xplain

A set of articles and custom demos to explain how the X Window System works.

Demo framework / xserver.js

It's not just a set of jury-rigged DOM nodes and custom JavaScript. It's actually an accurate implementation of how the X server works, by implementing most of the same requests that the X server normally takes, and sending the same sorts of events back to clients. The clients written here actually look mostly like clients that use raw Xlib to do a lot of their work.

It's not 100% complete, of course. Instead of emulating a state-of-the-80s graphics API, I simply give clients the full power of <canvas> by giving them a context that's already clipped to the area of the front buffer they need to paint.

Some other optimizations or simplifications have been made either so that it seems "more JavaScript-y" or for the sake of my sanity. For instance, SelectInput is an actual request rather than being the only per-client window attribute, and the request takes a list of event strings rather than a mask. Instead of a "changed" mask like ConfigureWindow usually has, clients just leave the property out of the property bag when they send the request over.

I also commonly add custom requests and events simply to make my life easier. For instance, for the inspector, I added a X-CursorWindowChanged event so that it can track the cursor window, rather than have to have to select for Enter/ Leave events on every window through CreateNotify.

Grab semantics are slightly different (they're more like XI2 in that you can overwite an existing grab instead of having to use ChangeActivePointerGrab).

I could go on.

Since this is a mostly from-scratch implementation of a 40 year old codebase, there's probably plenty of bugs to be found. Please don't think this anything but a learning project and a demo framework. If you send me a pull request because you're basing your new VNC clone on this, I reserve rights to laugh at you and close it.

Getting around

Most of the server code is in src/server/server.js

All the demo code is in src/article-demos/

Credits

This has taken me the better part of a year, off-and-on, to write. I've found browser bugs, X server bugs, gotten my name in the HTML5 specification, and lots more.

As I say in the article, I didn't do this alone. There's a lot of people who helped me tremendously with this work. A big thanks goes to Keith Packard, Alan Coopersmith, Adam Jackson, Peter Hutterer, and Owen Taylor who all helped me figure a lot of the hairier parts of X11.