Question

I am using a iOS library called Restkit, which automatically maps remote objects to local objects. You do that by creating a mapping using KVC pattern, and then loading with the mapping.

While I understand how it works, I don’t understand why some parts are implemented in certain ways. For example it uses a singleton pattern where you create an instance of RKObjectMapping like below:

RKObjectMapping* articleMapping = [RKObjectMapping mappingForClass:[Article class]];
[articleMapping mapKeyPath:@"title" toAttribute:@"title"];
[articleMapping mapKeyPath:@"body" toAttribute:@"body"];
[articleMapping mapKeyPath:@"author" toAttribute:@"author"];
[articleMapping mapKeyPath:@"publication_date" toAttribute:@"publicationDate"];

However, when it comes time to actually use it, you access the manager by calling sharedManager of the class RKObjectManager, like following:

[[RKObjectManager sharedManager].mappingProvider setMapping:articleMapping
                                                 forKeyPath:@"articles"];

[[RKObjectManager sharedManager] loadObjectsAtResourcePath:@"/articles"
                                                  delegate:self];

What I don’t understand is, why even bother instantiating the class as articleMapping, when you will just access it using a class method/variable like sharedManager? Why not do something like:

[RKObjectMapping initWithClass:[Article class]];

I am a newbie so think there would be a good reason for taking this pattern that I am not aware of. Anyone know or understand why this would be implemented in this way?

Answer 1

When an object needs to access data associated to itself, it uses instance variables (ivars) to store that data. However, instance variables are part of a specific instance of the class and not the class itself – from a class method, they cannot be accessed. So, when the manager class you’re talking about was to store/access its private data, it uses a shared instance (singleton) to create instance variables. If it was used only as a class, it had to store its private data in global variables, which is considered bad practice in C.

Also, if you look closely, the RKObjectManager class has properties. In Objective-C, properties are always backed by an underlying instance variable (be it created at runtime, dynamically, or at compile time, in a declared manner) – that’s why classes can’t have properties, and that’s why a singleton (an actual instance of a class) is needed.

Example without singleton (ugly):

static int uglyGlobalVar;

@interface UglyClass: NSObject

+ (void)storeData:(int)data;

@end

@implementation UglyClass: NSObject

+ (void)storeData:(int)data
{
    uglyGlobalVar = data;
}

@end

Example with singleton (much better):

@interface GoodClass: NSObject {
    int data;
}

+ (GoodClass *)sharedInstance;

@property (nonatomic, assign) int data;

@end

@implementation GoodClass: NSObject

@syntehsize data;

// also implement sharedInstance here

@end

// then this class is usable as:
[GoodClass sharedInstance].data = 1337;