Do you need to handle compressed files on your iOS project? Then you may find the following links to compression libraries useful:

• ZipKit is an Objective-C framework for reading and writing Zip archives in Mac OS X and iOS apps.
• Unrar4iOS The main goal of this project is provide a port of Unrar library to iOS platform.
• 7-Zip LZMA SDK LZMA is the default and general compression method of 7z format. The LZMA SDK provides the documentation, samples, header files, libraries, and tools you need to develop applications that use LZMA compression.

If you are using TouchXML or GDataXML for reading XML documents, you may have run into a problem with your XPath queries. I was working with the Freshbooks API to return a list of projects and it returned some fairly simple XML (I have abbreviated all the attributes of a project element for clarity):

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<?xml version="1.0" encoding="utf-8"?>
<response xmlns="http://www.freshbooks.com/api/" status="ok">
  <projects page="1" per_page="15" pages="1" total="5">
    <project>
      <name>Super Fun Project</name>
    </project>
  </projects>
</response>

So now I wanted to simply retrieve all the project elements in the response using XPath. Seemed simple enough, so this is what I wrote:

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// doc is an instance of GDataXMLDocument
NSArray *projects = [doc nodesForXPath:@"//projects/project" error:nil];
NSLog(@"%d", projects.count);

My NSLog statement always returned 0 results. This started to drive me nuts. I tried several different XPath queries and still no luck. There were no errors even when I used the error outlet, and I could even loop through the elements manually and see that there were indeed project elements being returned. So what’s the problem?

The problem had to do with namespaces. The XPath query needed to be written using the correct XML namespace, so something like this:

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// doc is an instance of GDataXMLDocument
NSArray *projects = [doc nodesForXPath:@"//ns:projects/ns:project" error:nil];
NSLog(@"%d", projects.count);

Great, but what is the namespace name? According to my XML response it was http://www.freshbooks.com/api/. However the GDataXMLDocument knew nothing about it.

I started digging into the source of GDataXML and came across this in the nodesForXPath method:

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int result = xmlXPathRegisterNs(xpathCtx, prefix, nsPtr->href);
if (result != 0) {
#if DEBUG
    NSCAssert1(result == 0, @"GDataXMLNode XPath namespace %@ issue",prefix);
#endif
}

Ok… some simple debugging showed it was actually registering a namespace, but what was it called? That lead me to some code just before the xmlRegisterNs call:

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if (prefix == NULL) {
    prefix = (xmlChar*) kGDataXMLXPathDefaultNamespacePrefix;
}

Now we’re getting somewhere. Next stop the definition of the kGDataXMLXPathDefaultNamespacePrefix constant:

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// when no namespace dictionary is supplied for XPath, the default namespace
// for the evaluated tree is registered with the prefix _def_ns
_EXTERN const char* kGDataXMLXPathDefaultNamespacePrefix _INITIALIZE_AS("_def_ns");

THAT WOULD HAVE BEEN HELPFUL AN HOUR AGO… Anyway, I went ahead and changed my original XPath query to:

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// doc is an instance of GDataXMLDocument
NSArray *projects = [doc nodesForXPath:@"//_def_ns:projects/_def_ns:project" error:nil];
NSLog(@"%d", projects.count);

BOOM. Now we’re cooking with gas. It correctly returned all my project elements. So the lesson learned here is that GDataXML and TouchXML will not register your namespace or use it by default. You have to use the default, or register it manually.

How do you register a namespace with GDataXML manually you ask?

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// doc is an instance of GDataXMLDocument
NSDictionary *ns = [NSDictionary dictionaryWithObjectsAndKeys:@"http://www.freshbooks.com/api/", @"fb", nil];
NSArray *projects = [doc nodesForXPath:@"//fb:projects/fb:project" namespaces:ns error:nil];
NSLog(@"%d", projects.count);

And finally, if you want to programatically register a namespace using GDataXml, then you would populate your NSDictionary by iterating through the namespaces found by GDataXMLElement:

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// doc is an instance of GDataXMLDocument
NSArray *namespaceURIs = [doc.rootElement namespaces];

Hopefully that saves you some time and sanity.

Just came across this great demo of using Core Animation in your iOS application. Its basically a demo application that allows you to try out each technique and then you can review the source. The project is on Github. You should also check out Bob McClune’s Blog, the creator of the demo.

If your looking to build a community around your iPhone or iPad application, then you should check out Socialize on Github. The Socialize website explains all the features.

You’ve probably seen those iPhone applications that refresh the content when you pull down on the TableView. I have found three libraries that you can use to implement this in your own apps:

• Pull to Refresh
• EGOTableViewPullRefresh
• Pull to Reload

Leah Culvers’s Pull to Refresh seems to be the most straight forward of the group.

If you are looking for a collection of UI classes to plug into your iPhone project then you need to checkout Three20.

When I do web development I always have some database manager open so that I can quickly review the data used by my application. So naturally with Core Data development, I want to be able to do the same thing. Since Core Data is working with a SQLite database, all you need is a SQLite database manager on your Mac. I have found two that I like:

Base This one costs $30 on the App Store.

SQLite Manager for Firefox This one will cost you nothing and is an addon for Firefox.

I also like to set a shortcut to the following path on my Finder:

/Users/YourUsername/Library/Application Support/iPhone Simulator/

This will help you quickly drill into the various application directories. You’ll find the SQLite database inside the Documents folder of your application inside the iPhone Simulator directory.

If you would like to output the SQL being generated by Core Data during iOS development you need to add an entry to the Arguments Passed On Launch for your app. Its as easy as:

1. Go to Product -> Edit Scheme
2. Choose Run YourAppName.app on the left panel
3. Add -com.apple.CoreData.SQLDebug 1 to the Arguments Passed On Launch panel

I found this answer on StackOverflow.

I have been diving into Core Data recently and I came across this great collection of videos: Core Data for iPhone and iPad

UPDATE: There are a ton of new Core Data and other videos available now at iDeveloper.tv

Just came across this awesome roadmap for aspiring iOS Developers:
Pragmatic Studio iOS Developer Roadmap

Also, be sure to checkout out my previous posts on Beginning iOS Development:
Top 5 Video Training Resources for iOS Development
Top 5 Books for Getting Started with iOS Development