In this post, I am going to focus on ways to filter data. We won't be looking at the UI side of things - this is something I'll do in a subsequent blog post.

Old-skool filtering

If you look at an arbitrary code base, chances are you'll sooner or later run into a piece of code similar to this one:

NSMutableArray *oldSkoolFiltered = [[NSMutableArray alloc] init];
for (Book *book in bookshelf) {
    if ([book.publisher isEqualToString:@"Apress"]) {
        [oldSkoolFiltered addObject:book];
    }
}

It's a straight-forward approach to filtering an array of items (in this case, we're talking about books) using a rather simple if-statement. Nothing wrong with this, but despite the fact we're using a fairly simple expression here, the code is rather verbose. We can easily imagine what will happen in case we need to use more complicated selection criteria or a combination of filtering criteria.

Simple filtering with NSPredicate

Thanks to Cocoa, we can simplify the code by using NSPredicate. NSPredicate is the object representation of an if-statement, or, more formally, a predicate.

Predicates are expressions that evaluate to a truth value, i.e. true or false. We can use them to perform validation and filtering. In Cocoa, we can use NSPredicate to evaluate single objects, filter arrays and perform queries against Core Data data sets.

Let's have a look at how our example looks like when using NSPredicate:

NSPredicate *predicate =
  [NSPredicate predicateWithFormat:@"publisher == %@", @"Apress" ];
NSArray *filtered  = [bookshelf filteredArrayUsingPredicate:predicate];

Much shorter and better readable!

Filtering with Regular Expressions

Regular Expressions can be used to solve almost any problem so it's good to know you can use them in NSPredicates as well. To use regular expressions in your NSPredicate, you need to use the MATCHES operator.

Let's filter all books that are about iPad or iPhone programming:

predicate = [NSPredicate predicateWithFormat:@"title MATCHES '.*(iPhone|iPad).*'"];
filtered = [bookshelf filteredArrayUsingPredicate:predicate];
dumpBookshelf(@"Books that contain 'iPad' or 'iPhone' in their title", filtered);

You need to obey some rules when using regular expressions in NSPredicate: most importantly, you cannot use regular expression metacharacters inside a pattern set.

Filtering using set operations

Let's for a moment assume you want to filter all books that have been published by your favorite publishers. Using the IN operator, this is rather simple: first, we need to set up a set containing the publishers we're interested in. Then, we can create the predicate and finally perform the filtering operation:

NSArray *favoritePublishers = [NSArray arrayWithObjects:@"Apress", @"O'Reilly", nil];
predicate = [NSPredicate predicateWithFormat:@"publisher IN %@", favoritePublishers];
filtered  = [bookshelf filteredArrayUsingPredicate:predicate];
dumpBookshelf(@"Books published by my favorite publishers", filtered);

Advanced filtering thanks to KVC goodness

NSPredicate relies on key-value coding to achieve its magic. On one hand this means your classes need to be KVC compliant in order to be queried using NSPredicate (at least the attributes you want to query). On the other hand, this allows us to perform some very interesting things with very little lines of code.

Let's for example retrieve a list of books written by authors with the name "Mark":

predicate =
  [NSPredicate predicateWithFormat:@"authors.lastName CONTAINS %@", @"Mark" ];
filtered  = [bookshelf filteredArrayUsingPredicate:predicate];

In case we'd want to return the value of one of the aggregate functions, we don't need to use NSPredicate itself, but instead use KVC directly. Let's retrieve the average price of all books on our shelf:

NSNumber *average = [bookshelf valueForKeyPath:@"@avg.price"];

Conclusion

The Cocoa libraries provide some very powerful abstractions which can make your life and that of the people reading your code much easier. It pays off to know about them, so go ahead and browse the Cocoa documentation and hunt for those gems!

Apple's NSPredicate programming guide provides an in-depth documentation for NSPredicate containing all the things I didn't cover in this post.

NSPredicate also plays an important role when querying data in Core Data, something we will need to have a look at in one of the next blog posts. Stay tuned!

Code

The code for this post is available on my github page: get forking!

Samsung TV App SDK

The part about it that really sucks is the project management. How come tool vendors still think it's good style in 2010 to place the projects under Program Files\Samsung TV Apps SDK\Apps? There's no way to store your project files in a different location. The project open dialog just won't let you navigate to some other place:

Samsung TV App SDK Project Management Dialog

So I wanted to be able to edit my files on the Mac (using TextMate) while still using the good parts of the Samsung TV SDK (i.e., the Emulator).

I'm using VMware Fusion to run Windows 7 and the Samsung SDK (no, there is no version for the Mac). Most virtualization solutions offer a mechanism to share folders between the host and the guest OS. So I quickly set up folder sharing between my Mac and the guest OS, in this case Windows 7.

Now that I can see the project files both on the Mac and on the Windows machine, how can I make sure I can open the project in the Samsung TV SDK IDE? As I mentioned before, there's no way to tell the IDE to open projects form other locations than Program Files\Samsung TV Apps SDK\Apps!

