Keyboard input for a game in Java

Question

I'm writing a game in Java, right now it's Swing + JOGL - a JFrame with a GLCanvas.

I handle input using keyPressed etc. events (jframe.addKeyListener(...)) and it doesn't seem to work properly:

• when I have 3+ keys down at the same time, they don't register properly - apparently this is keyboard's fault, I have to find an alternate control scheme.
• after the window loses, then regains focus, input stops working completely...

What am I doing wrong?

Is there a better way of handling keyboard input in Java?

(I'd rather not switch to another library, like LWJGL... unless I have no choice).

Answer 1

Some of the tips in this article on global event listeners include catching key events with the KeyboardFocusManager, and might help with coming back from losing focus.

Regarding the 3+ keys, that's going to be tricky since the KeyEvent accounts for modifiers but not for multiple (regular) keys in its API. You might have to manage the press state yourself, in that if you get a KEY_PRESSED you store that key and build up the set of keys that are currently pressed. But if you simply don't get events at all when 3 or more keys are pressed, I'm not sure there's much you can do.

Edit: Also, the JGame library has a JOGL target. Looking at how it handles key events might help. I know it can handle at least 2 keys simultaneously.

Answer 2

To keep dependencies down, I would go for the "built-in" keyboard handling. It works just fine if you know what your doing. I'll paste some code from my games:

It handles key-repeats with custom repeat-delay / rate and has no issues with in which component keyboard focus lies.

public class GameKeyController implements KeyEventDispatcher {

    private final int MAX_REPEAT_RATE = 100; // Hz

    private final LocalGame game;
    private final GamingContext context;
    private final Account account;
    Timer keyRepeatTimer;
    Map<Move, TimerTask> repeatingTasks = new EnumMap<Move, TimerTask>(Move.class);

    public GameKeyController(LocalGame game, GamingContext context,
            Account account) {
        this.game = game;
        this.context = context;
        this.account = account;
    }


    public boolean dispatchKeyEvent(KeyEvent e) {

        assert EventQueue.isDispatchThread();

        int kc = e.getKeyCode();

        if (e.getID() == KeyEvent.KEY_PRESSED) {

            // If repeat is activated, ignore KEY_PRESSED events.
            // Should actually not occur, since KEY_RELEASED *should* have been
            // intercepted since last KEY_PRESSED.
            if (kc == account.getInt(KC_MOVE_LEFT)  && !isRepeating(LEFT))      move(LEFT);
            if (kc == account.getInt(KC_MOVE_RIGHT) && !isRepeating(RIGHT))     move(RIGHT);
            if (kc == account.getInt(KC_SOFT_DROP)  && !isRepeating(SOFT_DROP)) move(SOFT_DROP);

            // Regular moves
            if (kc == account.getInt(KC_ROT_CW))        move(ROT_CW);
            if (kc == account.getInt(KC_ROT_CW2))       move(ROT_CW);
            if (kc == account.getInt(KC_ROT_CCW))       move(ROT_CCW);
            if (kc == account.getInt(KC_ROT_CCW2))      move(ROT_CCW);
            if (kc == account.getInt(KC_HARD_DROP))     move(HARD_DROP);
            if (kc == account.getInt(KC_SLIDE_DROP))    move(SLIDE_DROP);
            if (kc == account.getInt(KC_FULL_LEFT))     move(FULL_LEFT);
            if (kc == account.getInt(KC_FULL_RIGHT))    move(FULL_RIGHT);
            if (kc == account.getInt(KC_HOLD))          move(HOLD);

            if (kc == account.getInt(KC_SEND_TO_ME))    useSpecial(0);
            if (kc == account.getInt(KC_SEND_TO_1))     useSpecial(1);
            if (kc == account.getInt(KC_SEND_TO_2))     useSpecial(2);
            if (kc == account.getInt(KC_SEND_TO_3))     useSpecial(3);
            if (kc == account.getInt(KC_SEND_TO_4))     useSpecial(4);
            if (kc == account.getInt(KC_SEND_TO_5))     useSpecial(5);
            if (kc == account.getInt(KC_SEND_TO_6))     useSpecial(6);
            if (kc == account.getInt(KC_SEND_TO_7))     useSpecial(7);
            if (kc == account.getInt(KC_SEND_TO_8))     useSpecial(8);
            if (kc == account.getInt(KC_SEND_TO_9))     useSpecial(9);


