Question

I want to cause the “main thread” (the thread started which runs main()) to do some work from the actionPerformed() method of a button’s ActionListener, but I do not know how to achieve this.

A little more context:

I am currently programming a 2D game using Swing (a flavour of Tetris).
When the application starts, a window opens which displays the main menu of the game.
The user is presented several possibilities, one of them is to start the game by pushing a “Start” button, which causes the game panel to be displayed and triggers the main loop of the game.

To be able to switch between the two panels (that of the main menu and that of the game), I am using a CardLayout manager, then I can display one panel by calling show().
The idea is that I would like my start button to have a listener that looks like this:

public class StartListener implements ActionListener {
    StartListener() {}
    public void actionPerformed(ActionEvent e) {
        displayGamePanel();
        startGame();
    }
}

but this does not work because actionPerformed() is called from the event-dispatch thread, so the call to startGame() (which triggers the main loop: game logic update + repaint() call at each frame) blocks the whole thread.

The way I am handling this right now is that actionPerformed() just changes a boolean flag value: public void actionPerformed(ActionEvent e) {
startPushed = true;
}
which is then eventually checked by the main thread:

while (true) {
    while (!g.startPushed) {
        try { 
            Thread.sleep(100); 
        } catch (Exception e) {}
    }
    g.startPushed = false;
    g.startGame();
}

But I find this solution to be very inelegant.

I have read the Concurrency in Swing lesson but I am still confused (should I implement a Worker Thread – isn’t that a little overkill?). I haven’t done any actual multithreading work yet so I am a little lost.

Isn’t there a way to tell the main thread (which would be sleeping indefinitely, waiting for a user action) “ok, wake up now and do this (display the game panel and start the game)”?.

Thanks for your help.

EDIT:
Just to be clear, this is what my game loop looks like:

long lastLoopTime = System.currentTimeMillis();
long dTime;
int delay = 10;
while (running) {
    // compute the time that has gone since the last frame
    dTime = System.currentTimeMillis() - lastLoopTime;
lastLoopTime = System.currentTimeMillis();

    // UPDATE STATE
    updateState(dTime);
    //...

    // UPDATE GRAPHICS
    // thread-safe: repaint() will run on the EDT
    frame.repaint()

    // Pause for a bit
    try { 
    Thread.sleep(delay); 
    } catch (Exception e) {}
}

Answer 1

you should be using a SwingWorker this will execute the code in doInBackground() in a background thread and the code in done() in the EDT after doInBackground() stops