Sunday, January 25, 2009

Objective-XML 5.0

I've just pushed out a new release of Objective-XML, with some pretty significant new features.

Incremental parsing

This feature, which was already discussed a little in an earlier post, is now available in an official release. In short, Objective-XML will now stream data from network data sources (specified by URL) and produce results incrementally, rather than reading all of the data first and then parsing it. This can make a huge difference in responsiveness and perceived performance for slow networks. CPU and memory consumption will be slightly higher because of extra buffering and buffer stitching required, so this should only be used when necessary.

Static iPhone library

Although Objective-XML has always been compatible with the iPhone, previous releases required copying the pre-requisite files into your project. This burden has now been eased by the inclusion of a static library target. You still need to copy the headers, either MPWMAXParser.h or MPWXmlParser.h (or both).

Unique keys

Previous releases of Objective-XML had an -objectForTag:(int)tag method for quickly retrieving attribute or element values.

enum songtags {
  item_tag=10, title_tag, category_tag	
  [parser setHandler:self forElements:[NSArray arrayWithObjects:@"item",@"title",@"category",nil]
          inNamespace:nil prefix:@"" map:nil tagBase:item_tag];
-itemElement:(MPWXMLAttributes*)children attributes:(MPWXMLAttributes*)attributes parser:(MPWMAXParser*)p
   [song setTitle:[children objectForTag:title_tag]];

Objective-XML adds an -objectForUniqueKey:aKey method that removes the need for these additional integer tags.
  [parser setHandler:self forElements:[NSArray arrayWithObjects:@"item",@"title",@"category",nil]
          inNamespace:nil prefix:@"" map:nil];
-itemElement:(MPWXMLAttributes*)children attributes:(MPWXMLAttributes*)attributes parser:(MPWMAXParser*)p
   [song setTitle:[children objectForUniqueKey:@"title"]];

In addition to providing faster access, the integer tags also served to disambiguate tag names that might occur in multiple namespaces. To handle these conflicts, there now is a -objectForUniqueKey:aKey namespace:aNamespace method. The namespace objects required for this disambiguation process are now returned by the -setHandler:... and -declareAttributes:... methods, which were previously void.

Default methods

One of the attractive features of DOM parsers is that they do something useful "out of the box": point a DOM parser at some XML and you get back a generic in-memory representation of that XML that you can then start taking apart. However, once you go down that road, you are stuck with the substantial CPU and memory overheads of that generic representation.

Streaming parser like SAX or MAX can be a lot more efficient, but it takes a lot more time and effort until achieving a first useful result. Default methods overcome this hurdle by also delivering an immediately useful generic representation without any extra work. Unlike a DOM, however, this generic representation can be incrementally replaced by more specialized and efficient processing later on.

Tuesday, January 20, 2009

Cocoa HTML parsing with Objective-XML

Although Objective-XML's MPWSAXParser mostly provides NSXMLParser compatibility it also provides a number of useful additional features. Among these features is the ability to parse HTML files via the settings of two flags: enforceTagNesting and ignoreCase. By default, these are on and off, respectively, which gives you strict XML behavior. However, by setting enforceTagNesting to NO and ignoreCase to YES, you get a SAX parser that will happily and speedily process HTML.

Saturday, January 17, 2009

Semantic Noise

Martin Fowler and Gilad Bracha write about Syntactic Noise, making similar points and using similar typographical techniques as I did in my HOM paper.
By Syntactic Noise, what people mean is extraneous characters that aren't part of what we really need to say, but are there to satisfy the language definition. Noise characters are bad because they obscure the meaning of our program, forcing us to puzzle out what it's doing.
Couldn't have said it better myself, so I'll just quote Martin Fowler. Syntactic noise is one of the reasons I think neither the for(each) statement nor the blocks added to Objective-C are particularly good replacements for Higher Order Messaging.
newArray = [existingArray map:^(id obj){ return [obj  stringByAppendingString:@"suffix"]; }];
newArray = [[existingArray map] stringByAppendingString:@"suffix"]];

To me, that extra syntax is quite noisy, though the noise isn't, in fact, just syntactic. We also have to introduce, name and even correctly type a completely redundant stand-in (obj) that we don't really care about. Introducing extra entities is semantic noise. Apart from having to puzzle out what that extra entity is (and that it is, in fact, redundant) every time we read the code, it also brings us back to "element at a time" programming and thinking.

