Assigning RootViewController to UIViewController gives warning - why?

Question

I want to access the RootViewController of my App in one of its classes in order to present a modal view controller. I do this by getting the ApplicationDelegate and asking it for the RootViewController and store it in a UIViewController

AppDelegate *appDelegate = (AppDelegate *) [UIApplication sharedApplication].delegate;
UIViewController* presentingViewController = appDelegate.viewController;

In my opinion this should work without a warning as RootViewController inherits from UIViewController. However I receive this warning:

Incompatible pointer types initializing 'UIViewController *__strong' with an expression of type 'RootViewController *'

Can someone explain to me why I see this warning?

If it helps - this is the AppDelegate where I define the RootViewController:

@interface AppDelegate : NSObject <UIApplicationDelegate> {
    UIWindow            *window;
    RootViewController  *viewController;
}

@property (strong) RootViewController *viewController;

I defined my RootViewController like this:

@interface RootViewController : UIViewController {

}

score 1 · Accepted Answer · answered Nov 20 '11 at 16:58

You can assign an object to a variable declared as its superclass. That is no problem and is very useful when you only want to use superclass methods over a set of your own subclasses, especially common with view controllers in a navigation stack when the specific type of next view controller is unknown.

Also think about it. Methods like

[self presentModalViewController:customController animated:YES];

wouldn't work without being able to do this. This method is declared as taking a UIViewController * but you pass in a custom UIViewController subclass with no complaints. Finally

[rootViewController isKindOfClass:[UIViewController class]];

will return YES. QED.

Have you forward declared your class RootViewController in the header for your app delegate? i.e.

@class RootViewController;
@interface AppDelegate : NSObject <UIApplicationDelegate> {
....

Did you spell it correctly? This is a common area to mistype as xCode doesn't autocomplete forward declarations. It will then autocomplete your typo in the rest of the header file.

Did you remember to import the header file for your RootViewController into the .m file for the AppDelegate? You will still need to do that so the compiler knows about the inheretance.

Your code looks correct at the moment but we don't have all of it.

thank you, I already figured it out, but forgot to update this post. The problem was that it did not use the "correct" RootViewController class, because I did not import it. Because of this it used another class with the same name that existed in the cocos2d library. — gebirgsbärbel, Nov 23 '11 at 12:08

score 0 · Answer 2 · edited May 23 '17 at 12:12

0

The problem is that RootViewController is not the same class as UIViewController.

In the AppDelegate, you declare viewController to be of type RootViewController. Then, in these lines:

AppDelegate *appDelegate = (AppDelegate *) [UIApplication sharedApplication].delegate;
UIViewController* presentingViewController = appDelegate.viewController;

You are creating presentingViewController, which is of type UIViewController, and setting it to an instance of RootViewController. This is the source of the error.

Fix this by using a consistent type.

Read What's the difference between the RootViewController, AppDelegate and the View Controller classes that I may create? for a nice explanation of the difference between these two types.

edited May 23 '17 at 12:12

Community

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answered Sep 21 '11 at 21:47

PengOne

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I know that, however RootViewController inherits from UIViewController, shouldn't that be enough? In Java you can always assign a subclass (lets name it `B` here) of a class to the superclass (`A`) without problems or casts. So `B b = new B(); A a = b;` would work. – gebirgsbärbel Sep 21 '11 at 21:50
@gebirgsbaerbel: Any logic that begins "In Java..." isn't going to get you very far. Obj-C is a very different language. – PengOne Sep 21 '11 at 21:53
Yes I know that the language is very different. But for some time I even saw Objective-C code where people assigned the superclass to the subclass and one did not even get a warning. So I was very surprised that this gives a warning. And I would like to know whether this warning is an issue or whether I can savely ignore it. – gebirgsbärbel Sep 21 '11 at 21:55
You can never safely ignore warnings in Objective-C. What benefit do you think you're going to get by using the wrong type? – jlehr Sep 21 '11 at 22:00
I want to call the method `presentModalViewController` on it. This is a method that exists in UIViewcontroller and should therefore be available for my RootviewController which should inherit it. But somehow it isn't. When I assign the RootViewController to a UIViewController everything works as I want it to. – gebirgsbärbel Sep 21 '11 at 22:15

Assigning RootViewController to UIViewController gives warning - why?

2 Answers2