为什么 keyPressed 方法不会被调用？

Question

我正在为一个项目用 Java 制作一个格斗游戏，并试图让图片移动并在面板上重新绘制，以响应键盘（keyEvents）。我试图通过在 keyPressed 方法中设置一个开关，同时将 keyListener 添加到面板来实现此目的。我一直在遵循我的 Java 书中的一个示例，我编写的代码几乎相同，但它不起作用。

我真正想知道的是为什么它似乎根本没有对 keyEvents 做出反应。程序编译得很好，但什么也没发生。我不知道出了什么问题。如果我创建一个，它不会到达 keyPressed() 方法中的断点，如果我将其放入其中，它也不会执行 println() 。因此 keyPressed() 方法根本没有反应。我还测试并确保面板可聚焦，因此我确定它具有键盘焦点。

public class MovePanel extends JPanel implements KeyListener {
    private ImageIcon currentImage, facingLeft, facingRight;
    private int position;
    private final int MOVEMENT;
    private GameFrame gameFrame;
    private URL lefturl, righturl;

    public MovePanel(GameFrame gameFrame) {
        // Taking in a gameFrame to be able to swap the active panel 
        // (not really relevant).
        this.gameFrame = gameFrame;

        // Adding the key listener here.
        addKeyListener(this);

        // These are just the Images I'm using to test. 
        // Trying to get it to swap from one to the other.
    lefturl = getClass().getResource("/Images/facingLeft.jpg");
    righturl = getClass().getResource("/Images/facingRight.jpg");

    facingLeft = new ImageIcon(lefturl);
    facingRight = new ImageIcon(righturl);

    currentImage = facingLeft;
    position = 50;
    MOVEMENT = 30;

    setBackground(Color.red);
    setPreferredSize(new Dimension(600,300));

        // Calling this method so that the panel will react 
        // to the keyboard without having to be clicked.
    setFocusable(true);
    }

    // This is just the paintComponent method which works fine to paint the image
    // when starting the game.
    public void paintComponent(Graphics page) {
    super.paintComponent(page);
    currentImage.paintIcon(this, page, position, 170);
    }

    // No matter what I try to do inside the keyPressed method
    // it doesnt seem to react at all.
    public void keyPressed(KeyEvent e) {

        // This switch is to make the method react accordingly to the keys pressed.
        switch (e.getKeyCode()) {
        case KeyEvent.VK_LEFT: 

            // Here I'm changing the "active" image and the position 
            // by changing the position variable which is used 
            // to determine the x placement of the image.
            // This case is suppused to react if the left arrow key is pressed.
            currentImage = facingRight;
            position -= MOVEMENT;
            break; 
        case KeyEvent.VK_RIGHT: 
            currentImage = facingRight;
            position += MOVEMENT;
            break; 

        // This case is to exit to the menu when escape is pressed.
        case KeyEvent.VK_ESCAPE: 
            gameFrame.setMenuPanelActive();
            break; 
        }
        // After reacting to any of the proper keys pressed 
        // I'm trying to repaint which will use the
        // paintComponent method to paint the new image in its new position.
        repaint();
    }
    // I have empty definitions for the other 
    // implemented methods but won't be posting them.
}

有谁知道为什么这不起作用？为什么 keyPressed() 方法没有反应？

Answer 1

我没有看到如下代码，

您应该调用下面的行，在其中创建 MovePanel 实例

 MovePanel.requestFocus();      // Give the panel focus.



public class demo extends JFrame 
{
    MovePanel  panel;

    public demo () 
    {
        panel= new MovingTextPanel();
        this.getContentPane().setLayout(new BorderLayout())
        this.setTitle("Demo");
        this.pack();
        panel.requestFocus();      // Give the panel focus.
    }
}

在您的 MovePanel 中将 setFocusable 添加为 true

 public MovePanel(GameFrame gameFrame) {

        this.setFocusable(true);   // Allow this panel to get focus.
        // Adding the key listener here.
        addKeyListener(this);

更多跟踪

- Characters (a, A, #, ...) - handled in the keyTyped() listener.
- Virtual keys (arrow keys, function keys, etc) - handled with keyPressed() listener.
- Modifier keys (shift, alt, control, ...) - Usually their status (up/down) is tested by calls in one of the other listeners, rather than in keyPressed().

  public void keyTyped(KeyEvent e) {
    System.out.println(e.toString());
  }
  public void keyPressed(KeyEvent e) {
   System.out.println(e.toString());
  }
  public void keyReleased(KeyEvent e) {
    System.out.println(e.toString());
  }