Question

I have a superclass of UIViewController – MasterViewController which declares a property called itemsViewController. This declares a method called from the MasterViewController, and is wired up via a storyboard in IB.

I have a subclass of MasterViewController which redeclares this property as a specific iPad version, but I can’t access the redeclared property from the parent class.

MasterViewController

@interface MasterViewController : UIViewController {
}

@property (nonatomic, strong) IBOutlet ItemsViewController *itemsViewController;
@end

@implementation MasterViewController
@synthesize itemsViewController;

-(void)viewDidLoad {
// I can access itemsViewController in viewDidLoad.

}
@end 

MasterViewController_iPad

@interface MasterViewController_iPad : MasterViewController {
    IBOutlet ItemsViewController_iPad *_itemsViewController;
}

@property (nonatomic, strong) IBOutlet ItemsViewController_iPad *itemsViewController;
@end

@implementation MasterViewController_iPad
@synthesize itemsViewController = _itemsViewController;

-(void)viewDidLoad {
    [super viewDidLoad];
// when I call super viewDidLoad,  itemsViewController is nil, as though the property hasn't been overriden
// _itemsViewController is not nil in viewDidLoad.
}
@end 

Am I misunderstanding the way property inheritance works in Objective-C?

Answer 1

You can’t change the type signature of a method when you override a superclass method.

MasterViewController has these methods:

(void)setItemsViewController:(ItemsViewController *)foo
(ItemsViewController *)itemsViewController

But you’re trying to give MasterViewController_iPad these methods:

(void)setItemsViewController:(ItemsViewController_iPad *)foo
(ItemsViewController_iPad *)itemsViewController

Which you can’t do: you can’t overload the same method name but have different types for the arguments.

If ItemsViewController_iPad is a subclass of ItemsViewController, a quick solution would be to keep the same signature as in MasterViewController but simply use an ItemsViewController_iPad when you set the property.