Static variable in superclass - objective-c

Question

I have two classes which inherit from the same class. Each class has a corresponding JSON file with the the same name as the class. To avoid loading the JSON every time an instance is created, I added a class method and static variable:
static NSArray *map = nil;
+(NSArray *)map {
if (!map) {
map = [NSJSONSerialization JSONObjectWithData:
[NSData dataWithContentsOfFile:
[[NSBundle mainBundle] pathForResource:NSStringFromClass([self class])
ofType:#"json"]]
options:0
error:nil];
}
return map;
}
I added this method (literally copied and pasted) to both subclasses.
I'd like to move this up to the superclass, however if I do then the static variable will be shared between instances of both subclasses, and only the JSON map corresponding to the class that has an instance created first will be load and all subsequent instances of the other class will be returned the wrong map.
So how can I load the corresponding JSON file only once for each subclass and each subclass has its own map? (Ideally without copying and pasting code as I have)

Answer 1

In the base class keep a static NSMutableDictionary. Use as a key the name of the class (ie with NSStringFromClass(childClass)).
#interface BaseClass : NSObject
+(NSArray*)map;
#end
#interface OneChild : BaseClass
#end
#interface TwoChild : BaseClass
#end
#implementation BaseClass
+(NSArray*)map
{
static NSMutableDictionary *_mapStore;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
_mapStore = [[NSMutableDictionary alloc]init];
});
NSString *name = NSStringFromClass([self class]);
NSArray *map = [_mapStore objectForKey:name];
if(map == nil)
{
map = [NSJSONSerialization JSONObjectWithData:[NSData dataWithContentsOfFile:[[NSBundle mainBundle] pathForResource:name ofType:#"json"]] options:0 error:nil];
[_mapStore setObject:map forKey:name];
}
return map;
}
#end
#implementation OneChild
#end
#implementation TwoChild
#end

Answer 2

IMO, the cleanest way to do this is to declare the static variable in each subclass, like you already are. It is duplication of the code that loads the map, but the map needs to be different for each class scope so I don't find that to be too much of an inconvenience.
If you really want to put the loading logic and storage in the superclass, make the static variable a dictionary rather than just a array, like this:
static NSMutableDictionary *maps = nil;
+(NSArray *)map {
if (!maps) {
maps = [[NSMutableDictionary alloc] initWithCapacity:2];
}
if (![maps objectForKey:[self class]]) {
[maps setObject:[NSJSONSerialization JSONObjectWithData:[NSData dataWithContentsOfFile:[[NSBundle mainBundle] pathForResource:NSStringFromClass([self class]) ofType:#"json"]] options:0 error:nil] forKey:[self class]];
}
return [maps objectForKey:[self class]];
}