Open Radar

 
27-Mar-2012 01:54 PM Gregory Combs:
Summary:

I'd like to see an expansion of the Core Data framework that allows us to add a transient property's underlying code via a block assigned on an NSPropertyDescription, in order to provide an easy means to use transient properties without subclassing the managed object.

I create my Core Data model, entities, attributes, and relationships at runtime using a JSON model description.  However, in the event that I want to use an NSFetchedResultsController with sections based on (for example) a User object's lastName *initial* letter.  Right now, since I don't subclass the objects at design-time, if I want to use sections in the FRC I have to include the actual letter value in a separate "sectionBy" property, then make sure to change it's value whenever someone changes the User's lastName.

However, whenever I'm building the NSEntityDescription, it would be far cleaner if I merely needed to create an NSAttributeDescription (call it "sectionBy") and then set it up like follows:

NSAttributeDescription *transient = [[NSAttributeDescription alloc] init];
[transient setName:@"sectionBy"];
[transient setTransient:YES];
[transient setAttributeType:NSUndefinedAttributeType];
[transient setAttributeValueClassName:@"NSString"];

// The Good Stuff
[transient setTransientValueWithBlock:(id (^)(void)){
    NSString *input = [self valueForKey:@"lastName"];
    if (!input || !input.length)
        return nil;
    return [input substringToIndex:1];
}];


https://gist.github.com/grgcombs/2219225