Manipulating properties in init that are set up by a sub-class in Objective-C

Question

I have an abstract interface in Objective-C where every sub-class needs to set up a property and then do the exact same thing with that property at the end of init. I'm trying to avoid duplicated code with something like this:

Interface File

@interface Shape : NSObject

@property (nonatomic) PropertyType *prop;

- (id)init;
- (void)initProperty;

@end

Implementation File

@implementation Shape

- (id)init
{
    if(self = [super init]) {
        [self initProperty];
        [prop doSomething];
    }
    return self;
}

- (void)initProperty
{
}

@end

My problem is that every sub-class will need a different set of parameters passed to initProperty in order to implement the method correctly:

@implementation Rectangle

- (void)initPropertyWithRect:(CGRect)rect
{
    prop = [RectangleStuff rectangleWithRect:rect];
}

@end

@implementation Circle

- (void)initPropertyWithRadius:(CGFloat)radius
{
    prop = [CircleStuff circleWithRadius:radius];
}

@end

Is there a clean way to do what I'm trying to do in Objective-C? So far, my options seem to be:

1. Create a "property bag", and just pass around an NSDictionary.
2. Duplicate the [property doSomething]; code in every subclass.
3. Somehow pass in a factory object to init, and have the factory object create prop. This approach seems the cleanest, but I'd need the factory object to keep the rect and/or radius as internal state somehow, and that doesn't seem clean to me.

Any thoughts?

Answer 1

I would probably choose #2 (to keep it simple). If the property is only set once (in the subclass init method), you could override the property setter method in the superclass, and do the additional stuff there.

Untested code:

- (void)setProp:(PropertyType *)prop
{
    _prop = prop; // (Assuming ARC)
    [_prop doSomething];
}