Sci-Fi Hi-Fi

I haven’t posted any code to this site for awhile, and I think that’s a shame. So I’m going to remedy that tonight by publishing a handy little NSString category I wrote for use in my personal projects. This category, which I have christened “NSString+Templating,” arose out of two things: first, my frequent observation that a quick and dirty text templating mechanism is a very useful thing to have around; and, second, the huge appreciation I’ve recently gained for the “Key-Value Coding” design pattern.

So, where to start…

For those who aren’t familiar with Objective-C, the language has a rather unique feature in the notion of categories. Essentially, categories allow programmers to add functionality to existing classes—not by subclassing them, as is common in OO languages, but rather by grafting new methods onto existing classes. Thus, anyone can teach Cocoa’s standard string class (NSString) new tricks rather than creating a subclass called “MyFancyString” or something (this approach has its pros and cons, natch).

As it happens, my category adds one method to NSString (yes, I realize the selector is rather long, but I couldn’t think of a more succinct way of expressing what the thing does!):


- (NSString *) stringByParsingTagsWithStartDelimeter: (NSString *) startDelim endDelimeter: (NSString *) endDelim usingObject: (id) object;


The first two parameters (startDelim and endDelim) specify what will be recognized as a tag (I’ve been using “< $" and "/>”), and the last specifies an object that will provide the values to replace the tags.

This last bit is where the key-value coding comes into play. To determine the replacement value for a given tag, this code will use the tag’s “name” as a key to query the object. Thus, if the text contains a tag called “< $firstName/>,” the code will query the object using the “firstName” key. If the object responds to that method, the tag will be replaced with its return value (if not, the tag will be replaced with nothing).

The value returned from the key method must be coercible by NSString stringWithFormat: (using the ”%@” format string) into text. Fortunately (and surprisingly), this seems to include all of the “primitive types” (int, double, etc.) and any object that implements a “description:” method. In most cases, it “just works!”

(Update: It seems I was correct about the primitive types “just working,” but I was wrong about the reason why. See my addendum for more information.)

The source and header files (I didn’t put them into a project, since I’m using Xcode exclusively now) can be downloaded here:

- NSString+Templating.dmg (9k)

If anyone has any improvements or suggestions, I’d be interested in hearing them. I can think of a lot of interesting places this could be taken in the future (adding conditional branching, à la Rael Dornfest’s “Interpolate Fancy” Blosxom plugin, would be one example).

This entry was posted on Wednesday, October 22nd, 2003 at 7:43 am and is filed under Software. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.