CODEWITHSUNDEEP
objective-cinitialization
benjist
benjist

Reputation: 2881

Create a subclass of a class using parent's init - from another class

Edit: This downvoting syndrom here sucks big time. I asked a question where I in my opinion showed that I did my homework, and asked for advice. The upvoted answers implied going with compile time warnings, whereas my own and probably most clean OOP way didn't receive any interest.

Brief overview in order to understand why I need this and what I try to do: I'm writing an ORM that implements the data mapper pattern. A mapper (i.e. for SQLite results) must create subclasses of an entity class - using the initializer of the base entity class. So there is the problem.

The mapper does not, and should not, know about specific classes. Mapping descriptions and specific mappers for different data sources are abstracted away from the entity class, and by design part of the entity description.

Entities are similar to NSManagedObject, though the ORM follows a different pattern. The description any entity is created with is similar to NSEntityDescription (but also following a different pattern and purpose).

So my goal is to create entities that I know are subclasses of ManagedEntity, using the init method of ManagedEntity.

So the init of my mapper looks like this:

- (id)initWithEntityClass:(Class)EntityClass entityDescriptor:(EntityDescription*)entityDescriptor
{
self = [super init];
if (self)
{
    _EntityClass = EntityClass; 
    _entityDescription = entityDescription;

    ... (assert that class is of subclass of ManagedEntity)
}

And some time later in my mapper I then want to create the concrete entity:

-(void)createEntityWithSQLiteResultSet:(sqlite3_stmt*)resultSet
{
    // Problem: How to init a class known to be a subclass of ManagedEntity?  
    ManagedEntity *newEntity = [[_EntityClass] alloc]     initWithEntityDescription:_entityDescription];
}

So how do I create this child class of ManagedEntity, using the init of ManagedEntity?

Sure, I could use respondsToSelector() for initWithEntityDescription and invoke that. But something tells me there should be a more elegant way where the class kind is already known. Also, respondsToSelector and selector invocation will do a runtime check only. Even though the entity initializer should not change, it seems a bad choice to lose compile time checking if this method exists.

Upvotes: 1

Views: 162

Answers (2)

Holly
Holly

Reputation: 5290

As part of your mapping, you must know what subclass you need. Then use 

ManagedEntity *newEntity = [[NSClassFromString(className) alloc] initWithEntityDescription:_entityDescription];

EDIT: I was building out this in a GitHub project as I promised and realized why it may not compile. You must have -initWithEntityDescription: declared in a known class that is accessible within the scope. In this case, it would mean that you must declare and implement ManagedEntity -initWithEntityDescription: and have have `#import "ManagedEntity.h" at the top of your file.

Upvotes: 5

Daij-Djan
Daij-Djan

Reputation: 50089

To reinforce Neal's correct answer that the OP claims cant work because he knows objC :)

#import <CoreData/CoreData.h>

@interface TestMapper : NSObject

- (NSManagedObject*)createClassForEntity:(NSEntityDescription*)entity context:(NSManagedObjectContext*)ctx;

@end

#import "TestMapper.h"

@implementation TestMapper

- (NSDictionary*)entityToClassMap {
    return nil; //TODO ;)
}

- (NSManagedObject*)createClassForEntity:(NSEntityDescription*)entity context:(NSManagedObjectContext*)ctx {
    NSString *className = self.entityToClassMap[entity.name];
    assert(className);

    return [[NSClassFromString(className) alloc] initWithEntity:entity insertIntoManagedObjectContext:ctx];
}

@end

alternative using the runtime you want

id cls = NSClassFromString(className);
id alloced_cls = objc_msgSend(cls, @selector(alloc));
id newEntity = objc_msgSend(alloced_cls, @selector(initWithEntity:insertIntoManagedObjectContext:), entity, ctx);
return newEntity;

Upvotes: 0

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