Try overriding +[NSObject(NSKeyValueObserving)
automaticallyNotifiesObserversForKey:] to return NO for the properties you want
to handle manually.
On 2012-01-16, at 01:30, Gideon King wrote:
> Hi all, when I declare a property, something like:
>
> @property(readwrite, copy, nonatomic) NSString *foo;
>
> I will synthesize it with:
>
> @synthesize foo;
>
> But then I want to do some special processing when the value is set, so I
> implement my setter:
>
> - (void)setFoo:(NSString *)aFoo {
> [self willChangeValueForKey:@"foo"];
> [foo autorelease];
> foo = [aFoo retain];
> [self didChangeValueForKey:@"foo"];
> [self
> doSomeMoreProcessingThatShouldHappenAfterAllTheObserversKnowTheNewValue];
> }
>
> I assumed from the documentation and examples that since I was providing my
> own implementation method, that I would have to include the willChange and
> didChange methods at appropriate points in my code, but I have just
> discovered that when I do:
>
> anObject.foo = @"something";
>
> I get two prior notifications - one from the internals of the system, and one
> from my willChange call, and two notifications after the change - one from my
> didChange and one from the system.
>
> So I thought this must because I used @synthesize as well as providing my own
> method, so I implemented a getter and removed the @synthesize. The behavior
> didn't change.
>
> I double checked this by removing my willChange and didChange methods, and
> now the prior and after notifications only got sent once.
>
> One purpose of mentioning this is that given the fact that I have come across
> code from several places where they did what I did and had the willChange and
> didChange code in their setters, others may need to look back at their code
> and make appropriate changes.
>
> But the big thing for me is that I have a number of places throughout my
> application where I need to have the changes made, the didChange notification
> acted on by the observers, and then some other code run.
>
> I therefore wanted to use the setter, but not have to change the name of it,
> so that bindings would still work, so I changed my property declaration to be
> readonly, and declared a setFoo: method. I thought that declaring it would
> make it not call the willChange and didChange behind the scenes, but
> unfortunately, even with a readonly declaration, when I used anObject.foo =
> @"something", it still called the prior and after KVO methods. So I thought I
> would explicitly call setFoo: instead of using dot notation, but that made no
> difference. On the off chance there was something else going on, I also
> turned off accessesInstanceVariablesDirectly, but that made no difference.
>
> So I thought I would just remove the property declarations altogether, and
> just code my accessors myself, but it still called the built in KVO methods.
>
> Therefore my conclusion is that as soon as you add a KVO observation on
> anything, it triggers the prior and after notifications whenever the setter
> is called. This of course makes sense, but still leaves me with the original
> problem: I need some way to have the KVO notifications called *once only*,
> and yet have my setters able to call other methods after the KVO
> notifications have been sent. It is not possible for this to be done with
> performSelector:withObject:afterDelay: or anything similar, since these
> methods need to be called immediately after the after change notification.
>
> I know I could get it to call the KVO notifications at only the right places
> by coding my own methods like -(void)changeFooTo:, but I do not want to
> change the name of my setters because that would break my bindings.
>
> Are there any recommendations on the best approach for being able to have the
> setter able to do what it needs with the KVO and then calling other methods,
> without breaking bindings?
>
> I can't believe I have misunderstood this for so long, and never noticed what
> was happening until now!
>
> Thanks
>
> Gideon
>
>
>
>
>
>
>
>
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