• I want the data to be processed  and shown to the user as quickly as possible.
• If the file is large I don’t want to have to store the entire contents in memory.

Using NSXMLParsers initWithURL: method the xml file is downloaded synchronously with NSURLConnection before parsing. If NSXMLParser was able to begin as soon as stream buffer began to fill, even though the overall parsing time wouldn’t be reduced significantly if the updates are immediately displayed the user perceived time decreases significantly.

Using the excellent objective c expat wrapper library by Robbie Hanson as a base iPhoneExpat uses CFNetwork & CFHTTPMessage to create and feed a http stream gradually into Expat. If the server supports gzip compression zlib is used to decompress the stream on fly.

iPhone Expat offer 3 methods for initialization

- (id)initWithContentsOfURL:(NSURL *)url;
- (id)initWithContentsOfFile:(NSString *)path;
- (id)initWithData:(NSData *)data;

The delegate messages are also a drop in replacement for NSXMLParser

@protocol ExpatXMLParserDelegate 
 
@optional
- (void)parserDidStartDocument:(ExpatXMLParser*)parser;
- (void)parserDidEndDocument:(ExpatXMLParser*)parser;
- (void)parser:(ExpatXMLParser*)parser didStartMappingPrefix:(NSString *)prefix toURI:(NSString *)namespaceURI;
- (void)parser:(ExpatXMLParser*)parser didEndMappingPrefix:(NSString *)prefix;
- (void)parser:(ExpatXMLParser*)parser foundComment:(NSString *)comment;
- (void)parser:(ExpatXMLParser*)parser foundProcessingInstructionWithTarget:(NSString *)target data:(NSString *)data;
- (void)parser:(ExpatXMLParser*)parser parseErrorOccurred:(NSError *)parseError;
- (BOOL)parser:(ExpatXMLParser*)parser shouldProcessAttributesForElement:(NSString *)elementName;
 
@required
- (void)parser:(ExpatXMLParser*)parser didStartElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qualifiedName attributes:(NSDictionary *)attributeDict;
- (void)parser:(ExpatXMLParser*)parser didEndElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qName;
- (void)parser:(ExpatXMLParser *)parser foundCharacters:(NSString *)string;
 
@end

However

- (void)parser:(ExpatXMLParser *)parser foundCharacters:(NSString *)string;

Will report the entire contents of a tag so you don’t need an NSMutablestring to buffer fragments in your delegate. e.g.

- (void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string
{
	[buffer release];
	buffer = string;
	[buffer retain];
}

Should be good in 99% of cases to get the entire contents of a tag

The project can be found here

http://github.com/zootreeves/iPhoneExpat

Alternatively checkout the code using git: http://github.com/zootreeves/iPhoneExpat

Benchmark test output:

I’ve included a test application which sequentially downloads and parses the following feeds:

@"http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topalbums/sf=143441/limit=300/explicit=true/xml",
@"http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/ws/RSS/topalbums/sf=143441/limit=300/explicit=true/xml",
@"http://feeds.feedburner.com/DilbertDailyStrip",
@"http://designsponge.blogspot.com/atom.xml",
@"http://www.slate.com/rss/",
@"http://rssfeeds.usatoday.com/UsatodaycomBooks-TopStories",
@"http://googleblog.blogspot.com/atom.xml",
@"hhttp://api.flickr.com/services/feeds/groups_pool.gne?id=61057342@N00〈=en-us&format=rss_200",
@"http://phobos.apple.com/WebObjects/MZStore.woa/wpa/MRSS/topsongs/limit=25/rss.xml",
@"http://www.readwriteweb.com/rss.xml",
@"http://rssfeeds.usatoday.com/UsatodaycomNation-TopStories",
@"http://dictionary.reference.com/wordoftheday/wotd.rss",
@"http://www.quotationspage.com/data/qotd.rss",
@"http://sports.espn.go.com/espn/rss/news"

Time to reach the first element Expat: 8.611217 — NSXMLParser: 15.404100
Total time for Expat: 10.902272 — NSXMLParser: 17.108309

Memory is also reduced by a factor of about x4 (see screenshots)

Further Considerations

• It would be great if http requests were persistent, however after setting kCFStreamPropertyHTTPAttemptPersistentConnection to true using httpscoop I will still able to intercept “connection close” messages between requests to the same server. I think I must be doing something wrong here.
• It would be preferable if as many objects as possible were released manually rather than using an autorelease pool. For example the delegate methods that pass an elementName it is common for the client to use this string to determine their current tag, but rare that they actually want to retain it. The code could be altered so these strings are not placed in a pool and are released immediately after the delegate message is sent. Not only can the overhead of collection be avoided but CFStringCreateWithCharactersNoCopy can be used as well.
• With UTF8 creating CFStrings with the [NSString stringwithutf8string:] was no problem. However when I configured Expat to use UTF-16 characters internally I found that CFStringCreateWithCharacters would often return garbage. The problem is I was passing in UINT16_MAX as the buffer length hoping that the CFString would be terminated at null, however it doesn’t work that way and I had to change everything to calculate the buffer size using UniCharStrlen(buffer) before hand.
• FTP (CFFTPStream) and file streams could easily be supported.

