decodeObjectForKey: always returns nil

Question

I am trying to save my object graph to a file and then reload it at a later time, however decodeObjectForKey: always returns nil for any key I specify.

A binary file is created and does have the occasional human readable text in it, i.e., titleTextColor so I think the archiving process is working.

Have I miss-understood how NSKeyedArchiver and NSKeyedUnarchiver work? Any help would be appreciated.

- (void)encodeWithCoder:(NSCoder *)encoder {
    [encoder encodeFloat:titleFontSize forKey:@"titleFontSize"];
    [encoder encodeObject:[UIColor orangeColor] forKey:@"titleTextColor"];
    [encoder encodeObject:lineSeparatorColor forKey:@"lineSeparatorColor"];
    [encoder encodeObject:bodyTextColor forKey:@"bodyTextColor"];
    [encoder encodeFloat:bodyTextFontSize forKey:@"bodyTextFontSize"];
    [encoder encodeObject:backgroundColor forKey:@"backgroundColor"];
    [encoder encodeObject:tintColor forKey:@"tintColor"];
    [encoder encodeInteger:bodyTextAlignment forKey:@"bodyTextAlignment"];

    [encoder encodeObject:@"Text" forKey:@"Text"];
}

+ (void) saveToFile {
    // Get the shared instance
    PSYDefaults *sharedInstance = [PSYDefaults sharedInstance];    

    // Serialise the object
    NSData *serializedObject = [NSKeyedArchiver archivedDataWithRootObject:sharedInstance];

    // Get the path and filename of the file
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *pathAndFileName = [documentsDirectory stringByAppendingPathComponent:ksDefaultsFileName];

    // Write the defaults to a file
    if (serializedObject) [serializedObject writeToFile:pathAndFileName atomically:YES];
}

+ (void) loadFromFile {
    // Get the shared instance
    PSYDefaults *sharedInstance = [PSYDefaults sharedInstance];    

    // Get the path and filename of the file
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *pathAndFileName = [documentsDirectory stringByAppendingPathComponent:ksDefaultsFileName];

    NSData *codedData = [[[NSData alloc] initWithContentsOfFile:pathAndFileName] autorelease];
    NSKeyedUnarchiver *defaults = [[NSKeyedUnarchiver alloc] initForReadingWithData:codedData];

    // Set the properties of the shared instance
    NSString *test = [defaults decodeObjectForKey:@"Text"];
    NSLog (@"%@", test);
    sharedInstance.titleTextColor = [defaults decodeObjectForKey:@"titleTextColor"];

    [defaults release];
}

EDIT: Based on advice from DarkDust:

- (id)initWithCoder:(NSCoder *)aDecoder {
    self = [super init];
    if (self) {
        self.titleFontSize = [[aDecoder decodeObjectForKey:@"titleFontSize"] floatValue];
        self.titleTextColor = [aDecoder decodeObjectForKey:@"titleTextColor"];
        self.lineSeparatorColor = [aDecoder decodeObjectForKey:@"lineSeparatorColor"];
        self.bodyTextColor = [aDecoder decodeObjectForKey:@"bodyTextColor"];
        self.bodyTextFontSize = [[aDecoder decodeObjectForKey:@"bodyTextFontSize"] floatValue];
        self.backgroundColor = [aDecoder decodeObjectForKey:@"backgroundColor"];
        self.tintColor = [aDecoder decodeObjectForKey:@"tintColor"];
        self.bodyTextAlignment = [[aDecoder decodeObjectForKey:@"bodyTextAlignment"] intValue];
    }
    return self;
}

and creating a new instance just to test:

+ (void) loadFromFile {
    // Get the shared instance
    PSYDefaults *sharedInstance = [PSYDefaults sharedInstance];    

    // Get the path and filename of the file
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *pathAndFileName = [documentsDirectory stringByAppendingPathComponent:ksDefaultsFileName];

    NSData *codedData = [[[NSData alloc] initWithContentsOfFile:pathAndFileName] autorelease];
    PSYDefaults *newInstance =  [NSKeyedUnarchiver unarchiveObjectWithData:codedData];

    sharedInstance.titleTextColor = newInstance.titleTextColor;
}

EDIT - Update (needed to encode floats and ints as NSNumbers)

- (void)encodeWithCoder:(NSCoder *)encoder {
    [encoder encodeObject:[NSNumber numberWithFloat:titleFontSize] forKey:@"titleFontSize"];
    [encoder encodeObject:titleTextColor forKey:@"titleTextColor"];
    [encoder encodeObject:lineSeparatorColor forKey:@"lineSeparatorColor"];
    [encoder encodeObject:bodyTextColor forKey:@"bodyTextColor"];
    [encoder encodeObject:[NSNumber numberWithFloat:bodyTextFontSize] forKey:@"bodyTextFontSize"];
    [encoder encodeObject:backgroundColor forKey:@"backgroundColor"];
    [encoder encodeObject:tintColor forKey:@"tintColor"];
    [encoder encodeObject:[NSNumber numberWithInt:bodyTextAlignment] forKey:@"bodyTextAlignment"];
}

Answer 1

You serialized using a single root object, but you try to deserialize using a key which doesn't exist at that level.

You want unarchiveObjectWithData:, as in:

NSData *codedData = [[[NSData alloc] initWithContentsOfFile:pathAndFileName] autorelease];
PSYDefaults *decodedDefaults = [NSKeyedUnarchiver unarchiveObjectWithData:codedData];

Note that you'll also need to implement initWithCoder: as counterpart to encodeWithCoder:. Even though it seems you want to have a singleton here, NSCoding demands to create a new object here due to [NSKeyedArchiver archivedDataWithRootObject:sharedInstance].

If you want to en/decode the fields without creating a new instance of PSYDefaults, then you need to use -[NSKeyedArchiver initForWritingWithMutableData:] and pass that archive to a method similar to your encodeWithCoder: (but you should give it a different name then). Then you'd write a counterpart which reads the fields back via a NSKeyedUnarchiver where you do use decodeObjectForKey: and friends for each field.

You also might want to read Apple's Archives and Serializations Programming Guide.