This Week’s Program: Sep 18 - Sep 22

Happy Friday and Shanah Tovah! Last week I dipped my toe in the waters of OOP and this week I dove in like an apple into honey on a sweet new year.

If you recall, I’ve been stuck on this dilemma. I’m binding a bunch of Object-oriented C code to Racket. I want this Racket code to be idiomatic and follow typical Racket conventions. But Racket is a Lisp. It’s got lambda. How to I bridge the Object-oriented GObject C code with Racket and not lose something in translation?

I’ve been orbiting Racket’s own Object-oriented faculties for some time. Last week I finally had an a-ha moment and created a few different things: prop:gobject and gobject? — mechanisms for extending Racket types to behave like C pointers to GObjects, and the gobject<%> interface and the gobject% class to bridge the world of GObjects with Racket’s own OOP tools.

Why go through this whole rigmarole? Well eventually I know I’ll want to bring the Racket Drawing Toolkit into Overscan, and that uses a lot of Racket’s OOP stuff. Feels like it will reduce cognitive load to just stick with one OOP system.

This week, I jumped back into my GStreamer module and rewrote some of the code around GStreamer Elements to use some of this new thinking.

Oh, and in case you were wondering, I’m pronouncing “gobject” with a hard g, like “goblin”. Not “jee object” and not “jobject” (although that sounds funny too).

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The first thing I do is move the element% and element-factory% gi-objects to be classes that implement gobject<%>.

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Then I define methods on those. I also create two factory functions: element-factory%-find and element-factory%-make which are facades to the GStreamer factory functions. I also do a thing like get-metadata, which combines two different C function calls into one to provide a good interface. Objects can be good!

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I create a new macro. Originally called define-gobject-mixin, in this commit I rename it to make-gobject-delegate, which is a little bit more descriptive. This will create a mixin in Racket, and is designed to quickly define methods that should just delegate to the underlying C implementation. So:

(make-gobject-delegate get-name get-factory)

Will create a mixin around a gobject<%> that defines two methods called get-name and get-factory that delegates their arguments on to the gobject method. In short, it means I don’t have to type as much.

After this I write a bunch more documentation. Here’s make-gobject-delegate.

Next week, more GStreamer bindings with new OOP power.

🍎🍯 Mark