Can transformable attributes in Core Data entities be pointers to c structs? My transformer is not being called

Question

I'm trying to make a CoreData-persisted entity. This entity has a few properties, one of which is a non-standard attribute, state. The state attribute is a pointer to a C struct with a few properties in it. Here's what my data model looks like:

Entities:

MDInstance

Attributes:

• duration: Integer 16
• moves: Integer 16
• name: String
• state: Transformable. I set the Transformer to MDStateTransformer.

I generated my class and edited the state property. This is what the interface looks like:

#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>
#import "MDState.h" // this is where the State structure is defined

@interface MDInstance : NSManagedObject

@property (nonatomic, retain) NSNumber * duration;
@property (nonatomic, retain) NSNumber * moves;
@property (nonatomic, retain) NSString * name;
@property (nonatomic) State *state; // note that this is a pointer to a struct

@end

This is what the implementation looks like:

#import "MDInstance.h"

@implementation MDGameInstance

@dynamic duration;
@dynamic moves;
@dynamic name;

@synthesize state;

@end

I created a value transformer called MDStateTransformer. The implementation of this is (probably) not important. Suffice it to say I have allowsReverseTransformation returning YES, transformedValueClass returning [NSValue class], and I've implemented transformedValue: and reverseTransformedValue:

Lastly, I registered the MDStateTransformer in my AppDelegate's application:didFinishLaunchingWithOptions: like this:

MDStateTransformer *transformer = [[MDStateTransformer alloc] init];
[NSValueTransformer setValueTransformer:transformer forName:@"MDStateTransformer"];

If I create a new instance of my MDInstance, set it's attribute -including the state attribute- and then try to save the entity, my transformer is never called.

I put a stop point in my transformer's init method and it is being instantiated. I put another in transformedValue: and it the function is not being called.

However, if I update my MDInstance so that the state attribute is not a pointer, but is simply a State structure, and I update the transformer to work with a struct and not a pointer, the transformedValue: is called.

Is it possible to have a custom attribute on an object which is a pointer to a c struct? If so, any ideas what I'm doing wrong?

Answer 1

Your transformable attribute also needs to be specified as @dynamic and not @synthesize in your implementation file. By synthesizing that property you provide an getter and setter outside of core data and thus the Core Data provided accessors will not be used.