Best way to implement Enums with Core Data

Question

What is the best way to bind Core Data entities to enum values so that I am able to assign a type property to the entity? In other words, I have an entity called Item with an itemType property that I want to be bound to an enum, what is the best way of going about this.

Solution 1

You'll have to create custom accessors if you want to restrict the values to an enum. So, first you'd declare an enum, like so:

typedef enum {
    kPaymentFrequencyOneOff = 0,
    kPaymentFrequencyYearly = 1,
    kPaymentFrequencyMonthly = 2,
    kPaymentFrequencyWeekly = 3
} PaymentFrequency;

Then, declare getters and setters for your property. It's a bad idea to override the existing ones, since the standard accessors expect an NSNumber object rather than a scalar type, and you'll run into trouble if anything in the bindings or KVO systems try and access your value.

- (PaymentFrequency)itemTypeRaw {
    return (PaymentFrequency)[[self itemType] intValue];
}

- (void)setItemTypeRaw:(PaymentFrequency)type {
    [self setItemType:[NSNumber numberWithInt:type]];
}

Finally, you should implement + keyPathsForValuesAffecting<Key> so you get KVO notifications for itemTypeRaw when itemType changes.

+ (NSSet *)keyPathsForValuesAffectingItemTypeRaw {
    return [NSSet setWithObject:@"itemType"];
}

Solution 2

You can do this way, way simpler:

typedef enum Types_e : int16_t {
    TypeA = 0,
    TypeB = 1,
} Types_t;

@property (nonatomic) Types_t itemType;

And in your model, set itemType to be a 16 bit number. All done. No additional code needed. Just put in your usual

@dynamic itemType;

If you're using Xcode to create your NSManagedObject subclass, make sure that the "use scalar properties for primitive data types" setting is checked.

Solution 3

An alternative approach I'm considering is not to declare an enum at all, but to instead declare the values as category methods on NSNumber.

Solution 4

If you're using mogenerator, have a look at this: https://github.com/rentzsch/mogenerator/wiki/Using-enums-as-types. You can have an Integer 16 attribute called itemType, with a attributeValueScalarType value of Item in the user info. Then, in the user info for your entity, set additionalHeaderFileName to the name of the header that the Item enum is defined in. When generating your header files, mogenerator will automatically make the property have the Item type.

Solution 5

I set the attribute type as 16 bit integer then use this:

#import <CoreData/CoreData.h>

enum {
    LDDirtyTypeRecord = 0,
    LDDirtyTypeAttachment
};
typedef int16_t LDDirtyType;

enum {
    LDDirtyActionInsert = 0,
    LDDirtyActionDelete
};
typedef int16_t LDDirtyAction;


@interface LDDirty : NSManagedObject

@property (nonatomic, strong) NSString* identifier;
@property (nonatomic) LDDirtyType type;
@property (nonatomic) LDDirtyAction action;

@end

...

#import "LDDirty.h"

@implementation LDDirty

@dynamic identifier;
@dynamic type;
@dynamic action;

@end

View more solutions

Comments

[Michael Gaylord]

Interesting. It definitely seems doable.

[TheLearner]

brilliant idea! so much easier than creating tables in the db, unless your db is filled from a web service then its probably best to use a db table!

[Constantino Tsarouhas]

Thank you — too bad Core Data doesn't support this natively. I mean: Xcode generates class files, why not enums?

[port5432]

Here's an example: renovatioboy.wordpress.com/2011/10/06/…

[Anonymous White]

The last code is if you want to observe item itemTypeRaw. However, you can simply observe item itemType instead of itemTypeRaw right?

[Daniel Eggert]

With Xcode 4.5 you don't need any of this. Take a look at my answer. You just need to define the enum as an int16_t and you're set.

[ArtOfWarfare]

Are there any limits with which iOS / OS X versions this will work with?

[Daniel Eggert]

It’s been working for as long as I remember, but I can't make any promises. I remember using this on iOS 5, but I don't know why it wouldn't work with iOS 4...

[Ivan Vučica]

Note, this is a C++11 feature called "enum classes". cprogramming.com/c++11/…

[Daniel Eggert]

No, this has nothing to do with C++11. It's part of clang 3.3 supporting Enumerations with a fixed underlying type for ObjC. C.f. clang.llvm.org/docs/…

[Rob]

How do you avoid losing this code every time you regenerate the model class? I have been using Categories so that the core domain entities can be regenerated.

[Jeff]

Since this doesn't have the retain keyword, will it still get stored in the database?

[Daniel Eggert]

The retain is related to memory management, not whether it gets stored into the database or not.

[Kyle Redfearn]

I agree with Rob. I don't want this to have to be regenerated over and over again. I prefer the category.

[malhal]

Sure looks like C++ with all the underscores

[DonnaLea]

@Rob To avoid the enum being overwritten every time the model class is regenerated, I wrote my typedef in a separate .h header file. So only two things need changing when the file is regenerated, the property type to the enum and re-including the header file with the enum. Not perfect, but the best I could come up with if following this pattern.

[DonnaLea]

I like it. I'm going to use this approach in my project. I like that I can also contain all my other meta information about the meta data within the NSNumber category. (i.e. linking strings to the enum values)

[Gregarious]

Really great idea! Very useful for associating string identifiers, using directly in JSON, Core Data, etc.

[tapmonkey]

@Rob Categories is a way to do it, but instead you could also use mogenerator: github.com/rentzsch/mogenerator. Mogenerator will generate 2 classes per entity, where one class will always be overwritten on data model changes and the other subclasses that class for custom stuff and never gets overwritten.

[malhal]

I'm getting a bad access with this

[Wizard of Kneup]

How can you do this in Swift? If I am correct then this "trick" is not reproducible in Swift. See as well my question here: stackoverflow.com/questions/27656219/…

[Daniel Eggert]

Swift enum is something very, very different from a C / ObjC enum. I'm not sure if there's any good way to fudge it…

[p0lAris]

I have been using this in my code but this throws me errors every time: CoreData: error: Property <> is a scalar type on class <> that does not match its Entity's property's scalar type. Dynamically generated accessors do not support implicit type coercion. Cannot generate a setter method for it. I just want to use the accepted answer after trying so much.

[Gagan Joshi]

@Daniel Eggert , i try to use this but i am getting an error, Declaration of 'int16_t" must be imported from module . Can you help me on that. Many thanks in advance

[Daniel Eggert]

In ObjC int16_t is already defined if you import Foundation or CoreData. Are you using Swift? You need to make sure that your scalar type in the entity definition matches the int16_t -> 16 bit.

[malhal]

I was using this technique until I discovered a major problem that doing it this way prevents changedValues from containing the key.

[LolaRun]

@p0lAris I've been having the same error , because i thought i was doing the same implementation, but there's one thing you and I didn't notice at first. "typedef enum Types_e : int16_t {" this line, you probably have something like that instead "typedef enum {"

[malhal]

Any chance you could edit Types_e and Types_t to something clearer? thanks

[robmathers]

If you're getting BAD_ACCESS errors or similar, be sure that the proper type (i.e. int16_t) is set in your model file. If you're using Xcode to create your NSManagedObject subclass, make sure that the "use scalar properties for primitive data types" setting is checked.

[trapper]

@malhal is right! this seems to mess up CoreData change notifications