java - 为什么这不是始终如一地绘制？

Question

我编写了一个快速测试程序，它接受一个字符串输入并将其绘制在 JFrame 上。 super 简单。但由于某种原因，该程序的绘制不一致。就像在接受输入后，它并不总是将字符串绘制到屏幕上。为了尝试调试，我添加了一个 repaint();到 paintComponent(它反复要求绘制输入，因此我添加了双缓冲区)。当程序决定不绘制到框架时，它没有进行 repaint()；并且没有出现新的输入对话框。我无法理解为什么。

package test;
import java.awt.*;
import javax.swing.JFrame;
import javax.swing.JOptionPane;

public class Main extends JFrame {

private Image dbImage;
private Graphics dbg;

public Main() {
    setTitle("Test Project");
    setSize(300, 300);
    setVisible(true);
    setResizable(false);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

}

public void paint(Graphics g) {
    dbImage = createImage(getWidth(), getHeight());
    dbg = dbImage.getGraphics();
    paintComponent(dbg);
    g.drawImage(dbImage, 0, 0, this);
}

public void paintComponent(Graphics g) {
    g.drawString(input(), 140, 130);
}

public static String input() {
    String x = ((String)JOptionPane.showInputDialog (
        null, "Enter a string", "Test Project",
        JOptionPane.QUESTION_MESSAGE,
        null, null, null));
    return x;
}

public static void main(String[] args) {
    Main main = new Main();

}
}

请注意:最初，输入法位于一个单独的类中，因为它将在我正在测试的项目中位于该类中。为了简洁起见，我将它移到了这里，因为无论方法在哪里，问题仍然会出现。

Answer 1

从我的评论中获取理论 :-)

为我的评论添加更多积分:

• 不是在 JFrame 上使用 setSize()，而是为任何 JComponent/JPanel< 覆盖 getPreferredSize() 方法 你扩展。现在只需在 JFrame 上调用 pack()，让它计算自己的大小。
• 请阅读 Concurrency in Swing

这是我所说的工作示例:

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

public class Main extends JPanel {

    private JButton button; 
    private String message;

    private ActionListener buttonAction = new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent ae) {
            message = input();
            repaint();
        }
    };

    public Main() {
        message = "Nothing to display yet...";
    }

    private void displayGUI() {
        JFrame frame = new JFrame("Painting Example");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

        JPanel contentPane = new JPanel(new BorderLayout(5, 5));
        button = new JButton("Get Message");
        button.addActionListener(buttonAction);

        contentPane.add(this, BorderLayout.CENTER);
        contentPane.add(button, BorderLayout.PAGE_END);

        frame.setContentPane(contentPane);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

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

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawString(message, 50, 50);
    }

    public String input() {
        String x = ((String)JOptionPane.showInputDialog (
                null, "Enter a string", "Test Project",
                            JOptionPane.QUESTION_MESSAGE,
                                        null, null, null));
        return x;
    }

    public static void main(String[] args) {
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                new Main().displayGUI();
            }
        };
        EventQueue.invokeLater(runnable);
    }
}

编辑 1:

如果您不喜欢使用JButton 询问输入，您可以将MouseListener 添加到您在其上绘制的JPanel，如下例所示:

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

public class Main extends JPanel {

    private String message;
    private int x;
    private int y;

    private MouseAdapter mouseAction = new MouseAdapter() {
        @Override
        public void mouseClicked(MouseEvent me) {
            message = input();
            x = me.getX();
            y = me.getY();
            repaint();
        }
    };

    public Main() {
        message = "Nothing to display yet...";
        x = y = 0;
    }

    private void displayGUI() {
        JFrame frame = new JFrame("Painting Example");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

        JPanel contentPane = new JPanel(new BorderLayout(5, 5));

        addMouseListener(mouseAction);
        contentPane.add(this, BorderLayout.CENTER);

        frame.setContentPane(contentPane);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

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

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawString(message, x, y);
    }

    public String input() {
        String x = ((String)JOptionPane.showInputDialog (
                null, "Enter a string", "Test Project",
                            JOptionPane.QUESTION_MESSAGE,
                                        null, null, null));
        return x;
    }

    public static void main(String[] args) {
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                new Main().displayGUI();
            }
        };
        EventQueue.invokeLater(runnable);
    }
}