在singleton中设置nsmutabledictionary条目?

我正在使用一个singleton来存储应用程序状态信息。我将singleton包含在一个实用程序类中(最终还有其他东西)。该实用程序类依次包含在各种视图控制器等中并从中使用。实用程序类的设置如下:

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

@interface Utilities : NSObject {

}

+ (id)GetAppState;
- (id)GetAppDelegate;

@end

// Utilities.m
#import "Utilities.h"
#import "CHAPPAppDelegate.h"
#import "AppState.h"

@implementation Utilities

CHAPPAppDelegate* GetAppDelegate() {
    return (CHAPPAppDelegate *)[UIApplication sharedApplication].delegate;
}

AppState* GetAppState() {
    return [GetAppDelegate() appState];
}

@end

…AppState Singleton如下所示:

// AppState.h
#import <Foundation/Foundation.h>
@interface AppState : NSObject {
    NSMutableDictionary *challenge;
    NSString  *challengeID;
}
@property (nonatomic, retain) NSMutableDictionary *challenge;
@property (nonatomic, retain) NSString *challengeID;
+ (id)appState;
@end

// AppState.m
#import "AppState.h"

static AppState *neoAppState = nil;

@implementation AppState
@synthesize challengeID;
@synthesize challenge;

# pragma mark Singleton methods
+ (id)appState {
    @synchronized(self) {
        if (neoAppState == nil)
            [[self alloc] init];
    }
    return neoAppState;
}

+ (id)allocWithZone:(NSZone *)zone {
    @synchronized(self) {
        if (neoAppState == nil) {
            neoAppState = [super allocWithZone:zone];
            return neoAppState;
        }
    }
    return nil;
}

- (id)copyWithZone:(NSZone *)zone {
    return self;
}

- (id)retain {
    return self;
}

- (unsigned)retainCount {
    return UINT_MAX; //denotes an object that cannot be released
}

- (void)release {
    // never release
}

- (id)init {
    if (self = [super init]) {
        challengeID = [[NSString alloc] initWithString:@"0"];
        challenge = [NSMutableDictionary dictionary];
    }
    return self;
}

- (void)dealloc {
    // should never be called, but just here for clarity
    [super dealloc];
}
@end

…然后,从一个视图控制器,我可以这样设置singleton的“challengeid”属性:

[GetAppState() setValue:@"wassup" forKey:@"challengeID"];

…但当我尝试设置“挑战”字典条目值时,如下所示:

[[GetAppState() challenge] setObject:@"wassup" forKey:@"wassup"];

…它无法给我一个“已发送无法识别的选择器…”错误。如有任何见解/建议,将不胜感激。