On Feb 15, 2011, at 05:11, Remco Poelstra wrote:
> I've a "PresetsController" which holds a dictionary containing preset
> settings for my application. The presets contain trees (Mutable Dictionaries)
> of keys.
> To save the GUI code from bothering with tracking the current preset, I want
> to give my PresetController the option to replace a keyPath like
> @"current.parameter.subparameter.value" to
> @"preset2.parameter.subparameter.value".
> I implemented it with a valueForUndefinedKey:
> - (id) valueForUndefinedKey:(NSString *)key {
> if ([key isEqual:@"current"])
> return [presets valueForKey:currentPreset]; //presets is a
> NSMutableDicitonary, currentPreset a NSString
> else
> return [presets valueForKey:key];
> }
>
> , returning the dictionary belonging to the current preset. This works for
> setting and reading using keyPaths. It does not work for observing a keyPath
> like @"current.parameter.subparameter.value". How should I implement that?
I think you're making this too hard. If you need to observe a key path that
includes the "current" key, then just define a [derived] "current" property for
the presets controller (assuming that's the class that has the above code). The
getter would look like this:
- (NSDictionary*) current {
return [presets valueForKey:currentPreset];
}
The only thing you have to do is make sure that "current" is KVO compliant,
which means that notifications need to be sent out whenever the underlying
value changes:
[self willChangeValueForKey: @"current"];
currentPreset = ...
[self didChangeValueForKey: @"current"];
wherever you change the current preset. There are other possible variations,
depending on what's most convenient with your class:
1. Obviously you could vary this by keeping a pointer to the current preset
dictionary in an instance variable too.
2. If "currentPreset" is itself a KVO compliant property, you don't need to
generate KVO notifications manually (the second code fragment). Instead, you'd
use:
+ (NSSet*) keyPathsForValuesAffectingCurrent {
return [NSSet setWithObject: @"currentPreset"];
}
and write:
self.currentPreset = ...
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