Weird KVO problem

FROM : Mike R. Manzano
DATE : Sun Nov 25 09:29:22 2007

Sorry for all the KVC/KVO/KVB postings lately, but I'm finding it hard 
to understand their behavior. Here is my latest problem (don't even 
ask me how I got here; most of this code is the result of a desperate 
attempt to figure all this out):

- (void) ping
   {
   NSLog( @"selection:%@", [_blogAccountsController
       valueForKeyPath:@"selection.serviceURL"] ) ;
   }
   
- (void) awakeFromNib
   {
   [_blogAccountsController addObserver:self
       forKeyPath:@"selection.serviceURL"
       options:0 context:nil] ;

//    observeValueForKeyPath:ofObject:change:context: is only ever called
//    if the following two lines are uncommented.
//    [self    performSelector:@selector(ping)
//        withObject:nil afterDelay:0] ;

   }


- (void)observeValueForKeyPath:(NSString *)keyPath
       ofObject:(id)object
       change:(NSDictionary *)change
       context:(void *)context
   {
   NSLog( @"attr at keyPath changed: %@ on object: %@" , keyPath , 
object ) ;
   }

_blogAccountsController is an NSArrayController bound to a Core Data 
context to an entity named "blog" which in turn contains a serviceURL 
attribute.

Without the performSelector:withObject:afterDelay: message in -
awakeFromNib, my observeValueForKeyPath is never called. However, if I 
uncomment those lines, observeValueForKeyPath is called just fine.

What's going on here?

Thanks,

Mike