Make method that returns when JButton is pressed

问题

I need to make a method that returns only when a JButton is pressed. I have a custom JButton class

public class MyButton extends JButton {


   public void waitForPress() {
       //returns only when user presses this button
   }

}

and I want to implement waitForPress. Basically, the method should only return when the user presses the button with their mouse. I have achieved similar behavior for JTextField (to return only when user presses Space):

public void waitForTriggerKey() {
        final CountDownLatch latch = new CountDownLatch(1);
            KeyEventDispatcher dispatcher = new KeyEventDispatcher() {
                public boolean dispatchKeyEvent(KeyEvent e) {
                    if (e.getID() == KeyEvent.KEY_PRESSED && e.getKeyCode() == KeyEvent.VK_SPACE) {
                        System.out.println("presed!");
                        latch.countDown();
                    }
                    return false;
                }
            };
            KeyboardFocusManager.getCurrentKeyboardFocusManager().addKeyEventDispatcher(dispatcher);
            try {
                //current thread waits here until countDown() is called (see a few lines above)
                latch.await();
            } catch (InterruptedException e1) {
                e1.printStackTrace();
            }  
            KeyboardFocusManager.getCurrentKeyboardFocusManager().removeKeyEventDispatcher(dispatcher);

    }

but I would like to do the same thing with JButton.

In advance: Please, if you wish to comment saying that this is not a good idea and that one should simply wait for actionPerformed event on a JButton and then do some action, please realize I already know that and have a good reason for doing what I'm asking here. Please try to only help with what I've asked. Thanks!!

In advance: Please, also realize that implementing actionPerformed also will not directly solve the problem. Because the code will progress even without the button being pressed. I need the program to stop, and only return when the button has been pressed. Here is a terrible solution if I were to use actionPerformed:

public class MyButton extends JButton implements ActionPerformed {
   private boolean keepGoing = true;

   public MyButton(String s) {
       super(s);
       addActionListener(this);
   }

   public void waitForPress() {
       while(keepGoing);
       return;
   }

   public void actionPerformed(ActionEvent e) {
       keepGoing = false;
   }

}

回答1:

As you asked for an implementation with a mutex, here's what it would be like. I'm using an ActionListener though, but there's no busy wait in it. If that isn't what you desire, you atleast saw what Burkhard meant ;)

public class MyButton extends JButton implements ActionListener
{
    private Semaphore sem = new Semaphore(1);

    public MyButton(String text) throws InterruptedException
    {
        super(text);
        addActionListener(this);
        sem.acquire();
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        sem.release();
    }

    public void waitForPress() throws InterruptedException {
        sem.acquire();
        //do your stuff
        sem.acquire();
        //or just
        //waitForPress()
            //if you dont want it to end.
    }

    public static void main(String[] args) throws InterruptedException {
        JFrame frame = new JFrame();
        MyButton m = new MyButton("test");
        frame.add(m);
        frame.pack();
        frame.setVisible(true);
        m.waitForPress();
        //another time, if you only want it to block twice
        m.waitForPress();
    }
}

But I don't think this is a clean approach, but it doesn't consume CPU-time like a while(isStatementTrue)-implementation.
An important thing here is: you're blocking the main thread with m.waitForPress() but as you wrote you're quite experienced and you know how to handle that.

回答2:

For what it's worth, here is how you can do it with wait() and notify() but yet I feel that there is a deeper problem here. I would not consider this as a satisfying solution:

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.SwingUtilities;

public class TestBlockingButton {

    boolean clicked = false;
    private Object toNotify;

    private void initUI() {
        JFrame frame = new JFrame(TestBlockingButton.class.getSimpleName());
        JButton button = new JButton("Click me");
        button.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                clicked = true;
                if (toNotify != null) {
                    synchronized (TestBlockingButton.this) {
                        toNotify.notify();
                    }
                }
            }
        });
        frame.add(button);
        frame.setSize(400, 400);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public void waitForProcess() {
        toNotify = this;
        while (!clicked) {
            synchronized (this) {
                try {
                    wait();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }
        System.out.println("continuing work");
    }

    public static void main(String[] args) {
        final TestBlockingButton test = new TestBlockingButton();
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                test.initUI();
            }
        });
        ScheduledExecutorService pool = Executors.newScheduledThreadPool(1);
        pool.execute(new Runnable() {
            @Override
            public void run() {
                System.out.println("I was doing something and now I will wait for button click");
                test.waitForProcess();
                System.out.println("User has now cliked the button and I can continue my work");
            }
        });

    }
}

来源：https://stackoverflow.com/questions/14187235/make-method-that-returns-when-jbutton-is-pressed