Re: KVO one-step listening but two-step notifying?
Re: KVO one-step listening but two-step notifying?
- Subject: Re: KVO one-step listening but two-step notifying?
- From: Daniel Jalkut <email@hidden>
- Date: Fri, 23 Dec 2005 09:23:06 -0500
Thanks! I guess it makes sense now.
Is it worth questioning *which* components of bindings rely on the
old value? Looking at the KVO docs, I could imagine deciding (if I
was implementing all aspects of notifier and observed) for a
particular key that it's "OK" for it not to have an old value. This
would be in exchange for writing observer methods that either don't
subscribe to or disregard the NSKeyValueChangeOldKey.
I suppose once any "pre-fab" Apple bindings are introduced it's risky
business to assume whether they do or don't depend on the "old
value," but for instance I could imagine feeling pretty safe about
binding a single boolean value to a checkbox with disregard for the
"old value". Would even that be foolish?
Daniel
On Dec 23, 2005, at 4:21 AM, mmalcolm crawford wrote:
I'm assuming that *both* in the second essentially means "will/
didChange". If it's OK to do when nothing has happened, how can it
be wrong (dangerous, not just inefficient) to do so some time
after a change has already occurred? Isn't that the same (safety-
wise) as "nothing has happened"?
No, because a change has actually happened in the second case, and
the KVO machinery may need the old value to calculate a delta.
Some parts of bindings *require* the old and new values...
(See <http://developer.apple.com/documentation/Cocoa/Reference/
Foundation/ObjC_classic/Protocols/NSKeyValueObserving.html#//
apple_ref/doc/c_ref/NSKeyValueChangeKindKey>.)
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