Re: How to make KVO setup persistent?

FROM : Jon Hull
DATE : Fri Jul 28 22:54:58 2006

It looks like your problem is not with KVO, but with retain/release 
(Memory management).  Making sure that you have proper accessors (and 
always use them) will go a long way towards fixing your problem. 
myAge also needs to be an instance variable of the Person class. 
Right now, it is a local variable which is going away when 
awakeFromFetch
ends, though it is now being transfered to a class variable before 
that for some reason(???).  I suggest dumping the class variable and 
using an instance variable w/ accessors.

in the Person class:

-(void)setAge:(Age*)newAge
{
   if(myAge != newAge){
       [myAge autorelease];
       myAge = [newAge copy];
   }
}

-(Age*)age
{
   return myAge;
}

This will also give you KVC/KVO compliance!  I suspect that this is 
part of why your KVO wasn't working... How is supposed to know the 
value has changed if it isn't changed in a KVO compliant way?

It also sounds like you might just want to use a dependent key rather 
than KVO in this situation (but I don't know your exact situation).

BTW, you can also use [self setAge:nil]  in the dealloc method... you 
should never call dealloc directly (it will only lead to headaches).

Thanks,
Jon

On Jul 28, 2006, at 6:13 AM, Arthur C. wrote:

> Finally I have a working solution. It sets up KVO again after 
> application restart. It is as follows:
>
> @implementation Person
> NSMutableArray *myAgeArray;
>
> +(void) initialize
> {
> myAgeArray = [[NSMutableArray arrayWithCapacity: 5] retain]; // or 
> any number
> }
>
> -(void) awakeFromFetch
> {
> // first, fetch the 'Age' object with index equal to [self index] 
> using NSFetchRequest
>
>      Age * myAge = [[Age alloc] init];
>      myAge = [fetchResults objectAtIndex: 0]; // only one object 
> gets fetched
>      [myAge index]; // to get rid of the 'fault' state (!)
>      [myAgeArray addObject: myAge];
>
>      [self addObserver: [myAgeArray lastObject] 
> forKeyPath:@"yearofbirth" options: NSKeyValueObservingOptionNew 
> context:managedObjectContext];
> }
>
> -(void) dealloc
> {
>  [myAgeArray dealloc];
>  [super dealloc];
> }
>
> The point is that the fetched 'Age' needs to be retained, or else 
> it will get autoreleased and dealloced. The global array takes care 
> of that. The solution appears to work OK, but maybe there is a more 
> efficient way?
>
> I believe this problem is covered in the KVO programming Guide, 
> where it (only) says:
> "Note: The key-value observing 
> addObserver:forKeyPath:options:context method does not retain the 
> observing object or the observed objects. You need to review your 
> application's requirements and manage retain and release for the 
> observing, and observed objects."
>
> This looks not particularly enlightening to me, so I will file a 
> documentation enhancement request (in particular a piece of sample 
> code would do well).
>
>
> Thanks for your time,
>
> Arthur C.
>
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