On 12/11/2010, at 8:45 PM, Remco Poelstra wrote:
> If I ommit the valueForKey method, how does the KVC logic now, that it should
> check the properties variable?
> Further more, NSDictionary seems to always return a value, so all keys are
> "defined", they just return nil sometimes. How can valueForUndefinedKey: then
> ever be called?
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.
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.
@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).
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.
--Graham
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