<?xml version="1.0" encoding="UTF-8"?>
<Books>
<Book id="1">
<title>Circumference</title>
<author>Nicholas Nicastro</author>
<summary>Eratosthenes and the Ancient</summary>
</Book>
<Book id="2">
<title>Copernicus Secret</title>
<author>Jack Repcheck</author>
<summary>How the scientific revolution began</summary>
</Book>
<Book id="3">
<title>Angels and Demons</title>
<author>Dan Brown</author>
<summary>Robert Langdon is summoned to a Swiss</summary>
</Book>
</Books>
从其文档结构我们可以看出，要定义一个Book实体类描述具体的书籍信息，其中用于存储的相关xml文档元素的实例变量与对应元素同名（本例：title、author、summary），另外还有个bookID实例变量存储Book元素标签的属性id值。具体Book实体类的定义如下：
// Book.h
#import <UIKit/UIKit.h>
@interface Book:NSObject {
NSInteger bookID;
NSString *title;
NSString *author;
NSString *summary;
} @property (nonatomic, readwrite) NSInteger bookID;
@property (nonatomic, retain) NSString *title;
@property (nonatomic, retain) NSString *author;
@property (nonatomic, retain) NSString *summary; @end //Book.m
#import "Book.h"
@implementation Book
@synthesize title,author,summary,bookID; - (void)dealloc {
[summary release];
[author release];
[title release];
[super dealloc];
}
@end
由于例子中的xml文档结构表明会有很多书籍，即Book实体类的实例对象会有很多。所以我们要在应用程序中声明一个可变数组来存储这些信息，如下代码表明：在应用程序委托类中声明了一个可变数组books。
// AppDelegate.h
@interface AppDelegate : NSObject <UIApplicationDelegate> {
//默认缺省的实例变量 NSMutableArray *books;
}
//默认缺省的属性定义 @property (nonatomic, retain) NSMutableArray *books;
@end
为了代码的清晰，我们可以再声明一个代理类XMLParser，在当NSXMLParser实例对象指定委托时会用到。
// XMLParser.h
#import <UIKit/UIKit.h> @class AppDelegate, Book;
@interface XMLParser : NSObject {
NSMutableString *currentElementValue; //用于存储元素标签的值
AppDelegate *appDelegate;
Book *aBook; //书籍实例
} - (XMLParser *)initXMLParser; //构造器
@end
// XMLParser.m
- (XMLParser *)initXMLParser {
[super init];
appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
return self;
}
这整个XMLParser类的实现代码如下：
//XMLParser.m
#import "XMLParser.h"
#import "XMLAppDelegate.h"
#import "Book.h" @implementation XMLParser - (XMLParser *) initXMLParser { [super init]; appDelegate = (XMLAppDelegate *)[[UIApplication sharedApplication] delegate]; return self;
} - (void)parser:(NSXMLParser *)parser didStartElement:(NSString *)elementName
namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qualifiedName
attributes:(NSDictionary *)attributeDict { if([elementName isEqualToString:@"Books"]) {
//Initialize the array.
//在这里初始化用于存储最终解析结果的数组变量,我们是在当遇到Books根元素时才开始初始化，有关此初始化过程也可以在parserDidStartDocument 方法中实现
appDelegate.books = [[NSMutableArray alloc] init];
}
else if([elementName isEqualToString:@"Book"]) { //Initialize the book.
//当碰到Book元素时，初始化用于存储Book信息的实例对象aBook
aBook = [[Book alloc] init]; //Extract the attribute here.
//从attributeDict字典中读取Book元素的属性
aBook.bookID = [[attributeDict objectForKey:@"id"] integerValue]; NSLog(@"Reading id value :%i", aBook.bookID);
} NSLog(@"Processing Element: %@", elementName);
}
/* 可以看出parser:didStartElement:namespaceURI:qualifiedName:attributes方法实现的就是在解析元素开始标签时，进行一些初始化流程 */ - (void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string {
// 当用于存储当前元素的值是空时，则先用值进行初始化赋值
// 否则就直接追加信息
if(!currentElementValue)
currentElementValue = [[NSMutableString alloc] initWithString:string];
else
[currentElementValue appendString:string]; NSLog(@"Processing Value: %@", currentElementValue); } // 这里才是真正完成整个解析并保存数据的最终结果的地方
- (void)parser:(NSXMLParser *)parser didEndElement:(NSString *)elementName
namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qName { if([elementName isEqualToString:@"Books"])
return; //There is nothing to do if we encounter the Books element here.
//If we encounter the Book element howevere, we want to add the book object to the array 遇到Book元素的结束标签，则添加book对象到设置好的数组中。
// and release the object.
if([elementName isEqualToString:@"Book"]) {
[appDelegate.books addObject:aBook]; [aBook release];
aBook = nil;
}
else
// 不是Book元素时也不是根元素，则用 setValue:forKey为当前book对象的属性赋值
[aBook setValue:currentElementValue forKey:elementName]; [currentElementValue release];
currentElementValue = nil;
} - (void) dealloc { [aBook release];
[currentElementValue release];
[super dealloc];
} @end
只要没有碰到文档结束符，解析器会一而再，再而三的不断呼叫此三个回调函数进行循环：开始初始化book对象并读取有关元素属性，读取子元素和设置它们的值给实体对象，最后不断将实体对象添加到数组中保存。