JPanel 过度绘制重叠的 JComcoBox

Question

我正在用 Java 编写一个小型“游戏”，其中我使用 JPanel 作为主渲染屏幕。 我还在渲染 JPanel 上方添加了一个 JComboBox 用于下拉选项。但是当我尝试扩展 JComboBox 时，它会从 JPanel 中获得 overdrawn。

我尝试过 setComponentZOrder() 但这只会使组件在 LayoutManager 中互相吞噬。而且 JLayeredPane 似乎也不起作用。

如何制作“实时重新渲染” JPanel 不过度绘制重叠组件？

简化示例：

public static void main(String[] args) {
    JFrame frame = new JFrame();
    frame.setLayout(new BorderLayout());

    JPanel renderScreen = new JPanel();
    Thread drawingThread = new Thread(() -> {
        while (true) {
            Graphics graphics = renderScreen.getGraphics();
            graphics.setColor(Color.MAGENTA);
            graphics.fillRect(0, 0, renderScreen.getWidth(), renderScreen.getHeight());

            try {Thread.sleep(5);} catch (InterruptedException ie) {}
        }
    });
    frame.add(renderScreen, BorderLayout.CENTER);

    JComboBox<String> dropdown = new JComboBox<>(new String[]{"Hello", "StackOverflow", "please", "help"});
    frame.add(dropdown, BorderLayout.NORTH);

    frame.setComponentZOrder(renderScreen,1);
    frame.setComponentZOrder(dropdown,0);

    frame.setVisible(true);

    drawingThread.start();

}

这是结果：

Answer 1

您的代码违反了几项 Swing 规则，包括：

• 不要在组件上使用 getGraphics 来获取 Graphics 对象。该对象的生命周期很短，并且可能在某个时候为空。
• 不要从后台线程或任何其他非 Swing 事件调度线程中进行 Swing 调用。

相反，请使用 Swing Timer 并在 PaintComponent 方法中进行绘图。例如

import java.awt.BorderLayout;
import java.awt.Dimension;

import javax.swing.*;

public class SwingGui01 extends JPanel {
    private JComboBox<String> comboBox = new JComboBox<>(new String[]{"One", "Two", "Three", "Four", "Five"});
    private AnimationPanel animationPanel = new AnimationPanel();

    public SwingGui01() {
        setLayout(new BorderLayout());
        add(comboBox, BorderLayout.PAGE_START);
        add(animationPanel);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            JFrame frame = new JFrame("Swing GUI 01");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.add(new SwingGui01());
            frame.pack();
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
        });
    }
}

class AnimationPanel extends JPanel {
    private static final int DELTA_X = 5;
    private static final int DELTA_Y = DELTA_X;
    private static final int PREF_W = 400;
    private static final int PREF_H = 300;
    private static final int ANIMATION_DELAY = 20;
    private int x = 0;
    private int y = 0;
    private int deltaX = DELTA_X;
    private int deltaY = DELTA_Y;

    public AnimationPanel() {
        Timer timer = new Timer(ANIMATION_DELAY, e -> {
            if (y > getHeight() - 50 ) {
                deltaY = -DELTA_Y;
            } else if (y < 0) {
                deltaY = DELTA_Y;
            }
            if (x > getWidth() - 50) {
                deltaX = -DELTA_X;
            } else if (x < 0) {
                deltaX = DELTA_X;
            }
            x += deltaX;
            y += deltaY;
            repaint();
        });
        timer.start();
    }

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

    @Override
    protected void paintComponent(java.awt.Graphics g) {
        super.paintComponent(g);
        g.fillOval(x, y, 50, 50);
    }
}