组件仅在悬停时绘制

Question

谁能解释为什么我的组件只有在我将鼠标悬停在它们应该在的位置时才会被绘制？

我设置了一个可以拖动到任何地方的无边框框架，并且我试图在右上角创建一个退出按钮，但直到我将鼠标悬停在它上面才会被绘制。我在 JFrame 上绘制一个背景图像，然后绘制我的按钮并将整个内容设置为可见。

import java.awt.*;
import java.awt.event.*;
import javax.imageio.ImageIO;
import javax.swing.*;

public class GUI extends JFrame
{
    private Image Background = null;
    private static Point Offset = new Point();

    public GUI() {
        this.setUndecorated(true);
        this.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        AddListeners();
        SetCustomTheme();
        LoadBackground();
        Layout();
        pack();
        this.setSize(300, 300);
        this.setVisible(true);
    }

    private void Layout() {
        GroupLayout Info = new GroupLayout(this.getContentPane());
        this.getContentPane().setLayout(Info);
        JButton Button = new JButton();

        Info.setHorizontalGroup(
            Info.createSequentialGroup()
               .addComponent(Button)
         );

        Info.setVerticalGroup(
            Info.createParallelGroup()
                .addComponent(Button)
        );
    }

    private void SetCustomTheme() {
        try {
            UIManager.setLookAndFeel("javax.swing.plaf.nimbus.NimbusLookAndFeel");
        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
        }
    }

    private void LoadBackground() {
        try {
            Background = ImageIO.read(getClass().getResource("Images/meh.png"));
        } catch (Exception Ex) {

        }
    }

    private void SetCustomIcon() {
        Image Icon = Toolkit.getDefaultToolkit().getImage("Images/lol.jpg");
        setIconImage(Icon);
    }

    private void AddListeners() {
        this.addMouseListener(new MouseAdapter() {
            @Override public void mousePressed(MouseEvent e) {
              Offset.x = e.getX();
              Offset.y = e.getY();
            }
          });

        this.addMouseMotionListener(new MouseMotionAdapter() {
            @Override public void mouseDragged(MouseEvent e) {
              Point p = getLocation();
              setLocation(p.x + e.getX() - Offset.x, p.y + e.getY() - Offset.y);
            }
          });
    }

    @Override public void paint(Graphics g) {
        g.drawImage(Background, 0,0,this.getWidth(),this.getHeight(), null);
    }
}

Answer 1

1. 与 UI 的所有交互都必须在事件调度线程中执行
2. 您应该避免从顶级容器扩展，例如JFrame，而是使用JPanel。
3. 未能遵守 paint 链合约会阻止任何子组件开始绘制
4. 执行自定义绘画的首选重写方法是paintComponent

您可能想通读一遍

• Painting in AWT and Swing
• The Event Dispatch Thread
• Performing Custom Painting

试试这样吧；

public class BadPaint01 {

    public static void main(String[] args) {
        new BadPaint01();
    }

    public BadPaint01() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame();
                Image Icon = Toolkit.getDefaultToolkit().getImage("Images/lol.jpg");
                frame.setIconImage(Icon);
                frame.setUndecorated(true);
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new GUI());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public static class GUI extends JPanel {

        private Image Background = null;
        private static Point Offset = new Point();

        public GUI() {
            AddListeners();
            SetCustomTheme();
            LoadBackground();
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(300, 300);
        }

        private void Layout() {
            GroupLayout Info = new GroupLayout(this);
            setLayout(Info);
            JButton Button = new JButton();

            Info.setHorizontalGroup(
                    Info.createSequentialGroup()
                    .addComponent(Button));

            Info.setVerticalGroup(
                    Info.createParallelGroup()
                    .addComponent(Button));
        }

        private void SetCustomTheme() {
            try {
                UIManager.setLookAndFeel("javax.swing.plaf.nimbus.NimbusLookAndFeel");
            } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
            }
        }

        private void LoadBackground() {
            try {
                Background = ImageIO.read(getClass().getResource("Images/meh.png"));
            } catch (Exception Ex) {
            }
        }

        private void AddListeners() {
            this.addMouseListener(new MouseAdapter() {
                @Override
                public void mousePressed(MouseEvent e) {
                    Offset.x = e.getX();
                    Offset.y = e.getY();
                }
            });

            this.addMouseMotionListener(new MouseMotionAdapter() {
                @Override
                public void mouseDragged(MouseEvent e) {
                    Point p = getLocation();
                    setLocation(p.x + e.getX() - Offset.x, p.y + e.getY() - Offset.y);
                }
            });
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g); //To change body of generated methods, choose Tools | Templates.
            g.drawImage(Background, 0, 0, this.getWidth(), this.getHeight(), null);
        }
    }
}

你也可能想通读Code Conventions for the Java Programming Language，你不会因为忽略他们而结交任何朋友；）

Answer 2

如果我没记错的话，ToolKit.getImage 返回一个可能没有完全加载的Image。当您将鼠标悬停在它上面时，它可能同时已在后台加载。 而是这样做（类似于您的背景行）：

ImageIcon Icon = new ImageIcon(ImageIO.read(getClass().getResource("Images/lol.png")));
setIconImage(Icon);

（为了更好地理解，您可能需要搜索MediaTracker，我相信这是用来确保图像已完全加载的。）