Customized initializers and read only properties in Core Data

Question

Before working with Objective-C and Core Data, I had occasions to create classes that needed to be initialized with certain parameters that, after initialization, could not be modified (though they could be read).

With Core Data, I believe I can create a customized init on my NSManagedObject derived class as long as it includes a way to insert the object into a context like such:

-(Cell*) initWithOwner:(CellList*)ownerCellList andLocation:(int)initLocation
{
    if (self = (Cell*) [NSEntityDescription insertNewObjectForEntityForName:@"Cell"
                        inManagedObjectContext:[ownerCellList managedObjectContext]])
    {
        self.location = [NSNumber numberWithInt:initLocation];
        self.owner = ownerCellList;
        [ownerCellList addCellListObject:self];
    }
    return self;
}

Normally, I'd have a location variable and the location property would be read-only (so once set at initialization, it could not be changed). Is there a way to get this sort of pattern with Core Data? Is there a better way I'm not thinking of?

Thanks!

Answer 1

You are correct. As long as your initializer calls the NSManagedObject's designated initializer, your approach is fine. You can also override the -[NSManagedObject awakeFromInsert] to perform some action after insertion (creation) or -[NSManagedObject awakeFromFetch] to perform an action (e.g. populating a cache) each time the object is faulted back into a managed object context.

Like the rest of Objective-C, there is no way to make a property truly readonly. Malicious code will likely be able to modify your property. However, in your custom class, you can declare a @property(readonly) for e.g. location. This will at least cause a warning if you try to modify the property and will signal your intent to client code.