After playing around with some more or less usable folder synchronization utilities, I came up with something most MacOS / Linux users should be familiar with: symbolic links! While symlinks have been around in Unix-like OSes for ages, they have been rarely known to Windows users for most of the time. However, starting with Windows NT, you can create symbolic links, hard links and junctions using a nifty little tool called mklink. Unfortunately, you're not allowed to run mklink if you're not an administrator. Using runas (which is Windows' equivalent of sudo), didn't help as the shared folders weren't visible to the admin user.

To cut a long story short, I found Symlinker, a UI tool that helps in creating symlinks on Windows. As it is a UI tool, you can run it with administrator privileges (by selecting Run as Admin from the context menu). Using a UNC path, you can create a symlink to a VMware shared folder and place this symlink in the location the Samsung IDE expects it to be.

Finally, I can edit my files on the Mac and run the app in Samsung's Emulator on my hosted Windows machine. And as the files on my Mac are mapped to the hosted Windows machine via a symlink, I do not suffer a synchronization lag - all files are updated instantaneously

tell application "name of your app" to launch

If you want to try this from the command line, osascript comes in handy:

osascript -e 'tell app "iTunes" to launch'

So far so good. Should be easy to do this from Java, shouldn't it? Turns out it's not so easy at all. Let's try this:

  String launchCmd = "osascript -e 'tell application \"iTunes\" to play'";
  process = Runtime.getRuntime().exec(launchCmd);
 
  BufferedReader bufferedReader = new BufferedReader(
    new InputStreamReader(process.getErrorStream()));
  while ((lsString = bufferedReader.readLine()) != null) {
    System.out.println(lsString);
  }

I'm not sure why, but it results in a nasty "0:1: syntax error: A unknown token can't go here. (-2740)" error message.

But there is another signature for Runtime.exec:

  String[] cmd = { "osascript", "-e",	"tell app \"iPhone Simulator\" to launch" };
  process = Runtime.getRuntime().exec(cmd);
 
  BufferedReader bufferedReader = new BufferedReader(
    new InputStreamReader(process.getErrorStream()));
  while ((lsString = bufferedReader.readLine()) != null) {
    System.out.println(lsString);
  }

... and this works out just fine!

I am going to try to post one tool recommendation per week - unless I am on vacation or speaking at a conference.

So without further ado, here is the first tool: Skitch!

Skitch is a tool that helps you to create screenshots. I need to create lots of screenshots: for documentation, to explain things to people by mail, and to annotate my bug reports. Of course, Mac OSX has several shortcuts to create screenshots, so what's the deal about Skitch?

First of all, Skitch allows you to view and edit your screenshot: press CMD+SHIFT+5, select the capture area and voilà - Skitch opens a window showing the screenshot just taken:

You can now use the tools at the left hand side of the Skitch window to highlight certain areas of the screenshot, which comes on handy if you're filing a bug report for your favorite open source tool.

When you're done with editing, you can either drag the image to another application (using the "drag me" tab at the bottom of the window) or you can post the image to the web. I have set up Skitch to use my Flickr account, so I can use the images in other tools right away (I am writing my blog posts in Mars Edit, which has a great Flickr integration, so I've got a complete tool chain here). Skitch supports a number of file formats (JPG, SVG, PDF, TIF, GIF, BMP and native Skitch), so you can select the desired file format before sending the image to the web or dragging it to another application.

All images are also stored in a local history, so if you need to go back to one of the screenshots you took a while ago, no problem with Skitch.

Skitch really has made my life on my Mac easier because it integrates with other tools (both online and offline) so well and because it eliminates many steps that made dealign with screenshots so cumbersome before.

You can download Skitch beta from http://www.skitch.com. You will be asked to sign up, however, both the download and the software will work without registering.

Happy screen shooting

However, the keyboard shortcuts are really strange. In case you didn't find out yourself, here are the most important ones:

• Send message - CMD+SHIFT+D (what does the D stand for? Deliver?)
• Apply rules to selected mails - CMD+ALT+L (L for ruLe ???)
• Select all messages in a thread - CMD+SHIFT+K (K for seleKt ???)
• Erase junk mail - CMD+ALT+J (now, this is obvious - J for Junk)

Apple developers, if you read this, please comment on the deeper meaning behind the shortcuts.

So I decided to give Fink a try. Here is what you need to do to get a CVS commandline client on your Mac:

1. Download Fink
2. Install Fink
3. Copy FinkCommander to your Applications folder
4. Start FinkCommander
5. In the search box, type "cvs"
   
   

6. Click on the "install binary package" button (it's the leftmost, with the blue plus sign)
7. In the lower pane, you can now watch Fink downloading and installing the CVS package.
   
   

8. Let's see if it works. Open a command line window and type "cvs":
   
   

Perfect!