Objective-C Fundamentals

Important protocols
159
For some applications, examining the structure of the XML they’re receiving will
change the manner in which the application parses. In this case, you say that the XML
contains an element called Author. An Author is defined by a name, age, and gender,
which are simple strings. An author also has a list of Book elements. A Book is defined
by a title, year, and level, which are all simple strings. This analysis ensures that the
NSXMLParser knows what to do.
Most of the time when you parse XML, you’ll be aware of its structure when writing
your parser class and won’t need to investigate the XML feed’s DTD. An example of
this would be the Twitter XML feed for a timeline. You’ll assume you know the XML
structure for your XML and only implement the parsing functions of the NSXMLParser
delegate to parse the Author XML you already looked at.
PARSING AN AUTHOR WITH NSXMLPARSER DELEGATE
The first step when implementing NSXMLParser is to create a class that contains the
parser object and implement its delegate methods. Let’s create a new View-based
project called Parser_Project and a new NSObject subclass called Parser. The only
instance variables you declare for the Parser class are an NSXMLParser and an
NSMutableString to help with some parsing logic you’ll implement. Make the
Parser.h file look like the following:
#import
@interface Parser : NSObject {
NSXMLParser *parser;
NSMutableString *element;
}
@end
You now need an XML file to parse. You can take the XML in listing 7.10 and place it in
a regular text file. Save the file as Sample.xml, and add it to the project. This gives you
a local XML file to parse.
Now you need to fill in Parser.m, which will contain an init method and the
implementation of the three most common NSXMLParser Delegate methods. Let’s
start with the init method; add the code shown in the following listing into Parser.m.
Listing 7.11
Parser.m initializer
- (id)init {
if ((self == [super init])) {
parser = [[NSXMLParser alloc]
initWithContentsOfURL:
[NSURL fileURLWithPath:[[NSBundle mainBundle]