Case-insensitive KVC in Cocoa? [closed]

Question

I'd appreciate some feedback on a particular approach I'm thinking of using. The scenario is below.

I have an object (lets call it MObject) that has a number of properties, say, x and y coordinates, height and width. The properties are named according to the KVC guidelines (MObject.x; MObject.height, etc). My next task, is to read in an XML file that describes this MObject. Unfortunately, the XML elements are named differently -- X and Y, Height and Width (note the capitalization).

Ideally, the XML elements would match up with MObject's properties. In this case, I could use KVC and avoid a whole whack of code:

for (xmlProperty in xmlElement)
{
    [MObject setValue:xmlProperty.value forKey:xmlProperty.name].
}

One way of approaching this would be to make use of case-insensitive keys. Where would I start with that? Are there any other, better solutions?

Suggestions very much appreciated.

Answer 1

Don't override -[NSObject valueForKey:] and -[NSObject setValue:forKey:] if you can at all help it.

Your best bet would be to convert the keys you get from the XML file on the fly. Use a separate method to do the conversion and you can also maintain a cache of names to property keys, so you only need to do each conversion once.

- (NSString *)keyForName:(NSString *)name {
    // _nameToKeyCache is an NSMutableDictionary that caches the key
    // generated for a given name so it's only generated once per name
    NSString *key = [_nameToKeyCache objectForKey:name];
    if (key == nil) {
        // ...generate key...
        [_nameToKeyCache setObject:key forKey:name];
    }
    return key;
}

- (void)foo:xmlElement {
    for (xmlProperty in xmlElement) {
        [myObject setValue:xmlProperty.value forKey:[self keyForName:xmlProperty.name]].
    }
}

Answer 2

You can use NSString's lowercaseString to convert the XML key name to lowercase, if that helps.

Answer 3

Override -valueForUndefinedKey: and -setValue:forUndefinedKey:

If you find a key with a different capitalization use it, otherwise call up to super.

Answer 4

Override -valueForKey: and -setValue:forKey:.

You should probably only accept keys (element/attribute names) you recognize, and call up to super for other keys.

Answer 5

So I implemented Chris Hanson's suggestion and here's what I ended up with. I put this in my Utils class. It keeps a dictionary for each class that we lookup. It could probably use a little refactoring but it has worked very well for me so far.

static NSMutableDictionary *keyCache;

+ (NSString *)keyForClass:(Class)klass column:(NSString *)column {
    if (!keyCache) { keyCache = [NSMutableDictionary dictionary]; }

    NSString *className = NSStringFromClass(klass);

    NSMutableDictionary *tableKeyCache = [keyCache objectForKey:className];

    if (!tableKeyCache) {
        tableKeyCache = [NSMutableDictionary dictionary];

        unsigned int numMethods = 0;
        Method *methods = class_copyMethodList(klass, &numMethods);
        NSMutableArray * selectors = [NSMutableArray array];
        for (int i = 0; i < numMethods; ++i) {
            SEL selector = method_getName(methods[i]);
            [selectors addObject:NSStringFromSelector(selector)];
        }
        [tableKeyCache setValue:selectors forKey:@"allSelectors"];
        free(methods);
        [keyCache setValue:tableKeyCache forKey:className];
    }

    NSString *keyToReturn = [tableKeyCache valueForKey:column];
    if (!keyToReturn) {
        for (NSString *columnKey in [tableKeyCache valueForKey:@"allSelectors"]) {
            if ( [column caseInsensitiveCompare:columnKey] == NSOrderedSame) {
                [tableKeyCache setValue:columnKey forKey:column];
                keyToReturn = columnKey;
                break;
            }
        }
    }

    if (!keyToReturn) { // Taking a guess here...
        NSLog(@"Selector not found for %@: %@ ", className, column);
        keyToReturn = [Utils keyForClass:[klass superclass] column:column];
    }

    return keyToReturn;
}