Question

I’m relatively new to Objective-C (mainly using cocoa/apple foundation framework), but a long-time C++ programmer, so I’ll start by explaining what I’m trying to accomplish; It’s quite possible my approach isn’t the best one, so I’d love to hear any suggestions for a different approach.

I’ve got a text file in json format, just made this quick example:

"section1" : {
    "director" : "Sample Name 1",
    "writers" : {
        "name" : "Example Name 1",
        "name" : "Example Name 2",
    },
},

And I’ve got a class with properties which I want to match with the data inside this file, since I’m planning to parse this file and store some of the values in an instance of this class. My class header would look roughly like this:

@interface SongData : NSObject
@property (nonatomic, copy) NSString *director;
@property (nonatomic, copy) NSArray *writers;
@end

So I’m trying to find the cleanest, somewhat “data-driven” way to store this json data into the correct properties. Basically I want a way to loop through the json “dictionary” of data, and somehow use the “keys” to match with the correct property in my class and assign the value to that property. My first crack at it, I created a dictionary where the “key” is the actual key to lookup the proper data inside the json data, and the “value” would be a pointer to the correct class property. Sort of like this:

NSDictionary *descriptionDictionary = [[NSDictionary alloc] initWithObjectsAndKeys:
                                       director, @"name",
                                       writers, @"writers",
                                       nil];

Then the idea was to loop through the json data (which is stored in another NSDictionary), use each key from my “descriptionDictionary” to lookup the appropriate value in the json data, then use the matching “value” of the description dictionary based on the same key (which I was hoping is sort of a pointer to the actual class property?) and set that properties value to the value from the json data. Perhaps an example will make it a bit more clear :-). Lets assume “jsonDictionary” is the result of parsing my json data, and I’m already inside section1, so there should be a 1-1 match between keys that make up the jsonData NSDictionary and the keys that make up my descriptionDictionary NSDictionary:

[descriptionDictionary enumerateKeysAndObjectsUsingBlock:^(id key, id value, BOOL *stop) {
    value = [jsonDictionary objectForKey:propertyName];
}];

Now “value” would technically be pointing to one of my classes properties, and I want to assign relevant json data to it (ie director = @”Sample Name 1″). I know there’s problems with what I’m doing (I realize that I probably can’t just dereference a pointer to a random class property and assign an arbitrary object of unknown type to it, hoping it all matches up 🙂 but hoping my example at least illustrates what I’m trying to do so someone can tell me a better way.

Answer 1

Have a look at KVC (source: http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/KeyValueCoding/Articles/BasicPrinciples.html)

Setting Attribute Values Using Key-Value Coding

The method setValue:forKey: sets the value of the specified key,
relative to the receiver, to the provided value. The default
implementation of setValue:forKey: automatically unwraps NSValue
objects that represent scalars and structs and assigns them to the
property. See “Scalar and Structure Support” for details on the
wrapping and unwrapping semantics.

If the specified key does not exist, the receiver is sent a
setValue:forUndefinedKey: message. The default implementation of
setValue:forUndefinedKey: raises an NSUndefinedKeyException; however,
subclasses can override this method to handle the request in a custom
manner.

The method setValue:forKeyPath: behaves in a similar fashion, but it
is able to handle a key path as well as a single key.

Finally, setValuesForKeysWithDictionary: sets the properties of the
receiver with the values in the specified dictionary, using the
dictionary keys to identify the properties. The default implementation
invokes setValue:forKey: for each key-value pair, substituting nil for
NSNull objects as required.

One additional issue that you should consider is what happens when an
attempt is made to set a non-object property to a nil value. In this
case, the receiver sends itself a setNilValueForKey: message. The
default implementation of setNilValueForKey: raises an
NSInvalidArgumentException. Your application can override this method
to substitute a default value or a marker value, and then invoke
setValue:forKey: with the new value.

As long as your class properties have the same names as the JSON fields you can use the setValuesForKeysWithDictionary: and pass in the JSON root dictionary.

For keys/properties that are named differently you can simply override the setValue:forUndefinedKey: and set the appropriate property yourself.

Finally there is the case of a value being represented by a different type in the JSON than in the property. For example NSURL would be an NSString instead. Here you can simply check the class of the passed parameter and if it does not match the IVAR, do a conversion.