Objective-C Fundamentals

214 CHAPTER 11 Key-Value Coding and NSPredicate
For the matching setValue:forKey: message to work correctly, the object must provide
one of the following messages:
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■
■
A setter method named setKey:
An instance variable named _key or _isKey
An instance variable named key or isKey
By conforming to these conventions, you enable the KVC framework to use Objective-C
class metadata to determine how to interact with your objects at runtime. Although
the framework supports instance variables beginning with an underscore (_) prefix,
they should be avoided because the underscore prefix is typically reserved for internal
use by Apple and the Objective-C runtime.
Most objects are KVC-compliant by default
It’s important to realize that almost all Objective-C objects, including those in Foundation
framework, such as NSString, are by default suitable for use with KVC techniques.
This is because every object ultimately inherits from NSObject, and the
@synthesize compiler feature discussed in chapter 5 produces suitable getter and
setter methods automatically by default. In general, you have to go out of your way
to produce an object that’s incompatible with KVC.
11.1.1 Accessing properties via KVC
Now that you know how to make objects compatible with KVC, let’s look at how the
KVC mechanism can be used to get and update the current value of properties.
To query an object for the current value of one of its properties, you can use the
valueForKey: message. A similar setValue:forKey: message allows you to specify a
new value for a property:
NSLog(@"House address is: %@", [house valueForKey:@"address"]);
[house setValue:@"42 Campbell Street" forKey:@"address"];
Notice the prototype of the valueForKey: method indicates that it accepts an object
(id). This means you can’t pass in values for primitive data types such as int, float, or
BOOL directly. You must box these via an instance of the NSNumber class. The KVC
framework automatically unboxes these values as required, as shown here:
NSNumber *price = [house valueForKey:@"rentalPrice"];
NSLog(@"House rental price is $%f ", [price floatValue]);
[house setValue:[NSNumber numberWithFloat:19.25] forKey:@"rentalPrice"];
As a convenience measure, KVC also provides the dictionaryWithValuesForKeys:
and setValuesForKeysWithDictionary: methods that enable you to pass in multiple
KVC requests at once. In the case of the dictionaryWithValuesForKeys: message,
you pass in an NSArray of property names and get back an NSDictionary of property
name/value pairs.