Java游戏点击Canvas后失去焦点，按键控制失效问题求助

问题根源 #

你把KeyListener绑定在了JFrame上，当点击Canvas区域时，Canvas会获取输入焦点，JFrame失去焦点，导致后续的按键事件无法被JFrame的监听器捕获，直接表现为按键操作失效。如果点击Canvas时角色正在移动，此时释放按键的事件也无法被监听器接收，对应的XXXPressed布尔值会一直保持true，角色就会持续移动停不下来。

解决方案 #

方案1：将KeyListener绑定到Canvas（快速修复）

修改initFrame方法，把按键监听器绑定到Canvas（也就是当前的Game实例）上，同时确保Canvas可以接收焦点：

1. 替换initFrame中给JFrame加KeyListener的代码：

// 移除frame.addKeyListener(...)，替换为：
this.addKeyListener(new KeyAdapter() {
    @Override
    public void keyPressed(KeyEvent e) {
        Game.this.keyPressed(e);
    }

    @Override
    public void keyReleased(KeyEvent e) {
        Game.this.keyReleased(e);
    }
});

2. 在Game构造函数中添加焦点设置：

public Game() {
    sheet = new Spritesheet("/spritesheet.png");
    player = new BufferedImage[2];
    player[0] = sheet.getSprite(0,  0, 16, 16);
    player[1] = sheet.getSprite(16, 0, 16, 16);
    this.setPreferredSize(new Dimension(WIDTH*SCALE, HEIGHT*SCALE));
    // 新增以下两行
    this.setFocusable(true); // 允许Canvas接收输入焦点
    this.requestFocusInWindow(); // 启动时自动获取焦点
    initFrame();
    image = new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB);
}

方案2：使用KeyBindings（推荐的AWT/Swing最佳实践）

KeyBindings比KeyListener更灵活，可设置为只要窗口激活就能捕获按键，不受组件焦点变化影响。示例代码如下：

首先需要导入额外的类：

import javax.swing.AbstractAction;
import javax.swing.JComponent;
import java.awt.event.ActionEvent;

然后在Game构造函数中添加按键绑定：

public Game() {
    sheet = new Spritesheet("/spritesheet.png");
    player = new BufferedImage[2];
    player[0] = sheet.getSprite(0,  0, 16, 16);
    player[1] = sheet.getSprite(16, 0, 16, 16);
    this.setPreferredSize(new Dimension(WIDTH*SCALE, HEIGHT*SCALE));
    
    // 绑定W键按下
    this.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke(KeyEvent.VK_W, 0), "upPressed");
    this.getActionMap().put("upPressed", new AbstractAction() {
        @Override
        public void actionPerformed(ActionEvent e) {
            upPressed = true;
            playerDirectionY = -1;
        }
    });
    // 绑定W键释放
    this.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke(KeyEvent.VK_W, 0, true), "upReleased");
    this.getActionMap().put("upReleased", new AbstractAction() {
        @Override
        public void actionPerformed(ActionEvent e) {
            upPressed = false;
        }
    });
    
    // 绑定S键按下/释放
    this.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke(KeyEvent.VK_S, 0), "downPressed");
    this.getActionMap().put("downPressed", new AbstractAction() {
        @Override
        public void actionPerformed(ActionEvent e) {
            downPressed = true;
            playerDirectionY = 1;
        }
    });
    this.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke(KeyEvent.VK_S, 0, true), "downReleased");
    this.getActionMap().put("downReleased", new AbstractAction() {
        @Override
        public void actionPerformed(ActionEvent e) {
            downPressed = false;
        }
    });
    
    // 绑定A键按下/释放
    this.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke(KeyEvent.VK_A, 0), "leftPressed");
    this.getActionMap().put("leftPressed", new AbstractAction() {
        @Override
        public void actionPerformed(ActionEvent e) {
            leftPressed = true;
            playerDirectionX = -1;
        }
    });
    this.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke(KeyEvent.VK_A, 0, true), "leftReleased");
    this.getActionMap().put("leftReleased", new AbstractAction() {
        @Override
        public void actionPerformed(ActionEvent e) {
            leftPressed = false;
        }
    });
    
    // 绑定D键按下/释放
    this.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke(KeyEvent.VK_D, 0), "rightPressed");
    this.getActionMap().put("rightPressed", new AbstractAction() {
        @Override
        public void actionPerformed(ActionEvent e) {
            rightPressed = true;
            playerDirectionX = 1;
        }
    });
    this.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke(KeyEvent.VK_D, 0, true), "rightReleased");
    this.getActionMap().put("rightReleased", new AbstractAction() {
        @Override
        public void actionPerformed(ActionEvent e) {
            rightPressed = false;
        }
    });
    
    // 同理可添加上下方向键的绑定
    initFrame();
    image = new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB);
}

添加完这些绑定后，记得移除原来给JFrame添加的KeyListener代码，避免重复处理事件。

内容的提问来源于stack exchange，提问作者Gabriel