Multilevel cocoa bindings - cocoa

Question

When I bind to a multiple level keypath, say objectValue.person.photo, it does not update when the person changes, only when the photo changes. This would seem to be a problem with only the last key in the path being observed for changes.
Is it possible to observe multiple levels of bindings? For instance, in SproutCore, if you place an asterisk in the path, everything after it will be observed for changes (objectValue*person.photo).

Answer 1

If your bindings are not updating when objectValue.person is changed, then that usually means that whatever object is in objectValue is not Key-Value Observing compliant for the key person. With properly implemented objects, non-leaf mutations along a keyPath work fine. For instance, starting from the base non-document Cocoa Application template, I cooked up the following example:
Header:
#interface Person : NSObject
#property (copy) NSString* name;
#end
#interface Car : NSObject
#property (retain) Person* driver;
#end
#interface SOAppDelegate : NSObject <NSApplicationDelegate>
#property (assign) IBOutlet NSWindow *window;
#property (retain) Car* car;
- (IBAction)replaceCar:(id)sender;
- (IBAction)replaceDriver:(id)sender;
- (IBAction)changeName:(id)sender;
#end
Implementation:
#implementation Person
#synthesize name;
#end
#implementation Car
#synthesize driver;
#end
#implementation SOAppDelegate
#synthesize car = _car;
#synthesize window = _window;
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
Person* person = [[[Person alloc] init] autorelease];
person.name = #"Default Name";
Car* car = [[[Car alloc] init] autorelease];
car.driver = person;
self.car = car;
}
- (IBAction)replaceCar:(id)sender
{
Person* person = [[[Person alloc] init] autorelease];
person.name = #"Replaced Car";
Car* newCar = [[[Car alloc] init] autorelease];
newCar.driver = person;
self.car = newCar;
}
- (IBAction)replaceDriver:(id)sender
{
Person* person = [[[Person alloc] init] autorelease];
person.name = #"Replaced Driver";
self.car.driver = person;
}
- (IBAction)changeName:(id)sender
{
self.car.driver.name = #"Changed Name";
}
#end
Then in the .xib, I added three buttons, calling each of the IBActions and added a label whose value property was bound to App Delegate with a keyPath of car.driver.name
Pushing any of the buttons will cause the bound label to update, despite the fact that only one of them actually modifies the exact value pointed to by the bindings keyPath (car.driver.name). KVO compliance comes for free with standard #synthesized properties, so we get proper updates no matter what level in the keyPath they come from.
In short, bindings work the way you want them to (i.e. they update for changes to non-leaf-node keys in a compound keyPath). There's something in the implementation of the objects in objectValue or person that's deficient and preventing this from working. I would look there.
Also note, in case one of these things is a collection, that observing a collection is not the same thing as observing all the objects in a collection. See this page for more info on that.
PS: Yes, I know the example leaks memory. You can imagine the relevant -dealloc methods for yourself.