i recently updated my IDE to XCode 4.0 and saw a strange change in the Utillity-Application boiler-plate-code:

First, the MainViewController.h-File:

#import <UIKit/UIKit.h>
@class MainViewController;

@interface UtilityAppDelegate : NSObject <UIApplicationDelegate> {
}

@property (nonatomic, retain) IBOutlet UIWindow *window;
@property (nonatomic, retain) IBOutlet MainViewController *mainViewControlle开发者_Go百科r;
@end

Question 1: Where is "mainViewController" declared in the first place? I didn't find it anywhere.

In the *.m-File there is a

@synthesize mainViewController=_mainViewController;

statement. So my second question: Where is "_mainViewController" hidden? Can't find a declaration anywhere. It comes somehow out of the main *.nib file I guess.

But there is another problem: I did add a UINavigationController to one of my recent projects and have no need for mainViewController anymore. But when I delete @property and @synthesize out of MainViewController.m/.h , I can't run the app anymore because of this exception:

setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key mainViewController.'

occurring at this line

    int retVal = UIApplicationMain(argc, argv, nil, nil);

in the main.m.

Thx for your help.

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Look at you info.plist it's should be declared there. If you created your app with a template it's configured by the plist. There is some implicit declaration done by this, the mainWindow.xib(in my example) contains more than a window. There are the connections to the appDelegate and the viewController, see second screenshot.

The last line of the screenshot:

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Question 1:

When you use a declaration like this, you don't also need to explicitly define the property.

.h

@property (nonatomic, retain) IBOutlet MainViewController *mainViewController;

.m

@synthesize mainViewController=_mainViewController;

Question 2:

_mainViewController is not hidden. It points to mainViewController which is implicitly defined in the @property statement in .h

@synthesize mainViewController=_mainViewController;

This format is used to distinguish between the ivar and other properties. It refers to mainViewController.

Question 3:

You deleted the declarations @property/@synthesize for mainViewController, but it still exists in the nib file (IB). Delete it from IB and you should be good to go.

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You're encountering the new ABI for the first time. It is no longer necessary to actually declare variables for properties. If you use @property and @synthesize, a backing ivar will automatically be generated for you.

You're probably getting the KVC error because the NIB still references the old property. You should see a warning about this during compile. In IB, look at your App Delegate; it probably still has an outlet for mainViewController, and you probably are still generating a MainViewController. You need to delete them from the NIB.