TestableUserDefaults.h

#import <Foundation/Foundation.h>

@interface TestableUserDefaults : NSObject {
    NSString  *storage;
}
@property (retain) NSString *storage;

+ (id)defaults;

- (NSArray *)arrayForKey:(NSString *)defaultName;
- (void)setObject:(id)value forKey:(NSString *)defaultName;
- (BOOL)synchronize;

@end

TestableUserDefaults.m

#import "TestableUserDefaults.h"
#import "NSDate+JRExtensions.h"

@interface TestableUserDefaults ()
- (NSMutableDictionary*)plist;
- (void)setPlist:(NSMutableDictionary*)plist;
@end

@implementation TestableUserDefaults
@synthesize storage;

+ (id)defaults {
    return [[[self alloc] init] autorelease];
}

- (void)dealloc {
    [storage release];
    [super dealloc];
}

- (NSString*)description {
    return [NSString stringWithFormat:@"%@<%p>:\n%@", [self className], self, self.storage];
}

- (NSMutableDictionary*)plist {
    NSMutableDictionary *result = nil;
    if (self.storage) {
        NSPropertyListFormat format = NSPropertyListXMLFormat_v1_0;
        NSError *error = nil;
        result = [NSPropertyListSerialization propertyListWithData:[self.storage dataUsingEncoding:NSUTF8StringEncoding]
                                                           options:0
                                                            format:&format
                                                             error:&error];
        if (!result && error) {
            NSLog(@"plist error: %@ %@ %@", error, [error userInfo]);
        }
    }
    return result;
}

- (void)setPlist:(NSMutableDictionary*)plist {
    NSError *error = nil;
    NSData *data = [NSPropertyListSerialization dataWithPropertyList:plist
                                                              format:NSPropertyListXMLFormat_v1_0
                                                             options:0
                                                               error:&error];
    if (!data && error) {
        NSLog(@"setPlist error: %@ %@ %@", error, [error userInfo], plist);
    }
    self.storage = [[[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding] autorelease];
}

- (NSArray *)arrayForKey:(NSString *)defaultName {
    NSMutableDictionary *plist = [self plist];
    NSArray *result = [plist objectForKey:defaultName];
    return result;
}

- (void)setObject:(id)value forKey:(NSString *)defaultName {
    NSMutableDictionary *plist = [self plist];
    if (plist) {
        [plist setObject:value forKey:defaultName];
    } else {
        plist = [NSDictionary dictionaryWithObject:value forKey:defaultName];
    }
    [self setPlist:plist];
}

- (BOOL)synchronize {
    return YES;
}

@end