Thursday, January 15, 2009

Simple HOM

While it is good to see that Higher Order Messaging is still inspiring new work, I feel a bit guilty that part of that inspiration are sentiments such as the following:

"Still I have yet to find a simple implementation that I like and that does not use private methods. The last thing I want is a relying on classes which can break at any time."
Mea culpa.

While I did explain a bit why the current HOM implementation is a bit gnarly, code probably speaks more loudly than repeated mea-culpas.

So, without further ado, a really simple HOM implementation. An NSArray category provides the interface and does the actual processing:

@interface NSArray(hom)



@implementation NSArray(hom)

-(NSArray* )collect:(NSInvocation*)anInvocation
  NSMutableArray *resultArray=[NSMutableArray array];
  for (id obj in self ) {
    id resultObject;
    [anInvocation invokeWithTarget:obj];
    [anInvocation getReturnValue:&resultObject];
    [resultArray addObject:resultObject];
  return resultArray;

-collect {
  return [HOM homWithTarget:self selector:@selector(collect:)];

The fact that NSInvocation deals with pointers to values rather than values makes this a bit longer than it needs to be, but the gist is simple enough: iterate over the array, invoke the invocation, return the result.

That leaves the actual trampoline, which is really just an implementation detail for conveniently creating NSInvocation objects.

@interface HOM : NSProxy {
  id xxTarget;
  SEL xxSelector;


@implementation HOM

  [xxTarget performSelector:xxSelector withObject:anInvocation];

  return [[xxTarget objectAtIndex:0] methodSignatureForSelector:aSelector];

-xxinitWithTarget:aTarget selector:(SEL)newSelector
  return self;

+homWithTarget:aTarget selector:(SEL)newSelector
  return [[[self alloc] xxinitWithTarget:aTarget selector:newSelector] autorelease];

This code compiles without warnings, does not use any private API, and runs on both Leopard and the iPhone. Github:

EDIT (Aug 15 2015): Changed SimpleHOM download link to github repo.

Sunday, January 11, 2009

iPhone XML performance

Shortly after becoming an iPhone developer, I found a clever little piece of example code called XML Performance (login required). Having done some high performance XML processing code that works on the iPhone, I was naturally intrigued.

The example pits Cocoa's NSXMLParser against a custom parser based on libxml2, the benchmark is downloading a top 300 list of songs from iTunes.

More responsiveness using libxml2 instead of NSXMLParser

Based on my previous experience, I was expecting libxml2 to be noticeably faster, but with the advantage in processing speed being less and less important with lower and lower I/O data rates (WiFi to 3G to Edge), as I/O would start to completely overwhelm processing. Was I ever wrong!

While my expectations were technically correct for overall performance, I had completely failed to take responsiveness into account. Depending on the network selected, the NSXMLParser sample would appear to hang for 3 to 50 seconds before starting to show results. Needless to say, that is an awful user experience. The libxml example, on the other hand, would start displaying some results almost immediately. While it also was a bit faster in the total time taken, this effect seemed pretty insignificant compared to the fact that results were arriving continually pretty much during the entire time.

The difference, of course, is incremental processing. Whereas NSXMLParser's -initWithContentsOfURL: method apparently downloads the entire document first and then begins processing, the libxml2-based code in the sample downloads the XML in small chunks and processes those chunks immediately.

Alas, going with libxml2 has clear and significant disadvantages, with the code that uses libxml2 being around twice the size of the NSXMLParser-based code, at around 150 lines (non-comment, non-whitespace). If you have worked with NSXMLParser before, you will know that that is already pretty painful, so just imagine that particular brand of joy doubled, with the 150 lines of code giving you the simplest of parsers, with just 5 tags processed. Fortunately, there is a simpler way.

