VK_VERYLONGKEYNAME (on reflection in Clojure)

So, I'm still writing a game. As it happens with games, I rely on keypresses for the user to tell me what he wants to do. That made me fall back on Java's KeyListener/KeyAdapter facilities.

The basic idea is that I get callbacks with information about the pressed key as argument (WARNING: GROSS OVERSIMPLIFICATION.), among other things the keycode, which is an integer. Now, I'm supposed to dispatch it using constants defined in KeyListener/KeyAdapter/KeyEvent/probably some other places.

Problem?

It's a lot of typing in Clojure. Especially since each constant is called VK_(something). I had large maps looking like that:

{ KeyEvent/VK_UP [some function]

  KeyEvent/VK_DOWN [some function]

  KeyEvent/VK_RIGHT [some function]

... }

You get the idea. Since I have several keymaps, that gets annoying quickly. So I started to define aliases for some frequently used ones in a seperate "keys" namespace. Now I only had to write, for example, "keys/up" instead of "KeyAdapter/VK_UP". That's a lot of saved space (and some gained readability).

But that introduced something else that annoyed me: Some of the more rare keys were still written out fully, and it created an aesthetic discrepancy, which upset my minor OCD. Besides, having to redefine tons of variables by hand isn't particularly elegant in itself.

(Disclaimer: I am aware of clojure.contrib/import-static. But I don't rely on that library enough to warrant including it in my project just for this.)

Sure enough, there is a better way.
The first nugget of information I needed was... how to get a list of all fields in a Java class?

Of course, Stack Overflow helped me there. It's

(clojure.reflect/reflect java.awt.event.KeyEvent)

It spews out a LOT of things like this:

{:bases #{java.awt.event.InputEvent},

 :flags #{:public},

 :members

 #{{:name VK_F17,

    :type int,

    :declaring-class java.awt.event.KeyEvent,

    :flags #{:static :public :final}}

   {:name VK_DEAD_DOUBLEACUTE,

    :type int,

    :declaring-class java.awt.event.KeyEvent,

    :flags #{:static :public :final}}

...}

This is the abridged pretty-print output of the stuff. We know all the VK_ fields are keycodes. Everything else falls in place itself, basically. Most of what I need I covered in a previous post already.

We want to:
1. From the list of members, filter out everything that is not a keycode.
2. Define the aliases over it.

Simple enough.

(->> (clojure.reflect/reflect java.awt.event.KeyEvent)

     (:members) (map :name) (filter #(.startsWith (str %) "VK_")))

This yields us a list of symbols (which I am grateful for, as it simplifies the next step) of all fields that contain keycodes. Now, as in my earlier post, we doseq over the list and define the aliases via eval in the loop. I also wanted the VK_ dropped and the entire name downcased, but that's just my whim.

(doseq [sym (->> (clojure.reflect/reflect java.awt.event.KeyEvent)

            (:members) (map :name) (filter #(.startsWith (str %) "VK_")))]

  (eval `(def ~(symbol (.toLowerCase (drop 3 (str sym))))

           (. java.awt.event.KeyEvent ~sym))))

That's it. Now, there are vars like "a", "up" and so on that hold the corresponding keycodes. I put the entire thing into a new namespace (game.keys) that I require like that:

(ns game.some-module

  (:require [game.keys :as keys])

Another nonproblem solved! :D