Question

I have some NSManagedObject classes created for use with CoreData I need to add some additional properties for formatting I am doing using GRMustache templates.

Here is an example property:

-(NSString *) PriceFormatted {
    NSNumberFormatter *nfm = [[[NSNumberFormatter alloc] init] autorelease];
    [nfm setNumberStyle:NSNumberFormatterCurrencyStyle];
    [nfm setCurrencyCode:[Helpers GetCurrencyCode]];
    [nfm setNegativeFormat:@"-¤#,##0.00"];
    [nfm setMaximumFractionDigits:2];

    return [nfm stringFromNumber:self.Price];
}

I currently have this in my generated NSManagedObject class but this will cause issues if I need to regenerate a new NSManagedObject class.

Can I define these properties in a secondary set of classes – similar to partials in C#?

Answer 1

Probably the easiest way is to add a category to your generated managed object.

Here is Apple’s documentation on it, it is pretty easy.

To quote:

You can add methods to a class by declaring them in an interface file
under a category name and defining them in an implementation file
under the same name. The category name indicates that the methods are
additions to a class declared elsewhere, not a new class. You cannot,
however, use a category to add additional instance variables to a
class.

The methods the category adds become part of the class type. For
example, methods added to the NSArray class in a category are included
as methods the compiler expects an NSArray instance to have in its
repertoire. Methods added to the NSArray class in a subclass, however,
are not included in the NSArray type. (This matters only for
statically typed objects because static typing is the only way the
compiler can know an object’s class.)

Category methods can do anything that methods defined in the class
proper can do. At runtime, there’s no difference. The methods the
category adds to the class are inherited by all the class’s
subclasses, just like other methods.

The declaration of a category interface looks very much like a class
interface declaration—except the category name is listed within
parentheses after the class name and the superclass isn’t mentioned.
Unless its methods don’t access any instance variables of the class,
the category must import the interface file for the class it extends:

    #import "ClassName.h"   

    @interface ClassName ( CategoryName ) 
        // method declarations 
    @end 

Note that a category can’t declare additional
instance variables for the class; it includes only methods. However,
all instance variables within the scope of the class are also within
the scope of the category. That includes all instance variables
declared by the class, even ones declared @private.

There’s no limit to the number of categories that you can add to a
class, but each category name must be different, and each should
declare and define a different set of methods.