Key handling - Java-Gaming.org

I'm more-or-less done with my first game and I'm thinking about a second. This would use keys, and I want to ensure that the key handling code is thread-safe. The topic of handling keys is discussed a bit in the tips thread, but I don't want to pollute that one, hence this thread. It might be appropriate to link to this thread, or a post in it, from that one once the dust settles.

The issue I have is this: the standard key handling discussed there is basically to detect key pressed and released events and update a boolean. That's fine if my game logic only cares whether keys are up or down, but not so fine if I care about detecting key presses: e.g. for jumping. If the pressed and released events both occur before the logic call then the character won't jump and may meet a frustrating doom.

My idea for approaching this is to borrow a trick from lock-free synchronisation (e.g. lock-free single-producer single-consumer, which is essentially what we have here: an event thread which produces events and another thread which does updates and probably renders). I would be grateful for critiques and improvements on this implementation, and suggestions of cheaper ways to obtain the same result.

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public class Game4k extends Applet
{
   // Constants. You can make this slightly larger if it will compress better, but making it
   // smaller risks ArrayIndexOutOfBoundsExceptions in the event thread unless you add
   // bounds testing to processKeyEvent(...).
   private static final int NUM_KEYS = 0x10000;

   // Shared state. In the spirit of lock-free synchronisation, this is written
   // only by the event thread, and it advances values rather than setting /
   // resetting.
   private int[] keyEventCount = new int[NUM_KEYS];

   // This can be adapted if you do stuff properly rather than hackily, or if you use a JFrame
   @Override
   public void start()
   {
      // Set up event handling. Note: keyDown[key] and keyPressed[key] are 0
      // for false; any other value should be interpreted as true.
      enableEvents(KeyEvent.KEY_EVENT_MASK);
      int[] prevKeyEventCounts = new int[NUM_KEYS];
      int[] keyDown = new int[NUM_KEYS];
      int[] keyPressed = new int[NUM_KEYS];

      // Other setup...

      // Main game loop. Against, adapt as necessary.
      while (isActive())
      {
         // Handle events.
         for (int i = 0; i < NUM_KEYS; i++)
         {
            int eventCount = keyEventCount[i];
            // An odd number of events means that it's down. Simple!
            keyDown[i] = eventCount & 1;
            // The key was pressed since the last loop if the number of events
            // since then is at least two, by the pigeonhole principle, or if
            // the number of events since then is positive and the key is down.
            // Since we only need to catch the second case when delta == 1,
            // we can simplify the key-is-down test a bit.
            int delta = eventCount - prevKeyEventCounts[i];
            keyPressed[i] = (delta / 2) + (delta & eventCount);
            prevKeyEventCounts[i] = eventCount;
         }

         // Do game logic...
         // Render...
         // Sleep...
      }
   }

   @Override
   public void processKeyEvent(KeyEvent evt)
   {
      // The valid key event ids are 400, 401, 402.
      // Of those we care about 401 (KEY_PRESSED) and 402 (KEY_RELEASED).
      int id = evt.getID() & 3;
      if (id != 0)
      {
         keyEventCount[evt.getKeyCode()]++;
      }
   }
}

Ideally I'd like to debounce both keyDown and keyPressed as well, to work around the issue with some window managers (e.g. KDE) doing auto-repeat. However, there's no sure-fire way of telling whether multiple key presses come from the user or the WM, so I think I'll have to live with that issue.

		Key handling (Read 942 times)
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