Threads -- Waiting for events

[Ray Muirhead]

Hi there

Got a slight problem (maybe) here with threads and waiting for action from a player in a game. The play() method keeps cycling through the NPCs, checking if each is faster than the player. If the player has not acted yet that round and is faster, the waitForPlayer() method is called until the player has acted (another method changes playerDone to true). What I am curious about is whether there is a way to make the main() thread just sit there and wait for an event to come in signalling that the player has acted, the same way the Swing event dispatch thread works? That way waitForPlayer() could just chill out instead of running through a while loop until the boolean is changed, and return when an event has come in signalling the players action. There seem to be some serious holes in my logic here, but I'm working on this myself so I don't have anyone to bounce ideas off. Kind of stuck in a vacuum.

  
 
    private ArrayList<NonPlayingCharacter> theCast ; // set by constructor
    private boolean playerDone = false ;
 
    private boolean waitForPlayer()
    {
        return !playerDone ;
    }
 
 
    public void play()
    {
        boolean playerCanAct = true ;
        boolean finished = false ;
 
        int currentNPC = 0 ;
        int lastNPC = theCast.size() - 1 ;
 
        while ( !finished )
        {
            while ( currentNPC <= lastNPC )
            {
                NonPlayingCharacter npc = theCast.get( currentNPC ) ;
                // check if the NPC should move before the player
                if ( !playerCanAct ||
                     npc.getBasicSpeed() > player.getBasicSpeed() ||
                     ( npc.getBasicSpeed() ==  player.getBasicSpeed() &&                   
                       npc.getDX() > player.getDX() ) )
                {
                    npc.act() ;
                    currentNPC++ ;
                }
                else  // it's the player's turn, wait for an event from the  
                         gui or player class (undecided)
                {
                    System.out.println( "It's the player's turn" ) ;
                    // wait for an action to be sent in signalling that the 
                       player has finished acting
                    while( waitForPlayer() )
                    {
                        // twiddle thumbs
                    }
                    // once that player has acted, they cannot act again 
                       until the next round
                    playerCanAct = false ;
                }
            }
            playerCanAct = true ;
            currentNPC = 0 ;
 
            if ( castReshuffled )  // if any character's speed values have 
                                      changed, re-order them
                {
                    sortTheCast() ;
                }
        }
    }
 

Muchas gracias!

[ September 17, 2004: Message edited by: Ray Muirhead ]
[ September 17, 2004: Message edited by: Ray Muirhead ]

[Ernest Friedman-Hill]

In a nutshell, the main loop can call Object.wait() on some object, and the player can call notify() on that same object. wait() sleeps forever or until some other thread calls notify() on that object. This is exactly what you're looking for.

You could read Sun's tutorial on threading, especially this part, to learn about this; c'mon back with questions.

[Ray Muirhead]

Thank you Ernest, I thought there would be something in the API, but I just couldn't find it, and it wasn't in Head First Java.

Cheers!

[Ray Muirhead]

Feeling a little humbled -- this threads stuff is hard! Got it working ok, but still figuring out how to synch it up tight so that the player can only act once and only act in her turn.

Thanks for the pointer Earnest.