Objective-C Fundamentals

218 CHAPTER 11 Key-Value Coding and NSPredicate
}
}
[super setValue:value forUndefinedKey:key];
- (id)valueForUndefinedKey:(NSString *)key {
if ([key isEqualToString:@"advert"]) {
return [NSString stringWithFormat:@"$%f, %@",
rentalPrice, address];
} else {
[super valueForUndefinedKey:key];
}
}
An important point to consider about custom setValue:forUndefinedKey: and value-
ForUndefinedKey: message implementations is that, internally, they should access setter
methods or properties rather than their backing ivars directly. If you access or
update an instance variable directly, Key-Value Observing won’t always correctly detect
changes in a key’s value and hence won’t notify interested observers. In listing 11.1,
therefore, it’s important that you refer to rentalPrice and address as self.rental-
Price and self.address because, instead of updating the instance variable, this syntax
(or the alternative [self rentalPrice], [self address]) will invoke the matching
setter methods giving KVO a chance to observe the change.
With the additional method implementations in listing 11.1, the CTRentalProperty
object now appears to have an additional property called advert that returns a string
in the format "$price, address"—for example, "$175, 46 Coleburn Street". Following
is an example of how this property would be accessed:
NSLog(@"Rental price is: %@", [house valueForKey:@"rentalPrice"]);
NSLog(@"Address is: %@", [house valueForKey:@"address"]);
NSString *advert = [house valueForKey:@"advert"];
NSLog(@"Advert for rental property is '%@'", advert);
This is not a very realistic implementation of the setValue:forUndefinedKey: and
valueForUndefinedKey: messages. It would have perhaps been easier, more efficient,
and cleaner to add a physical property named advert to the CTRentalProperty class. A
more realistic implementation could use something like an NSMutableDictionary to
enable a CTRentalProperty object to store arbitrary metadata about a rental property.
11.2.2 Handling nil values
A similar problem can occur when a nil value is passed to setValue:forKey:. As an
example, what happens if you execute the following statement?
[house setValue:nil forKey:@"rentalPrice"];
Because the rentalPrice property of the CTRentalProperty class is a primitive datatype
(float), it can’t store the value nil. In this case, the default behavior is for the KVC
infrastructure to throw an exception, as follows:
[ setNilValueForKey]: Could not set nil as the
value for the key rentalPrice.