On May 19, 2008, at 2:02 PM, Gordon Apple wrote:
1. What happened when you try to observe a property that doesn't
exist?
(e.g., is there is no shadow, there is no "shadow.angle". Do I
have to test
"hasShadow" to disallow observing "shadow.angle when the properties
list is
scanned?
2. What happens when you try to stop observing a property that
does not
exist (a la Q1)?
I was hoping both of these would simply not do anything if the
path was
not complete (e.g., "shadow.angle"). Unfortunately, it appears to be
blowing me out of the water (unless that is actually happening
elsewhere).
Is KVO supposed to have a benign response to these cases or am I
going to
have to filter them when scanning the properties list?
I believe it invokes the valueForUndefinedKey: method on the object
which is missing the property at that point along the key path. The
default implementation of that method throws an
NSUndefinedKeyException, but you can override it to do something
else. Or, you can catch the exception to learn that the attempt to
observe that key path failed.
See here: http://developer.apple.com/documentation/Cocoa/Conceptual/
KeyValueCoding/Concepts/BasicPrinciples.html#//apple_ref/doc/uid/
20002170-178726
That's discussing KVC rather than KVO specifically, but KVO is
implemented on top of KVC so it still applies.
Do a full-text search for "valueForUndefinedKey" to find more
discussion related to this subject.
By the way, if there are places where it makes sense for your code to
observe or bind to a key path including the "shadow" property on
objects which are not known to be anything more specific than Shapes,
then you might want to move the shadow property up to the Shape class
and have a default implementation which just returns nil.
Cheers,
Ken
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