java - KeyListener 无法与 PaintComponent 一起使用

Question

我的程序应该等待按下左箭头键或右箭头键，然后更改一个值，以便下次更新 PaintComponent 时，屏幕看起来会有所不同。但是，当我运行程序时，屏幕没有变化。

以下是变量声明:

static KeyListener listen;

public static int slide = 0;

这是主函数中的 KeyListener 声明:

listen = new KeyListener() {
        public void keyPressed(KeyEvent arg0) {
            if(arg0.getKeyCode() == KeyEvent.VK_LEFT && slide>0) {
                slide--;
                System.out.println("Left pressed. Slide is " + slide);
            }
            else if(arg0.getKeyCode() == KeyEvent.VK_RIGHT) {
                slide++;
                System.out.println("Right pressed. Slide is " + slide);
            }
        }
        public void keyReleased(KeyEvent arg0) {}
        public void keyTyped(KeyEvent arg0) {}

    };

以下是绘画所使用的方法:

public void paintComponent(Graphics g){
        super.paintComponent(g);
        this.setBackground(new Color(0, 0, 20));
        g.drawImage(exitButton, 20, 20, null);
        drawBoard(g);
    }



    public void drawBoard(Graphics g){

        if(slide == 0) {
            String[] text0 = {"This is the test",
                    "for the Presenter application",
                    "Here is a nicely framed picture of the Berlin Wall."
                    };
            makeTextBox(text0);
            g.drawImage(textBox, textX, textY,(boxW)*2,(boxH)*2, null);
            g.setColor(Color.blue);
            g.fillRect(wallX-10, wallY-10, wallW+20, wallH+20);
            g.drawImage(wall, wallX, wallY, null);
        }
        else if(slide == 1) {
            String[] text1 = {"Here is the second page...",
                                "Welcome."};
            makeTextBox(text1);
            g.drawImage(textBox, textX, textY,(boxW)*2,(boxH)*2, null);
        }

    }

当我运行程序时，程序会打印 if(slide == 0) 情况，但是当我按箭头键时，屏幕上没有任何变化。

Answer 1

您需要在组件被调用之前添加关键监听器:

addKeyListener(new KeyListener() {
    /* ... methods as before ... */
});

您还需要使组件可聚焦并为其提供焦点:

setFocusable(true);
requestFocusInWindow();

最后，如果您想在按下按键时重绘屏幕，则必须在按键处理程序中调用组件的 repaint() 方法。