iphone Programming : Handling Data - Parsing XML

The two widely used methods for parsing an XML document are SAX and DOM. A SAX (Simple API for XML) parser is event-driven. It reads the XML document incrementally and calls a delegate method whenever it recognizes a token. Events are generated at the beginning and end of the document, and the beginning and end of each element. A DOM (Document Object Model) parser reads the entire document and forms a tree-like corresponding structure in memory. You can then use the XPath query language to select individual nodes of the XML document using a variety of criteria.

Most programmers find the DOM method more familiar and easier to use; however, SAX-based applications are generally more efficient, run faster, and use less memory. So, unless you are constrained by system requirements, the only real factor when deciding to use SAX or DOM parsers comes down to preference.

1. Parsing XML with libxml2

NSArray *PerformHTMLXPathQuery(NSData *document, NSString *query);
NSArray *PerformXMLXPathQuery(NSData *document, NSString *query);

If you want to return the entire document as a single data structure, the following will do that. Be warned that except for the simplest of XML documents, this will normally generate a heavily nested structure of array and dictionary elements, which isn’t particularly useful:

NSString *xpathQueryString;
NSArray *nodes;

xpathQueryString = @"/*";
nodes = PerformXMLXPathQuery(responseData, xpathQueryString);
NSLog(@"nodes = %@", nodes );

The XML document had a structure that looked like the following snippet:

<forecast_conditions>
   ...
   <icon data="/ig/images/weather/chance_of_rain.gif"/>
</forecast_conditions>
<forecast_conditions>
   ...
   <icon data="/ig/images/weather/chance_of_rain.gif"/>
</forecast_conditions>
<forecast_conditions>
   ...
   <icon data="/ig/images/weather/chance_of_rain.gif"/>
</forecast_conditions>
<forecast_conditions>
   ...
   <icon data="/ig/images/weather/chance_of_rain.gif"/>
</forecast_conditions>

To extract the URL of the icons, we carried out an XPath query:

xpathQueryString = @"//forecast_conditions/icon/@data";
nodes = PerformXMLXPathQuery(responseData, xpathQueryString);

The nodes array returned by the PerformXMLXPathQuery method looked like this:

(   {
         nodeContent = "/ig/images/weather/mostly_sunny.gif";
         nodeName = data;
    },
         {
         nodeContent = "/ig/images/weather/chance_of_rain.gif";
         nodeName = data;
    },
         {
         nodeContent = "/ig/images/weather/mostly_sunny.gif";
         nodeName = data;
    },
         {
         nodeContent = "/ig/images/weather/mostly_sunny.gif";
         nodeName = data;
   }
)

This structure is an NSArray of NSDictionary objects, and we parsed this by iterating through each array entry and extracting the dictionary value for the key nodeContent, adding each occurrence to the icons array:

for ( NSDictionary *node in nodes ) {
    for ( id key in node ) {
        if( [key isEqualToString:@"nodeContent"] ) {
            [icons addObject:
                [NSString stringWithFormat:@"http://www.google.com%@",
                [node objectForKey:key]]];
        }
    }
}

2. Parsing XML with NSXMLParser

The official way to parse XML on the iPhone is to use the SAX-based NSXMLParser class. However, the parser is strict and cannot take HTML documents:

NSString *url = @"http://feeds.feedburner.com/oreilly/news";
NSURL *theURL = [[NSURL URLWithString:url] retain];

NSXMLParser *parser = [[NSXMLParser alloc] initWithContentsOfURL:theURL];
[parser setDelegate:self];
[parser setShouldResolveExternalEntities:YES];
BOOL success = [parser parse];
NSLog(@"Success = %d", success);

We use the parser by passing it an XML document and then implementing its delegate methods. The NSXMLParser class offers the following delegate methods:

parserDidStartDocument:
parserDidEndDocument:
parser:didStartElement:namespaceURI:qualifiedName:attributes:
parser:didEndElement:namespaceURI:qualifiedName:
parser:didStartMappingPrefix:toURI:
parser:didEndMappingPrefix:
parser:resolveExternalEntityName:systemID:
parser:parseErrorOccurred:
parser:validationErrorOccurred:
parser:foundCharacters:
parser:foundIgnorableWhitespace:
parser:foundProcessingInstructionWithTarget:data:
parser:foundComment:
parser:foundCDATA:

The most heavily used delegate methods out of all of those available methods are the parser:didStartElement:namespaceURI:qualifiedName:attributes: method and the parser:didEndElement:namespaceURI:qualifiedName: method. These two methods, along with the parser:foundCharacters: method, will allow you to detect the start and end of a selected element and retrieve its contents. When the NSXMLParser object traverses an element in an XML document, it sends three separate messages to its delegate, in the following order:

parser:didStartElement:namespaceURI:qualifiedName:attributes:
parser:foundCharacters:
parser:didEndElement:namespaceURI:qualifiedName:

Returning to the Weather application: to replace our XPath- and DOM-based solution with an NSXMLParser-based solution, we would substitute the following code for the existing queryService:withParent: method:

- (void)queryService:(NSString *)city
   withParent:(UIViewController *)controller {
     viewController = (MainViewController *)controller;
     responseData = [[NSMutableData data] retain];

     NSString *url =
      [NSString stringWithFormat: @"http://www.google.com/ig/api?weather=%@",
      city];
      theURL = [[NSURL URLWithString:url] retain];
      NSXMLParser *parser = [[NSXMLParser alloc] initWithContentsOfURL:theURL];
      [parser setDelegate:self];
      [parser setShouldResolveExternalEntities:YES];
      BOOL success = [parser parse];
}

					  

We would then need to delete all of the NSURLConnection delegate methods, substituting the following NSXMLParser delegate method to handle populating our arrays:

- (void)parser:(NSXMLParser *)parser
  didStartElement:(NSString *)elementName
  namespaceURI:(NSString *)namespaceURI
  qualifiedName:(NSString *)qName
  attributes:(NSDictionary *)attributeDict {

   // Parsing code to retrieve icon path
   if([elementName isEqualToString:@"icon"]) {
      NSString *imagePath = [attributeDict objectForKey:@"data"];
      [icons addObject:
         [NSString stringWithFormat:@"http://www.google.com%@", imagePath]];
   }

   // ... add remaining parsing code for other elements here

   [viewController updateView];
}

					  

Warning:

This example parses only the icon element; if you wanted to use NSXMLParser here, you’d need to look at connectionDidFinishLoading: in the original Weather app, and add parsing code for each of those elements before you call [viewController updateView] in this method (otherwise, it will throw an exception and crash the app because none of the data structures are populated).