A simpler way: Objective-XML's SAX

Assuming you have already written a Cocoa-(Touch-)based parser using NSXMLParser, all you need to do is include Objective-XML in your projects and replace the reference to NSXMLParser with a reference to MPWSAXParser, everything else will work just as before. Well, the same except for being significantly faster (even faster than libxml2) and now also more responsive on slow connections due to incremental processing.

I have to admit that not having incremental processing was a "feature" Objective-XML shared with NSXMLParser until very recently, due to my not taking into account the fact that latency lags bandwidth. This silly oversight has now been fixed, with both MPWMAXParser and MPWSAXParser sporting URL-based parsing methods that do incremental processing.

So that's all there is to it, Objective-XML provides a drop-in replacement for NSXMLParser that has all the performance and responsiveness-benefits of a libxml2-based solution without the coding horror.

Even simpler: Messaging API for XML (MAX)

However, even a Cocoa version of the SAX API represents a pretty low-bar in terms of ease of coding. With MAX, Objective-XML provides an API that can do the same job much more simply. MAX naturally integrates XML processing with Objective-C messaging using the following two main features:
  • Clients get sent element-specific messages for processing
  • The parser handles nesting, controlled by the client
The following code for building Song objects out of iTunes <item> elements illustrates these two features:
-itemElement:(MPWXMLAttributes*)children attributes:(MPWXMLAttributes*)attributes parser:(MPWMAXParser*)p
  Song *song=[[Song alloc] init];
  [song setArtist:[children objectForTag:artist_tag]];
  [song setAlbum:[children objectForTag:album_tag]];
  [song setTitle:[children objectForTag:title_tag]];
  [song setCategory:[children objectForTag:category_tag]];
  [song setReleaseDate:[parseFormatter dateFromString:[children objectForTag:releasedate_tag]]];
  [self parsedSong:song];
  [song release];
  return nil;
MAX sends the -itemElement:attributes:parser: message to its client whenever it has encountered a complete <item> element, so there is no need for the client to perform string processing on tag names or manage partial state as in a SAX parser. The method constructs a song object using data from the <item> element's child elements which it then passes directly to the rest of the app via the parsedSong: message. It does not return an value, so MAX will not build a tree at this level.

Artist, album, title and category are the values of nested child elements of the <item> element. The (common) code shared by all these child-elements gets the character content of the respective elements and is shown below:

-defaultElement:children attributes:atrs parser:parser
	return [[children combinedText] retain];
Unlike the <item> processing code, which did not return a value, this method does return a value. MAX uses this return value to build a DOM-like structure which is then consumed by the next higher-level, in this case the -itemElement:attributes:parser: method shown above. Unlike a traditional DOM, the MAX tree structure is built out of domain-specific objects returned incrementally by the client.

These two pieces of sample code demonstrate how MAX can act like both a DOM parser or a SAX parser, controlled simply by wether the processing methods return objects (DOM) or not (SAX). They also demonstrated both element-specific and generic processing.

In the iTunes Song parsing example, I was able to build a MAX parser using about half the code required for the NSXMLParser-based example, a ratio that I have also encountered in larger projects. What about performance? It is slightly better than MPWSAXParser, so also somewhat better than libxml2 and significantly better than NSXMLParser.

Summary and Conclusion

The slightly misnamed XML Performance sample code for the iPhone demonstrates how important managing latency is for perceived end user performance, while showing only very little in terms of actual XML processing performance.

While ably demonstrating the performance problems of NSXMLParser, the sample code's solution of using libxml2 is really not a solution, due to the significant increase in code complexity. Objective-XML provides both a drop-in replacement for NSXMLParser with all the performance and latency benefits of the libxml2 solution, as well as a new API that is not just faster, but also much more straightforward than either NSXMLParser or libxml2.

Best of Show, MacWorld 2009

Since I recently became the Mac tech lead for Livescribe, responsible for delivering the Mac desktop software, I am happy to report that not only did we meet all of our target dates, we also won Best of Show at MacWorld 2009.

Spending 3 days at the booth was both exhausting and rewarding, the enthusiasm exhibited by customers was absolutely mind-blowing.