Op 12-11-2010 12:13, Graham Cox schreef:
nil from NSDictionary means no value associated with the key
(dictionaries cannot store 'nil' as a value) so this is your cue to
go fetch. The problem is that the object on which
-valueForUndefinedKey: is invoked is the dictionary, not your
wrapper, and since you are not subclassing the dictionary, you won't
get that message (and the default will simply throw an exception). So
instead, the override to -valueForKey: in your wrapper is acceptable,
in this case. But instead of invoking -valueForKey: on the
dictionary, use -objectForKey: directly. That way, you get nil when
there's no key instead of an exception.
Unfortunatly this does not seem to be true. I use valueForKey on the
dictionary and for non-existing keys, nil is returned. No exception is
thrown. This is not a major problem I think, as I can just test for nil
as with objectForKey.
If you have properties that can legally be nil, you'll need to treat
those differently, using some sort of marker object that means the
property really should return nil (and hence can be stored in a
dictionary). For example [NSNull null] is a valid object which means
'nil'. Basically, you can't use nil itself to mean two different
things, both uninitialized and a legal value, and since NSDictionary
reserves the use of nil to mean undefined, you have no choice but to
deal with that in some way.
Fortunatly, this is not a problem, all my properties are non-nil.
@implementation MyWrapper
- (id) valueForKey:(NSString*) key { id value = [myDictionary
objectForKey:key];
if( value ) return value;
[self beginFetchForKey:key]; // assume starts a thread or
asynchronous task which calls -setValue:forKey: on completion
return nil; // OK, we have no value to return yet. }
- (void) setNilValueForKey:(NSString*) key { [myDictionary
setObject:[NSNull null] forKey:key]; // substitute a valid object for
nil values }
Because this does not override -setValue:forKey: you get all the
usual goodness that gives you, such as KVO notifications. As your
thread sets the value using KVC any observers get notified of the
value having been fetched automatically (if you are using a thread
take care to set the value on the main thread so that all
notifications take place on the main thread).
But if I do not override setValue:forKey: How does the KVC logic now
that it should not try to call setValue:forKey: on my wrapper object,
but on the enclosed dictionary instead? Do I not need to override
setValue:forKey: and call [myDictionary setValue:forKey] there? just as
with valueForKey:?
The override to -valueForKey: here does lose some of the 'special
powers' that Ken alluded to, one of them being the treatment of a
leading '@' in the key. If that matters, you need to emulate what
NSDictionary's implementation does. But since you are really
replacing NSDictionary's version with your own, which has already
discarded the vast majority of NSObject's 'special powers' I can't
see you are losing much in this case. It has the advantage that
subclassing NS(Mutable)Dictionary is not required.
Just out of curiosity, what is the use of the @ operator? I can't find
anything about it in the KVC Programming Guide.
Kind regards,
Remco Poelstra
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