gistfile1.m

@interface A : NSObject
- (NSDictionary *)someDictionaryForSomething;
@end

@interface B : A

@end

@implementation B

// ...

- (NSDictionary *)someDictionaryForSomething {
  static NSDictionary *myBDict = nil;
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{ 
    myBDict = ...;
  });
  return myBDict;
}

// ...

@end