java - KeyListener 方法 keyPressed 不起作用

Question

我正在尝试使用 java swing 制作一个小游戏，并且已经取得了很大的进展，但现在我的 KeyListener 没有使用 keyPressed 方法。

这是我的一些代码

public class Screen extends JPanel implements Runnable{

private static final int WIDTH = 300, HEIGHT = 300, RIGHT = 0, LEFT = 1, UP = 2, DOWN = 3, STILL = 4;
private Thread thread;
private boolean running = false;
private int direction = DOWN;

   public Screen() {
    setFocusable(true);
    addKeyListener(new Key());
    setPreferredSize(new Dimension(WIDTH, HEIGHT));
    start();
   }

   public void tick(){
     System.out.println(direction)  
   }

   public void start() {
    running = true;
    thread = new Thread(this, "Game Loop");
    thread.start();
}

   public void run() {
     while (running) {
       tick();
       repaint();
    }
}

   private class Key implements KeyListener{
    @Override
    public void keyTyped(KeyEvent e) {
        // TODO Auto-generated method stub

    }

    @Override
    public void keyPressed(KeyEvent e) {
        int key = e.getKeyCode();

        if (key == KeyEvent.VK_D) {
            direction = RIGHT;
        }

        if (key == KeyEvent.VK_A) {
            direction = LEFT;
        }

        if (key == KeyEvent.VK_W) {
            direction = UP;
        }

        if (key == KeyEvent.VK_S) {
            direction = DOWN;
        }

    }

    @Override
    public void keyReleased(KeyEvent e) {
        // TODO Auto-generated method stub

    }
   }
}

我正在查看控制台，并期望无论我按下什么按钮，输出都会从 3(向下)更改，但它从来没有这样做。一段时间后，我意识到 keyPressed 方法出了问题，我只是不知道是什么。

Answer 1

您不需要单独的“游戏循环”线程。 Swing 是事件驱动的。当用户按下计算机键盘上的某个键时，将调用您的按键监听器代码。

正如MadProgrammer在他的comment to your question中提到的那样

modifying he state of the UI from outside the context of the EDT may cause painting issues

换句话说，Swing 是 single threaded该线程称为事件调度线程 (EDT)，所有更改 GUI 的代码都应仅在 EDT 上执行。

下面的代码是一个独立的 Swing 应用程序，它显示可聚焦的 JPanel。当您按下相关键盘“方向”键之一(即 A 或 D 或 S 或 W)时，控制台会显示方向。按任何其他键都会导致方向仍然写入控制台。请注意，我使用了 enum对于各个方向，而不是整数常量。

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.WindowConstants;

public class Screen01 extends JPanel implements Runnable {
    public enum Directions {RIGHT, LEFT, UP, DOWN, STILL};

    private static final int WIDTH = 300, HEIGHT = 300;
    private Directions direction;

    public Screen01() {
        setPreferredSize(new Dimension(WIDTH, HEIGHT));
        setFocusable(true);
        addKeyListener(new Key());
    }

    public void run() {
        createAndDisplayGui();
    }

    private void createAndDisplayGui() {
        JFrame frame = new JFrame("Screen");
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.add(this, BorderLayout.CENTER);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    private class Key implements KeyListener {

        @Override
        public void keyTyped(KeyEvent e) {
            // Do nothing.
        }

        @Override
        public void keyPressed(KeyEvent e) {
            int key = e.getKeyCode();
            switch (key) {
                case KeyEvent.VK_D:
                    direction = Directions.RIGHT;
                    break;
                case KeyEvent.VK_A:
                    direction = Directions.LEFT;
                    break;
                case KeyEvent.VK_W:
                    direction = Directions.UP;
                    break;
                case KeyEvent.VK_S:
                    direction = Directions.DOWN;
                    break;
                default:
                    direction = Directions.STILL;
            }
            System.out.println("Direction = " + direction);
        }

        @Override
        public void keyReleased(KeyEvent e) {
            // Do nothing.
        }
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Screen01());
    }
}