unknown type name in objective c - objective-c

Question

I'm pretty new to objective c, and having some basic problems.
I wrote a simple program using a navigator, and everything worked fine.
then I added few lines of code (can't even remember what exactly, and it seems to have no connection to the problem) and the problem occurred. I tried ctrl+z, and the problem remained:
I run the program and get these errors:
1. unknown type name "mainController"
2. property with 'retain (or strong)' attribute must be of object type
while mainController is the first screen to be loaded.
This is the appDelegate.h file:
#import <UIKit/UIKit.h>
#import "mainController.h"
#import "WishesList.h"
#import "Wish.h"
#interface AppDelegate : UIResponder <UIApplicationDelegate>
#property (strong, nonatomic) UIWindow *window;
#property (strong, nonatomic) IBOutlet UINavigationController *navController;
#property (strong, nonatomic) IBOutlet mainController *viewController; // this line creates the errors
#property (strong, nonatomic) WishesList *WishesArray;
#property (strong, nonatomic) NSIndexPath *temp;
#end
this is the relevant part of the appDelegate.m file:
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions: (NSDictionary *)launchOptions
{
self.window = [[UIWindow alloc]initWithFrame:[[UIScreen mainScreen]bounds]];
WishesArray = [[WishesList alloc]init];
temp = nil;
[self setViewController:[[mainController alloc]init]];
[self setNavController:[[UINavigationController alloc]initWithRootViewController:self.viewController]];
[self.window setRootViewController:navController];
[self.window makeKeyAndVisible];
return YES;
}
And this is mainController.h:
#import <UIKit/UIKit.h>
#import "addWishController.h"
#import "displayWish.h"
#import "WishesList.h"
#import "Wish.h"
#interface mainController : UIViewController
#property (nonatomic, weak) WishesList *list;
#property (nonatomic, strong) IBOutlet UITableView *wishTable;
- (void)addWish;
#end
it already worked...
can you figure it out?
thanks

Answer 1

I figured out, that the same error appears if you have an import-cycle:
Class_A.h: #import "Class_B.h"
Class_B.h: #import "Class_A.h"
To fix: look for any imports of the offending class (the error tab is your friend, expand the relevant error for a list of imports). Remove #import's accordingly

Answer 2

This problem happen to me once.
I was importing the "APPDelegate.h" in my h file and in my APPDelegate.h I was importing the file too (it shouldn't be a problem but...)
What I did: I changed the Import from my own .h to .m and it worked :)

Answer 3

As others have mentioned, this is indeed caused by cyclic imports. To fix this remove the imports in one of the classes. But sometimes this is not sufficient. If the classes depend on each other, simply forward-declare the one class in the other:
Class A:
#import <UIKit/UIKit.h>
#class B; //<- this is essential here
#interface A: NSObject
#property(nonatomic, strong) B *b;
//...
In class B we have:
#import "A.h"
#interface B: NSObject
#property(nonatomic, strong) A *a;

Answer 4

#JustAStranger and #NathanielSymer, both are correct!
Anyway, worth remember that this case, below, has the same problem too:
Class_A.h: #import "Class_B.h"
Class_B.h: #import "Class_C.h"
Class_C.h: #import "Class_A.h"
This problem reveal to us how important is take care about owners at our Class relationships. Is very easy creates cycle problems using ObjC headers.

Answer 5

Check the target and the files it is compiling. Perhaps mainController has some how been removed from that target. If so, when building, you would get the message that it cannot be found.

Answer 6

This problem looks like a typo because class names usually start with an uppercase character. Therefore, mainController could/should be MainController. Check the class name to see if the error is indeed a typo, because the compiler is telling you it cannot find any class called mainController.