCoder:, etc.)

One useful rule of thumb for initialization of objects created or assigned within a nib file is to use -awakeFromNib to perform most of your initialization tasks. A corollary to this is that the app delegate can set up its state in response to the -applicationDidFinishLaunching: method. In this case, if there is a second instance of your AppDelegate class being allocated somewhere, only the one which is really set as the app's delegate will receive -applicationDidFinishLaunching:.

At the end of the day, stepping through in the debugger and looking at the call stack should show you if something isn't happening in quite the way it should.

Answer 2

Could there be an issue with your assignment of dataController = dc in TableViewController? In your log statement you use self.dataController, should your assignment directly above it be self.dataController = dc ?

Answer 3

I found the gotcha. Tanks to Jim!

Moved assignment from -init to -awakefromnib and now DataController is valid.

My mistake is that after putting the code initially in -viewDidLoad and -viewWillAppear which was wrong I thought that in -init is the place for the assignment.