The code below is provided under an MIT license, but if you do make any updates it would be great if you could send them back.

Usage:

   //Example to download google's source and print out the urls of all the images
   NSError * error = nil;
   HTMLParser * parser = [[HTMLParser alloc] initWithContentsOfURL:[NSURL URLWithString:@"http://www.google.com"] error:&error];
 
   if (error) {
     NSLog(@"Error: %@", error);
     return;
  }
  HTMLNode * bodyNode = [parser body]; //Find the body tag
 
  NSArray * imageNodes = [bodyNode findChildTags:@"img"]; //Get all the <img alt="" />
 
 for (HTMLNode * imageNode in imageNodes) { //Loop through all the tags
       NSLog(@"Found image with src: %@", [imageNode getAttributeNamed:@"src"]); //Echo the src=""
  }
 
  [parser release];

You can grab a copy of the code here on github:
http://github.com/zootreeves/Objective-C-HMTL-Parser
git@github.com:zootreeves/Objective-C-HMTL-Parser.git

Note:

-(NSString*)rawContents; does not work, you need to use NSString * rawContentsOfNode(xmlNode * node, htmlDocPtr doc); to dump the entire html contents of a node.

When the device is connected to a cellular network, iVersion does not load its contents.  After the user enters the URL (http://svn.collab.net/repos/svn/trunk/) and taps “connect,” an error message is received.

Apparently the error message is not clear enough or something, anyway this is not a rant, this is what apple recommends.

Please take a look at the Reachability iPhone program sample which demonstrates the use of the System Configuration Reachability API to detect the absence of WiFi and Wireless Wide Area Network (WWAN) services. Your application can then take appropriate action at the first point where network services are required.

The sample code can be found on Apple developer site, however the two file we are interested in are:

Download the Reachability Code:
Reachability.h

Reachability.m

How to use:
Lets say you have a simple UILabel which you want to update with either “(Offline)” or “(Online)” depending on whether the network is reachable. Your view controller would be defined something like this:

@interface MyStatusView : UIViewController
{
	UILabel * statusLabel;
}
 
@property(nonatomic, assign) IBOutlet UILabel * statusLabel;
 
- (void)reachabilityChanged:(NSNotification *)note;
- (void)updateStatus;
 
@end

Then we add the following code to the view controller .m file:

-(void)viewWillAppear:(BOOL)animated
{
	self.reachability = [Reachability reachabilityWithHostName:@"google.com"];
	[reachability startNotifer];
 
	[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(reachabilityChanged:) name:@"kNetworkReachabilityChangedNotification" object:nil];
 
	[self updateStatus];
}
 
-(void)viewWillDisappear:(BOOL)animated
{
	self.reachability = nil;
	[[NSNotificationCenter defaultCenter] removeObserver:self];
}
 
-(void)reachabilityChanged:(NSNotification*)notification
{
	[self updateStatus];
}
 
-(void)updateStatus
{
	NSLog(@"Reachability changed");
 
	if ([reachability currentReachabilityStatus] == NotReachable)
	{
		statusLabel.text = @"(Offline)";
	}
	else
	{
		statusLabel.text = @"(Online)";
	}
}

I think it’s fairly self explanatory. Basically when the view appears the reachability class is created and started using startNotifier. Then we hook in our reachabilityChanged: method into the notification centre, so it is automatically called when the network status changes. reachabilityChanged: calls updateStatus which simply changes our label depending on the connectivity.

iVersion uses the open source subversion 1.6 client library, written in plain C, with an Objective C wrapper for simpler interfacing with the Objective C UI. Using an objective C Wrapper the client can easily be abstracted to work with other version control protocols. In the future git, mercurialand bazaar support maybe included.

Features

• Multi-threaded non-locking ui
• OpenSSL support for https:// and svn+ssh:// connections
• svn commands: list, info, cat,
• Browse Directories
• View Commit Logs
• View file Info
• Bookmarks, History & Directory Cache through NSUserDefaults

** Commit Support
Subversion does not allow commiting of a single file without a working copy of the previous directory. A work around for this has been found and commit support should be available in a future version.