gistfile1.m

/* MyModelObject.h */
@interface MyModelObject : NSObject

// this data is "publicly" immutable
@property (nonatomic, readonly) NSArray *someData;

@end

gistfile2.m

/* MyModelObject.m */
@interface MyModelObject()

// allow this data to be "privately" mutable
@property (nonatomic, readwrite) NSMutableArray *someData;

// don't expose this method publicly
- (void) process;

@end

@implementation

- (void) process {
  // do some processing steps here
}

@end