Objective-C Fundamentals

Filtering and matching with predicates
219
In some cases, this is less than desirable. It’s possible to override how nil values (typically
used to represent the absence of a value) are handled. In the example of
CTRentalProperty’s rentalPrice property, a more sensible approach may be to set
the rentalPrice value to 0 whenever nil is received. You can do so by overriding
another message called setNilValueForKey:, as demonstrated here:
- (void)setNilValueForKey:(NSString *)key {
if ([key isEqualToString:@"rentalPrice"]) {
rentalPrice = 0;
} else {
[super setNilValueForKey:key];
}
}
With this implementation of setNilValueForKey:, attempting to use KVC to set the
rentalPrice property to nil instead causes the property to be updated to the value 0.
11.3 Filtering and matching with predicates
Now that you know how to reference and access object properties via string-based
key paths, it may not surprise you that a layer of other technologies is built on top of
this foundation.
Often, when provided with an object, you’ll want to compare it against a certain
condition and perform different actions based on the result of the comparison. Similarly,
if you have a collection of data, such as an NSArray or NSSet, you may want to use
a specific set of criteria to filter, or select, subsets of the data.
These forms of expressions are commonly called predicate conditions and are represented
in the Foundation framework by the NSPredicate class, which internally uses
KVC, and key paths in particular.
11.3.1 Evaluating a predicate
Suppose you want to quickly determine if a CTRentalProperty object’s rental price
is greater than $500. Mathematically, this can be expressed by the following predicate
expression:
rentalPrice > 500
You can programmatically perform this check with an Objective-C statement such as
the following:
BOOL conditionMet = (house.rentalPrice > 500);
The condition is hardcoded into the source code and can’t easily change during runtime
or in response to user input. In such a case, the NSPredicate class decouples
things in a manner similar to what KVC does for getter and setter operations. The
example condition could also be evaluated by the following code snippet, which uses
the NSPredicate class:
NSPredicate *predicate =
[NSPredicate predicateWithFormat:@"rentalPrice > 500"];
BOOL conditionMet = [predicate evaluateWithObject:house];