Question

The code:

UIImagePickerController *picker = [[UIImagePickerController alloc] init];
picker.delegate = self;
picker.sourceType = UIImagePickerControllerSourceTypeCamera;
[self.navigationController presentModalViewController:picker animated:YES];
[picker release];

The warning, which shows up on the picker.delegate = self; line:

Class 'CardEditor' does not implement the 'UINavigationControllerDelegate' protocol

Why does the UIImagePickerController care if my class implements the UINavigationControllerDelegate protocol?

Answer 1

UIImagePickerController inherits from UINavigationController. The delegate property is part of UINavigationController, so it requires that its delegate conforms to the UINavigationControllerDelegate protocol.

The delegate property is declared like so:

@property(nonatomic, assign) id<UINavigationControllerDelegate> delegate;

This tells the compiler to verify that your controller will implement the UINavigationConrollerDelegate protocol. The check is in place so that a compile-time warning can be generated if your controller class does not properly implement all of the methods that the UINavigationController might send to its delegate.

If you’re looking for a solution, you can indicate (in your controller class interface) that your controller is compliant with that protocol:

@interface MyController : NSObject <UINavigationControllerDelegate>
    ...
@end

Once this is done, the compiler will warn you about any required delegate methods that you haven’t implemented. This is a good thing, as it will prevent you from releasing code that might accidentally throw a “method not found” runtime error.