Re: Curious about copyWithZone message from NSArrayController in "GC required" project

FROM : Bill Bumgarner
DATE : Sun Jan 13 06:31:32 2008

On Jan 12, 2008, at 8:03 PM, Luke Evans wrote:
> I have a "gc required" project with some entities in a CoreData 
> model, each with requisite subclasses of NSManagedObject.
> In particular, I have an entity/class 'A' that declares a property 
> returning an NSArray (which actually returns an ordered list of 
> entity/class 'B' that is defined as a to-many relationship on the 
> entity A).
> Now, in my UI, I have a couple of NSArrayControllers.  One of these 
> obtains the 'A's from the managed object context, and provides 
> instances to UI that in turn uses an NSArrayController bound to the 
> property of 'A', such that all the 'B's of a given 'A' are returned 
> in the correct order.
>
> Now, all this is now working correctly (as far as I can tell), but 
> before I got it to work as expected, I had to fix a problem logged 
> in the console as:
> *** -[B copyWithZone:]: unrecognized selector sent to instance 
> 0x127f310
>
> The obvious fix of implementing a 'dumb' copyWithZone in the B class 
> (that


First and foremost, -copyWithZone: has absolutely nothing to do with 
the differences between GC and non-GC.  As its name implies, it is all 
about copying an object, something that must be done in either GC or 
non-GC.

> returns B's 'self' ) got past the problem, but it raises a number of 
> questions in my Cocoa n00b's mind:
> 1. Why am I being asked to create a copy of B instances anyway?  I 
> would have thought that NSArrayController could work quite happily 
> with a reference to my original B's - at least in the GC context. 
> Indeed, forcing this to happen with a trivial implementation of 
> copyWithZone seems to work happily.


If you are ever causing a -copyWithZone: to be sent to an 
NSManagedObject or subclass therein, then there is something seriously 
wrong with your code.

NSManagedObjects cannot be copied via -copyWithZone: for a reason;  it 
doesn't make sense to do so without taking the model into 
consideration -- without fully traversing the relationships and 
considering exactly how the copy should be done within the bounds of 
the managed object model.

So -- first question -- what is triggering the -copyWithZone: being 
sent to an instance of B in the first place?

b.bum