            // Reported bug: Key repeat "lags on releases", that is, the key
            // continues to repeat a few ms after it has been released.
            // The following two lines gives one "upper" approximation of
            // when someone really wants to release the key.
            if (kc == account.getInt(KC_MOVE_RIGHT)) stopRepeating(LEFT);
            if (kc == account.getInt(KC_MOVE_LEFT)) stopRepeating(RIGHT);
        }


        if (e.getID() == KeyEvent.KEY_RELEASED) {
            if (kc == account.getInt(KC_MOVE_LEFT)) stopRepeating(LEFT);
            if (kc == account.getInt(KC_MOVE_RIGHT)) stopRepeating(RIGHT);
            if (kc == account.getInt(KC_SOFT_DROP)) stopRepeating(SOFT_DROP);
        }

        return false;
    }


    private synchronized void stopRepeating(Move m) {
        if (!isRepeating(m))
            return;
        repeatingTasks.get(m).cancel();
        repeatingTasks.remove(m);
    }


    private synchronized boolean isRepeating(Move m) {
        return repeatingTasks.get(m) != null;
    }


    private synchronized void move(Move move) {
        assert EventQueue.isDispatchThread();

        context.notIdleSinceStart();

        PlayfieldEvent pfe = game.move(move);

        // Fake wall kicks
        if ((move == ROT_CW || move == ROT_CCW) &&
                account.getBool(USE_FAKE_WALL_KICKS) && !pfe.pfChanged) {

            // Try RIGHT and ROT, then LEFT and ROT.
            Playfield pf = game.getPlayfield();
            if (pf.isFakeRotPossible(true, move == ROT_CW)) {
                game.move(RIGHT);
                game.move(move);
            } else if (pf.isFakeRotPossible(false, move == ROT_CW)) {
                game.move(LEFT);
                game.move(move);
            }
        }


        // Initiate key repeats
        int delay = account.getInt(KEY_REPEAT_DELAY);
        int rate = account.getInt(KEY_REPEAT_RATE);
        if (delay > 0 && rate > 0 && isRepeatable(move))
            startRepeating(move);
    }


    private boolean isRepeatable(Move m) {
        return m == LEFT || m == RIGHT || m == SOFT_DROP;
    }


    private synchronized void startRepeating(Move move) {
        assert EventQueue.isDispatchThread();

        if (isRepeating(move))
            return;

        long delay = account.getInt(KEY_REPEAT_DELAY);
        int rate = account.getInt(KEY_REPEAT_RATE);

        Move repeatMove = move;
        if (rate >= MAX_REPEAT_RATE) {
            rate = MAX_REPEAT_RATE;
            repeatMove = move == LEFT      ? FULL_LEFT
                       : move == RIGHT     ? FULL_RIGHT
                       : move == SOFT_DROP ? SLIDE_DROP
                       : null; // not a repeatable move!
        }

        long period = (long) (1000.0 / rate);

        if (move == SOFT_DROP)
            delay = period;

        final Move m = repeatMove;
        TimerTask tt = new TimerTask() {

            // Should only be executed by keyRepeatTimer thread.
            public void run() {

                // Remove the if-branch below and you get old school GB behavior
                // With the if-branch it's more TDS-ish.
                // TODO: Make this depend on an account-setting
                if (m == SOFT_DROP && game.getPlayfield().isTetOnSurface()) {
                    stopRepeating(SOFT_DROP);
                    return;
                }

                game.move(m);

                // Attempt to make it more responsive to key-releases.
                // Even if there are multiple this-tasks piled up (due to
                // "scheduleAtFixedRate") we don't want this thread to take
                // precedence over AWT thread.
                Thread.yield();
            }
        };
        repeatingTasks.put(move, tt);
        keyRepeatTimer.scheduleAtFixedRate(tt, delay, period);
    }


    public synchronized void init() {
        if (!isInited()) {
            keyRepeatTimer = new Timer("Key Repeat Timer");
            KeyboardFocusManager.getCurrentKeyboardFocusManager().addKeyEventDispatcher(this);
        }
    }


    public synchronized boolean isInited() {
        return keyRepeatTimer != null;
    }


    public synchronized void uninit() {
        if (isInited()) {
            KeyboardFocusManager.getCurrentKeyboardFocusManager().removeKeyEventDispatcher(this);

            keyRepeatTimer.cancel();
            keyRepeatTimer = null;
        }
    }


    private void useSpecial(int target) {
        context.notIdleSinceStart();
        context.useSpecial(target);
    }

}