On Nov 12, 2010, at 01:45, Remco Poelstra wrote:
> @interface PropertiesController: NSObject {
> NSMutableDictionary *properties;
> }
>
> -valueForKey:
> -setValue:forKey:
> @end
> @implementation PropertiesController
> valueForKey {
> id retVal=[properties valueForKey:key];
> if (!retVal) {
> //fetch value from network
> //We do not wait for the value
> }
> return retVal;
> }
> @end
> 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?
As as been said several times in this thread, you *shouldn't* override
'valueForKey:', but instead override 'valueForUndefinedKey:' like this:
@implementation PropertiesController
- (id) valueForUndefinedKey: (NSString*) key {
id retVal=[properties objectForKey:key]; // note: NOT 'valueForKey:'
if (!retVal) {
//fetch value from network
//We do not wait for the value
}
return retVal;
}
@end
If you want to know how control reaches this method, read the documentation:
http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/KeyValueCoding/Concepts/SearchImplementation.html%23//apple_ref/doc/uid/20000955-CJBBBFFA
The only restriction here is that none of your dictionary keys should be the
same as existing NSObject property names, because KVC references to those keys
will go to the existing methods instead of 'valueForUndefinedKey:'. For
example, a dictionary key "class" isn't going to work. If there's a chance the
dictionary keys will conflict with NSObject property names, you are going to
have to take steps to ensure the keys are made unique.
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