Objective-C Fundamentals

Making the Rental Manager application data driven
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To store a nil value in an array or dictionary, you can use the null factory method
located on the NSNull class:
NSNull myValue = [NSNull null];
[myArray addObject:myValue];
An instance of the NSNull class is an object, so it can be stored in an NSArray or
NSDictionary instance or used anywhere else an object is expected. Because an
NSNull instance represents the absence of a value, the class doesn’t provide any way in
which to extract a value, so when it comes time to pull your NSNull value out of an
array, you can compare it against itself:
id value = [myArray objectAtIndex:5];
if (value == [NSNull null]) {
NSLog(@"The 6th element within the array was empty");
} else {
NSLog("@The 6th element within the array had the value %@", value);
}
Unlike the problems you had comparing NSString instances with the == operator,
this technique works for NSNull instances. The statement [NSNull null] doesn’t create
a new object each time it’s called. Instead, [NSNull null] always returns the
memory location of a sole instance of the NSNull class. This means that two variables
that are storing a pointer to an NSNull value will have an identical address, and
hence the == operator will consider them the same. This is an example of the Singleton
design pattern.
This completes our coverage of the basics of Foundation Kit’s core classes and the
data structure implementations they provide. With your newfound knowledge, you
can also resolve the issue with the Rental Manager application highlighted at the end
of chapter 3.
4.4 Making the Rental Manager application data driven
One outstanding problem with the Rental Manager application is that the mapping
of city names to geographical locations is hardcoded, when what you ideally want
is for it to be data driven and easily updateable without the need to recompile the
entire application.
These concepts nicely match the functionality provided by an NSDictionary object
(with the key being the city name and the value being its matching location type) that
obtains its initial contents from a plist file.
Let’s add a new plist file called CityMappings.plist to your project. As you may
expect, the New File dialog (located in File > New > New File… and shown in figure 4.1)
has a suitable template to get you started. Once the new file is added to your project,
Xcode presents a graphical plist file editor that quickly allows you to edit its contents
without worrying about balancing XML open and closing brackets, and so on. Using
this editor, populate your new plist file with the contents shown in figure 4.2.