在滚动窗格的左上角绘制箭头

Question

我在 JScrollPane 中有一个 JPanel。在我的程序中，JPanel 显示地图图像并显示车辆的当前位置。我的程序接收车辆的位置和当前方向。我需要在地图上显示这个。

我在显示车辆方向时遇到问题。我想在地图可见区域的左上角显示一个箭头来显示方向。以下是我尝试过的代码。但是当我向下滚动时它会引发异常。

Exception in thread "AWT-EventQueue-0" java.awt.image.RasterFormatException: Transformed height (-287) is less than or equal to 0.
    at java.awt.image.AffineTransformOp.createCompatibleDestImage(Unknown Source)
    at java.awt.image.AffineTransformOp.filter(Unknown Source)
    at Test.paintComponent(Test.java:62)
    at javax.swing.JComponent.paint(Unknown Source)
    at javax.swing.JComponent.paintToOffscreen(Unknown Source)


import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.geom.AffineTransform;
import java.awt.image.AffineTransformOp;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.URL;
import java.util.Random;

import javax.imageio.ImageIO;
import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.Timer;

public class Test extends JPanel implements ActionListener {
    public static void main(String[] args) {
        JFrame f = new JFrame();
        f.setSize(300, 400);
        Test view = new Test();
        Dimension minimumSize = new Dimension(1000, 1000);

        view.setBorder(BorderFactory.createLineBorder(Color.BLUE));
        JScrollPane comp = new JScrollPane(view);
        comp.setBorder(BorderFactory.createLineBorder(Color.RED));
        f.getContentPane().add(comp, BorderLayout.CENTER);
        f.setVisible(true);
        view.setPreferredSize(minimumSize);
    }

    private BufferedImage i;
    private Random r = new Random();

    public Test() {
        try {
            i = ImageIO
                    .read(new URL("https://cdn4.iconfinder.com/data/icons/cc_mono_icon_set/blacks/48x48/br_next.png"));
            new Timer(1000, this).start();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);

        Graphics2D g2d = (Graphics2D) g.create();
        AffineTransform transform = g2d.getTransform();

        transform.rotate(r.nextFloat() * Math.PI * 2, i.getWidth() / 2, i.getHeight() / 2);
        AffineTransformOp op = new AffineTransformOp(transform, AffineTransformOp.TYPE_BILINEAR);

        Rectangle cb = g.getClipBounds();
        g2d.drawImage(op.filter(i, null), (int) cb.getX(), (int) cb.getY(), null);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        repaint();
    }

}

即使用户使用滚动条滚动，我如何才能使箭头始终在左上角可见？

Answer 1

JScrollPane 和JViewport 都经过高度优化，在绘制时，剪切边界只设置在实际需要更新的区域（引入的区域）

一个更好的主意可能是创建自己的JViewport，您可以在其上绘制视口的内容

import java.awt.BorderLayout;
import java.awt.Color;
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.ActionListener;
import java.awt.geom.AffineTransform;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.URL;
import java.util.Random;
import javax.imageio.ImageIO;
import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JViewport;
import javax.swing.Timer;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;

public class Test extends JPanel {

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JFrame f = new JFrame();
                f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                f.setSize(300, 400);
                Test view = new Test();

                view.setBorder(BorderFactory.createLineBorder(Color.BLUE));
                JScrollPane comp = new JScrollPane();
                ViewPortDirection vp = new ViewPortDirection();
                comp.setViewport(vp);
                comp.setViewportView(view);
                comp.setBorder(BorderFactory.createLineBorder(Color.RED));
                f.getContentPane().add(comp, BorderLayout.CENTER);
                f.setVisible(true);
            }
        });
    }

    public static class ViewPortDirection extends JViewport {

        private BufferedImage i;
        private Random r = new Random();

        public ViewPortDirection() {
            try {
                i = ImageIO.read(new URL("https://cdn4.iconfinder.com/data/icons/cc_mono_icon_set/blacks/48x48/br_next.png"));
                new Timer(1000, new ActionListener() {
                    @Override
                    public void actionPerformed(ActionEvent e) {
                        repaint();
                    }
                }).start();
            } catch (IOException e) {
                throw new RuntimeException(e);
            }

            addChangeListener(new ChangeListener() {
                @Override
                public void stateChanged(ChangeEvent e) {
                    repaint(0, 0, i.getWidth(), i.getHeight());
                }
            });
        }

        @Override
        public void paint(Graphics g) {
            super.paint(g);

            Graphics2D g2d = (Graphics2D) g.create();
            AffineTransform transform = AffineTransform.getRotateInstance(r.nextFloat() * Math.PI * 2, i.getWidth() / 2, i.getHeight() / 2);

            int x = 0;
            int y = 0;
            g2d.setTransform(transform);
            g2d.drawImage(i, x, y, this);
            g2d.dispose();
        }

    }

    public Test() {
        setBackground(Color.BLUE);
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(1000, 1000);
    }

}