instance variable in NSManagedObject class acts like a class variable?

Question

IOS 5 only, with ARC. In my Core Data model class:

//  Planet.h //

#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>

@interface Planet : NSManagedObject

@property (nonatomic, retain) NSString *name;
@property (nonatomic, retain) NSNumber *diameter_km;
@property (nonatomic, retain) NSNumber *mass_kg;

-(void) setVisited:(BOOL)flag;
-(BOOL) isVisited;

@end

// Planet.m //
 
//#import "Planet.h"

@implementation Planet
 
@dynamic name;
@dynamic diameter_km;
@dynamic mass_kg;
 
BOOL visitedByHumans;       // not a core data entity; just an ivar
 
-(void)setVisited:(BOOL)flag {
    visitedByHumans = flag;
}
-(BOOL)isVisited {
    return visitedByHumans;
}
 
@end

I use MagicalRecord to create "Venus" and "Mars". In my view controller, I use labels and buttons to test the above. Testing shows that when I "visit" Mars, Venus also becomes visited. If I switch the ivar visitedByHumans into a non-Core-Data property, it works as expected. So I'm no longer 'stuck', but I want to understand the ivar thing.

Answer 1

vistedByHumans is not actually an ivar, but a global variable of your subclass Planet. So, any and every "planet" instance will appear to be visited regardless of which instance is actually visited. If you want to make it an actual ivar, you need to add a @property to your @interface much like name, diameter_km, and mass_kg (although, those three of course were generated for your models). e.g.:

@property (nonatomic,assign,getter=isVisited) BOOL visited;

and in your implementation:

@synthesize visited=visitedByHumans;

or just

@synthensize visited;

Since you appear to be using those methods (visited and setVisited:) anyhow, and not really directly accessing visitedByHumans.

Also, be sure to remove the line of code

BOOL visitedByHumans;

and the two method definitions isVisited and setVisited:. They will be generated for you when you @synthesize'd them.