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KVO: I get called on change, but then can't get the object

Subject: KVO: I get called on change, but then can't get the object
From: Rick Mann <email@hidden>
Date: Sat, 31 May 2008 00:21:09 -0700
Delivered-to: email@hidden
Delivered-to: email@hidden

I registered to observe changes in the "number" column of my tree controller:

	[mItemsController addObserver: self
		forKeyPath: @"selection.number"
		options: (NSKeyValueObservingOptionNew)
		context: NULL];

My -observe: action gets called, but this line

NSLog(@"Value changed %@: %@ (%@)", inKeyPath, inChange, [inObject selection]);


dumps this to the console:

Value changed selection.number: {
    kind = 1;
    new = <null>;
} (<_NSControllerObjectProxy: 0x177cb0>)

Note that "new" is null. I would expect it to be the value of the number.

Furthermore, this line:

	NSLog(@"selection: %@", [inObject valueForKey: @"selection"]);

dumps:

	selection: <_NSControllerObjectProxy: 0x177cb0>

But if I change it to @"selection.number", it throws the exception:

"[<NSTreeController 0x1722e0> valueForUndefinedKey:]: this class is not key value coding-compliant for the key selection.number."

From reading the docs:

"NSObjectController and its subclasses implement the selection method. This method returns a proxy object that represents the receiver’s current selection. The proxy is fully key-value-coding compliant. When you request a key’s value from the selection proxy it returns the value, or a selection marker. Placeholder markers provide additional information about the selection. There are three placeholder markers defined in the NSPlaceholders informal protocol:"

It seems that I should be able to get the number from the selection, but instead I get the exception:

"[<NSTreeController 0x170c60> valueForUndefinedKey:]: this class is not key value coding-compliant for the key selection.number."

I feel like I should be able to get the number. Elsewhere I read that I would get an array of numbers. What am I doing wrong?

TIA,


--
Rick

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