Re: Keyboard state (instead of keyboard events)

From:
"John B. Matthews" <nospam@nospam.com>
Newsgroups:
comp.lang.java.gui
Date:
Wed, 31 Dec 2008 18:18:58 -0500
Message-ID:
<nospam-0538BE.18185831122008@nntp.motzarella.org>
In article <495bc2ba$0$25409$b9f67a60@news.newsdemon.com>,
 Knute Johnson <nospam@rabbitbrush.frazmtn.com> wrote:

Philipp Gressly wrote:

Philipp Gressly wrote:

Hello everybody

I am programming a "moon-lander" and want to check every 100 ms if
a certain key is pressed. The keyboard events (key-pressed,
key-released and key-typed) are not helpful, because the operating
System (linux im my case) generates key-releases and key-presses
at its own (depending on the "key repeat speed").

Is there a command to satisfy the following interface easily?

public interface KeyState {
  boolean isKeyDown(char keyCode);
}


With all your help, I have implemented the code below.

It works in 99%, because the gnome "keyPressed.getWhen()" has
mostly the same value as a previous "keyReleased.getWhen()" in case
of the "key-repeat-sequence". Very rarely the below mentioned code
reports "down: false", but it should be "down: true". It would be
interesting to have some feedback about other operating systems.

Thanks

import javax.swing.JFrame;
import java.awt.event.*;

/**
 * @author Philipp Gressly (phi@gressly.ch)
 * after a code from Luther :
http://forums.sun.com/thread.jspa?threadID=698156
 */

public class IgnoreRepeats extends JFrame implements KeyListener,
Runnable {

    private long oldWhen = 0L;
    public boolean down;

    /* starter */
    public static void main(String[] _) {
        new IgnoreRepeats("Test Frame").top(); }

    public IgnoreRepeats(String name){
        super(name); }

    private void top() {
        super.addKeyListener (this) ;
        super.setDefaultCloseOperation (EXIT_ON_CLOSE);
        setSize (300, 300) ;
        setVisible (true) ;
        new Thread(this). start(); }

    public void run() {
        while (true) {
            System.out.println("down: " + down);
            try {
                Thread.currentThread().sleep(40);

                    Thread.sleep(40); // Class reference OK here

            } catch (InterruptedException e) { }
        }
    }

    public void keyReleased(final KeyEvent e) {
        if (oldWhen == e.getWhen()) return;
        down = false; }

    public void keyPressed(final KeyEvent e) {
        long now = e.getWhen();
        if (oldWhen == now) return;
        oldWhen = now;
        down = true; }

    /* ignore */
    public void keyTyped(KeyEvent e) {}

} // end "IgnoreRepeats"


Philipp : Works well on MacOS 10.5.6, Java 1.5, PPC. I get no false
negatives with limited testing at various rates. I'm wary of relying on
this sort of undocumented behavior, but I think I see why you're doing
it. In effect, auto-generated released-pressed pairs appear to share the
same time stamp. Would testing "delta < 1" be better than testing for
equality?

Throw out the getWhen() part, you don't need it. Just set the flag when
the key is pressed, and clear it when it's released.


Knute: On MacOS (Darwin/BSD) and (IIUC) on Linux, automatic key repeat
generates continual triplets: pressed, typed, released, [pressed, typed,
released]. This appears not to be the case on Windows.

--
John B. Matthews
trashgod at gmail dot com
http://home.roadrunner.com/~jbmatthews/

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