<div id="readability-page-1" class="page"><div data-astro-cid-57z6rh23="">Parsing XML with iPhone SDK 3.0, I ran into a few things worth noting.
I tried building an XMLParser class written for SDK 2.x, and the build failed with an error.
<pre tabindex="0" data-language="objective-c"><code>- (void)parseContents:(NSURL *)url {
 NSXMLDocument *xmlDoc = [[NSXMLDocument alloc] initWithContentsOfURL:url options:1 &lt;&lt; 10 error:nil];
 [self parseXMLData:[xmlDoc XMLData]];
 [xmlDoc release];
}</code></pre>
Trying to parse with this simple code gives an “NSXMLDocument undeclared” error.
I looked through the SDK API and couldn’t find an <code>NSXMLDocument</code> class anywhere.
Setting the SDK version to 2.0 builds fine with no errors. But since I need features only available in SDK 3.0, I had to find another way around the error.
<pre tabindex="0" data-language="objective-c"><code>-(void)parseXMLUrl:(NSURL *)url {
 NSXMLParser*p =[NSXMLParser alloc];
 [p initWithContentsOfURL:url];
 [p setShouldProcessNamespaces:YES];
 [p setDelegate:self];
 [p parse];
 [p release];
}</code></pre>
This still failed. The reason: the XML data doesn’t fully follow spec. The XML being fetched has issues, so it needs to be cleaned up first.
There were special characters inside an XML tag that needed to be wrapped in CDATA.
<pre tabindex="0" data-language="objective-c"><code>NSString *xmlString = [NSString stringWithContentsOfURL:url];
NSData *xmlData = [[self sanitizeXmlString:xmlString] dataUsingEncoding:NSUTF8StringEncoding];</code></pre>
The goal: make the code above work.
<ul>
<li>Write a sanitizing function that fixes the broken tag strings before parsing, as a preprocessing step.</li>
<li>Use NSData’s <code>dataUsingEncoding</code> to convert NSString to NSData (matching the charset).</li>
<li>Use NSString’s <code>stringWithContentsOfURL</code> to read the file contents.</li>
</ul>
In the end, a function that wraps the contents of <code>&lt;link&gt;&lt;/link&gt;</code> in CDATA.
<code>stringWithContentsOfURL</code> is synchronous. It blocks on a thread until it finishes, and doesn’t let you set an async delegate. If the file is large, storage I/O is slow, or you’re loading an external file, you’d want to run that block on a separate async thread. For now, let’s keep it simple.
The sanitizing function:
<pre tabindex="0" data-language="objective-c"><code>-(NSString*) sanitizeLinkTag:(NSString*)string {
 NSString* sanitizeLinkTag =[string stringByReplacingOccurrencesOfString:(NSString*)@"&lt;/link&gt;" withString:(NSString*)@"]]&gt;&lt;/link&gt;"];
 sanitizeLinkTag = [sanitizeLinkTag stringByReplacingOccurrencesOfString:(NSString*)@"&lt;link&gt;" withString:(NSString*)@"&lt;link&gt;&lt;![CDATA["];
 return sanitizedString;
}</code></pre>
For NSString’s <code>stringByReplacingOccurrencesOfString</code>, the first parameter is the target and the second, <code>withString</code>, is the replacement.
It’s not a mutating method — it returns a new String. I haven’t tried instantiating an NSString myself; the API docs just say it returns a new one.
Anyway, outside of this special case — i.e. when you’re fetching properly formed XML — you need to know the XML’s structure “beforehand” in order to parse it. For a URL, check it in a browser first; for a file, read and inspect it.
NSXMLParser has four key delegate methods that handle the parsing work,
and they don’t build a tree structure for you. Unlike AS3 or other libraries, they don’t give you a tree or hash-object structure automatically. More on that below.
The four most important delegate methods on NSXMLParser:
<pre tabindex="0" data-language="objective-c"><code>-(void)parser:(NSXMLParser*)parser parseErrorOccurred:(NSError*)parseError
-(void)parser:(NSXMLParser*)parser didStartElement:(NSString*)elementName namespaceURI:(NSString*)namespaceURI qualifiedName:(NSString*)qualifiedName attributes:(NSDictionary*)attributeDict
-(void)parser:(NSXMLParser*)parser didEndElement:(NSString*)elementName namespaceURI:(NSString*)namespaceURI qualifiedName:(NSString*)qName
-(void)parser:(NSXMLParser*)parser foundCharacters:(NSString*)string</code></pre>
The first, <code>parseErrorOccurred</code>, fires when a parsing error happens.
The second, <code>didStartElement</code>, fires when it hits a tag wrapped in <code>&lt;&gt;</code>.
The third, <code>didEndElement</code>, fires when it hits a tag wrapped in <code>&lt;/&gt;</code>.
The fourth, <code>foundCharacters</code>, returns a String after both <code>didStartElement</code> and <code>didEndElement</code>.
The important one is the fourth, <code>foundCharacters</code> —
it gets delegated both after <code>didStartElement</code> and after <code>didEndElement</code>.
What that means: when it’s delegated right after <code>didStartElement</code>, the parameter is the data inside the element. But when it’s delegated after <code>didEndElement</code>, the parameter is something else (not nil, not <code>@""</code>)
— basically a value you can’t use (maybe a <code>\n</code>?).
That’s why, in a lot of sample code, the delegate methods are written using a global variable called <code>xmlValue</code>:
reset <code>xmlValue</code> to empty inside <code>didStartElement</code>
(<code>xmlValue = @"";</code>, something like that),
then in <code>foundCharacters</code>,
assign <code>xmlValue = string</code>,
and in <code>didEndElement</code>, use <code>xmlValue</code> wherever it’s needed.
That’s the pattern you need to follow. If you use the <code>string</code> from <code>foundCharacters</code> directly wherever it’s needed, you’ll end up using the garbage string value that comes through on the <code>foundCharacters</code> call fired after <code>didEndParsing</code>.
Apparently this doesn’t happen with the SDK 2.x structure that uses <code>NSXMLDocument</code>.
In 2.x, with <code>NSXMLDocument</code>, <code>foundCharacters</code> only fires once.
Since the XML data can be malformed, this “only fires once” behavior can also happen in SDK 3.0 by accident, so it seems safer to manage the XML data by continuously updating a global <code>xmlValue</code>.
And since it’s not a tree structure,
you’d use the tag name you get from <code>elementName</code> in <code>didStartElement</code> to decide
which NSMutableDictionary, array, or class acting as a struct to migrate the data into.
If an NSMutableDictionary structure looks like
car
<ul>
<li>brand - model - color - grade</li>
</ul>
then regardless of whether the source XML was a tree or something else,
when the tag name is <code>model</code>, you need to directly set the <code>model</code> key on the dictionary inside <code>didEndParsing</code>.
Whether it’s <code>color</code> or <code>grade</code>, you need to convert the flat incoming data into a tree structure yourself.</div></div>

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Jeong-Un, Kim

Parsing XML with iPhone SDK 3.0, I ran into a few things worth noting. I tried building an XMLParser class written for SDK 2.x, and the build failed with…

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