On Apr 20, 2008, at 1:13 AM, Jake Carter wrote:
So the AppDelegate will contain an array of Foo object and each Foo
object will have an array of Bar objects. I want to be able to
observe the bars from the AppDelegate to see when new bars are added
or when a property changes in each bar object. I know I can use
addObserver:forKeyPath:options:context: and add it to each key I
want to observe, but is there a better way to do this? Maybe some
KVO magic way to watch a whole tree of objects and their properties?
Hey Jake,
Unfortunately, while KVO and bindings can often seem magical, the need
to observe changes to properties in an array brings you slightly back
to reality, requiring a bit more manual code to be introduced.
In your case, registering AppDelegate as an observer of Foo's array of
Bar objects will allow it to receive notification only when objects
are added and removed from the Bar array. Changes to specific
attributes of your Bar objects, name and number, do not trigger any
notification.
The general approach is to register as an observer of the array itself
first, and when notified about the addition of objects, register to
observe specific properties in each of them. You should also pay
attention to objects removed from the array, and unregister an an
observer of their properties.
Malcolm Crawford has written some excellent documentation about this
topic at <http://homepage.mac.com/mmalc/CocoaExamples/
controllers.html> under the heading "Observing a collection is not the
same as observing the properties of the objects in a collection."
Also be sure to take a look at his "Graphics Bindings" example,
specifically the observeValueForKeyPath:... and
startObservingGraphics: method implementations in GraphicsView.m.
Having your model objects use keyPathsForValuesAffectingValueForKey:
(or setKeys:triggerChangeNotificationsForDependentKey: pre 10.5)
allows for an elegant way to perform this task, simplifying your
controller's registration and observation in cases where you generally
need to respond identically to a group of properties no matter which
specific one was modified.
Hope that helps,
-Murph.
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