帆布子类未接收关键

Question

我有一个JFrame的子类，其中包含一个DrawingCanvas的实例（java.awt.Canvas的子类），我在DrawingCanvas中实现了KeyListener，但它没有收到任何关键事件。

我还使用setFocusable()方法使Canvas可以集中注意力，但是我仍然无法接收关键事件。

drawdcanvas.java

class DrawingCanvas extends Canvas implements TimePulseListener, KeyListener { 
    ...
    ...
    @Override
    public void keyPressed(KeyEvent e) {
        switch(e.getKeyCode()) {
            case KeyEvent.VK_KP_DOWN:
            case KeyEvent.VK_KP_UP:
                xFlag = false;
                yFlag = true;
                reverseY();
                break;
            case KeyEvent.VK_KP_LEFT:
            case KeyEvent.VK_KP_RIGHT:
                xFlag = true;
                yFlag = false;
                reverseX();
                break;
            default:
                System.out.println(" * NOT an action key ");
                break;
        }
    }
    @Override
    public void keyReleased(KeyEvent e) {
    }
    @Override
    public void keyTyped(KeyEvent e) {
    }
    ...
    ...
}

我在这里缺少什么？

Answer 1

您可能必须像下面更改代码。

    public DrawingCanvas() {
        super();
        this.addKeyListener(this);
    }
    @Override
    public void keyPressed(KeyEvent e) {
        switch(e.getKeyCode()) {
            case KeyEvent.VK_DOWN:
            case KeyEvent.VK_KP_DOWN:
            case KeyEvent.VK_UP:
            case KeyEvent.VK_KP_UP:
                xFlag = false;
                yFlag = true;
                reverseY();
                break;
            case KeyEvent.VK_LEFT:
            case KeyEvent.VK_RIGHT:
            case KeyEvent.VK_KP_LEFT:
            case KeyEvent.VK_KP_RIGHT:
                xFlag = true;
                yFlag = false;
                reverseX();
                break;
            default:
                System.out.println(" * NOT an action key " + e.getKeyChar());
                break;
        }
    }

Answer 2

正如 @jägermeister在评论中指出的那样，我确实错过了在另一类中注册KeyListener，并补充了解决我的问题。

class DrawThreads extends JFrame implements ActionListener {
    ...
    public DrawThreads(int width, int height) {
        ...
        drawingCanvas = new DrawingCanvas();
        drawingCanvas.addKeyListener(drawingCanvas); // register listener
        drawingCanvas.setFocusable(true);
        ...
    }
    ...
}