Question

I need to attach a method to all my UIViewControllers. The method just returns a pointer to my main app delegate class object. However, xcode 4 throws an error “parse issue expected a type” in header file at the declaration of output parameter type MyAppDelegate. If I change it to the other type, for example id, then the error goes away. But I’m using a dot syntax to access main app delegate properties and if I will change the type to id then xcode4 not recognize my main app delegate properties. I have included the definition file of category to those UIViewController class files where I’m accessing this method. Here is definition of my category:

#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>
#import "MyAppDelegate.h"

@interface UIViewController (MyCategory)

-(MyAppDelegate *) appDelegate; // xcode 4 complains about MyAppDelegate type, though it autocompletes it and show even in green color.

@end

Here is an implementation:

#import "MyCategory.h"

@implementation UIViewController (MyCategory) 

-(MyAppDelegate *)appDelegate{
  MyAppDelegate *delegate = (MyAppDelegate *)[[UIApplication sharedApplication] delegate]; 
  return delegate;
}

EDIT: The reason why I’m implementing this category is that I need to have handy shortcut for accessing my main app delegate from any place of the code (in my case from UIViewControler objects):

// handy shortcut :)
self.appDelegate.someMethod;

//not so handy shortcut :(
MyAppDelegate *delegate = (MyAppDelegate *)[[UIApplication sharedApplication] delegate]; 

Answer 1

I think you have a dependency cycle in your header files. If MyAppDelegate.h imports MyCategory.h either directly or indirectly, the first time the category declaration is compiled the compiler won’t know what a MyAppDelegate is. You should remove the import of MyAppDelegate.h from the MyCategory.h header and replace it with a forward class declaration:

#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>

@class MyAppDelegate

@interface UIViewController (MyCategory)

-(MyAppDelegate *) appDelegate; 

@end

Then put the import in the .m file instead. This is actually a good general principle. Where possible, use forward class declarations in the headers and put imports in the implementation file.