java - 如何使用箭头键在 JPanel 中移动对象

Question

我正在尝试使用 Windowbuilder 创建一个小程序，它只需在 JPanel 中绘制一个红色矩形(称为 car1)，然后通过按箭头键来移动它；为此，我将一个与箭头关联的方法来更改 x 位置，调用重绘方法，但矩形根本不移动 - 因此我可能用 KeyEvent 和/或重绘弄乱了一些东西。

每次按下正确的箭头键时，我应该怎么做才能使矩形移动并刷新面板？

public class Car extends JPanel {
    int x;
    int y;

    public Car(int x,int y){
        this.x=x;
        this.y=y;
    }


    public void paint(Graphics g) {
        g.setColor(Color.RED);
        g.fillRect(x, y, 20, 20);
    }

    public void move_right(){
        x=x+20;
    }

    public void move_left(){
        x=x-20;
    }

}



public class Form extends JFrame {

    //private JPanel contentPane;
    Car car1;

    /**
     * Launch the application.
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    Form frame = new Form();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    /**
     * Create the frame.
     */
    public Form() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setBounds(100, 100, 727, 550);
        getContentPane().setLayout(null);
        car1 = new Car(350, 480);
        car1.addKeyListener(new KeyAdapter() {
            public void keyTyped(KeyEvent e) {
                int key = e.getKeyCode();
                if (key == KeyEvent.VK_KP_LEFT) {
                    car1.move_left();
                    car1.repaint();
                }
                if (key == KeyEvent.VK_KP_RIGHT) {
                    car1.move_right();
                    car1.repaint();
                }
            }
        });
        car1.setBounds(0, 0, 700, 500);
        car1.setBackground(new Color(255, 255, 255));
        getContentPane().add(car1);
    }

}

Answer 1

至少有3个问题:

1. 使用KeyListener。 KeyListener 众所周知，它只响应可聚焦且具有键盘焦点的组件上发生的按键事件。默认情况下，JPanel 不可聚焦，因此它无法接收键盘焦点。更好的解决方案是使用键绑定(bind) API，它允许定义触发绑定(bind)之前组件必须具有的焦点级别，并允许您对多个键重复使用 Action，从而减少代码重复
2. 重写paint。强烈建议在执行自定义绘制时重写paintComponent而不是paint。你也未能维护油漆链，这将导致无穷无尽的奇怪而美妙的油漆工件。 Painting in AWT and Swing和 Performing Custom Painting了解更多详情
3. 使用 null 布局。避免使用 null 布局，像素完美布局是现代 UI 设计中的一种幻觉。影响组件个体尺寸的因素太多，您无法控制其中任何一个。 Swing 的设计目的是与核心的布局管理器一起工作，放弃这些将导致无休止的问题和问题，您将花费越来越多的时间来尝试纠正

例如...

import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;
import javax.swing.AbstractAction;
import javax.swing.Action;
import javax.swing.ActionMap;
import javax.swing.InputMap;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.KeyStroke;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class Test {

    public static void main(String[] args) {
        new Test();
    }

    public Test() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (Exception ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private int xPos;

        public TestPane() {
            Action leftAction = new AbstractAction() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    xPos -= 2;
                    if (xPos < 0) {
                        xPos = 0;
                    }
                    repaint();
                }
            };
            Action rightAction = new AbstractAction() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    xPos += 2;
                    if (xPos + 10 > getWidth()) {
                        xPos = getWidth() - 10;
                    }
                    repaint();
                }
            };

            bindKeyStroke(WHEN_IN_FOCUSED_WINDOW, "move.left", KeyStroke.getKeyStroke(KeyEvent.VK_LEFT, 0), leftAction);
            bindKeyStroke(WHEN_IN_FOCUSED_WINDOW, "move.left", KeyStroke.getKeyStroke(KeyEvent.VK_KP_LEFT, 0), leftAction);
            bindKeyStroke(WHEN_IN_FOCUSED_WINDOW, "move.left", KeyStroke.getKeyStroke(KeyEvent.VK_4, 0), leftAction);
            bindKeyStroke(WHEN_IN_FOCUSED_WINDOW, "move.left", KeyStroke.getKeyStroke(KeyEvent.VK_A, 0), leftAction);

            bindKeyStroke(WHEN_IN_FOCUSED_WINDOW, "move.right", KeyStroke.getKeyStroke(KeyEvent.VK_RIGHT, 0), rightAction);
            bindKeyStroke(WHEN_IN_FOCUSED_WINDOW, "move.right", KeyStroke.getKeyStroke(KeyEvent.VK_KP_RIGHT, 0), rightAction);
            bindKeyStroke(WHEN_IN_FOCUSED_WINDOW, "move.right", KeyStroke.getKeyStroke(KeyEvent.VK_6, 0), rightAction);
            bindKeyStroke(WHEN_IN_FOCUSED_WINDOW, "move.right", KeyStroke.getKeyStroke(KeyEvent.VK_D, 0), rightAction);
        }

        protected void bindKeyStroke(int condition, String name, KeyStroke keyStroke, Action action) {
            InputMap im = getInputMap(condition);
            ActionMap am = getActionMap();

            im.put(keyStroke, name);
            am.put(name, action);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            int yPos = (getHeight() - 10) / 2;
            g2d.drawRect(xPos, yPos, 10, 10);
            g2d.dispose();
        }

    }

}