getGraphics的图形对象和paintComponent之间的区别 [英] Difference between Graphics object of getGraphics and paintComponent

问题描述

如果我使用JPanel，getGraphics返回的Graphics对象和paintComponent方法中传递的Graphics对象有什么不同？

解决方案

getGraphics

• 可以是 null


• 是上一次绘制过程的快照


• 在下一个绘图周期中丢失

您应该避免使用 getGraphics 什么是过去 paintComponent 方法。

从理论上讲，它们之间没有区别，但如果你想为了在重绘之间生存，你应该使用 paintComponent

/ strong>

没有示例代码，我猜...但是...

JPanel ，它充当绘制图像的主要画布， ，当设置时，一个 Rectangle 作为选择。

它使用第二个类作为 MouseListener 实际上决定要绘制什么

  import java.awt.AlphaComposite ; 
 import java.awt.BorderLayout; 
 import java.awt.Color; 
 import java.awt.Composite; 
 import java.awt.Dimension; 
 import java.awt.EventQueue; 
 import java.awt.Graphics; 
 import java.awt.Graphics2D; 
 import java.awt.Point; 
 import java.awt.Rectangle; 
 import java.awt.event.MouseAdapter; 
 import java.awt.event.MouseEvent; 
 import java.awt.image.BufferedImage; 
 import java.io.File; 
 import java.io.IOException; 
 import javax.imageio.ImageIO; 
 import javax.swing.JFrame; 
 import javax.swing.JPanel; 
 import javax.swing.UIManager; 
 import javax.swing.UnsupportedLookAndFeelException; 
 
 public class MouseSelection {
 
 public static void main（String [] args）{
 new MouseSelection（）; 
 
 $ b public MouseSelection（）{
 EventQueue.invokeLater（new Runnable（）{
 @Override 
 public void run（）{
尝试{
 UIManager.setLookAndFeel（UIManager.getSystemLookAndFeelClassName（））; 
} catch（ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex）{
} 
 
 ImagePane imgPane = new ImagePane（）; 
 new MouseHandler（imgPane）; 
 
 JFrame frame = new JFrame（Testing）; 
 frame.setDefaultCloseOperation（JFrame.EXIT_ON_CLOSE）; 
 frame.setLayout（new BorderLayout（））; 
 frame.add（imgPane）; 
 frame.pack（）; 
 frame.setLocationRelativeTo（null）; 
 frame。 setVisible（true）; 
} 
}）; 
 
 
公共类MouseHandler扩展MouseAdapter {
 
私有ImagePane imgPane; 
 
私人点clickPoint; 
 
 public MouseHandler（ImagePane imgPane）{
 this.imgPane = imgPane; 
 imgPane.addMouseMotionListener（this）; 
 imgPane.addMouseListener（this）; 
} 
 
 @Override 
 public void mousePressed（MouseEvent e）{
 imgPane.clearSelection（）; 
 clickPoint = e.getPoint（）; 
} 
 
 @Override 
 public void mouseReleased（MouseEvent e）{
 clickPoint = null; 
} 
 
 @Override 
 public void mouseDragged（MouseEvent e）{
 if（clickPoint！= null）{
 Point dragPoint = e.getPoint ）; 
 
 int x = Math.min（clickPoint.x，dragPoint.x）; 
 int y = Math.min（clickPoint.y，dragPoint.y）; 
 int width = Math.max（clickPoint.x，dragPoint.x） -  x; 
 int height = Math.max（clickPoint.y，dragPoint.y） -  y; 
 
 imgPane.setSelection（new Rectangle（x，y，width，height））; 
 
 
 
 
 
 public class ImagePane extends JPanel {
 
 private BufferedImage img; 
专用矩形选择; 
 $ b $ public ImagePane（）{
 try {
 img = ImageIO.read（new File（C：\\\\\\\\\\\\\\'） .JPG））; 
} catch（IOException ex）{
 ex.printStackTrace（）; 
} 
} 
 
 @Override 
 public Dimension getPreferredSize（）{
 return img == null？新维（200,200）：新维（img.getWidth（），img.getHeight（））; 
 
 
 @Override 
 protected void paintComponent（Graphics g）{
 super.paintComponent（g）; 
 Graphics2D g2d =（Graphics2D）g.create（）; 
 if（img！= null）{
 int x =（getWidth（） -  img.getWidth（））/ 2; 
 int y =（getHeight（） -  img.getHeight（））/ 2; 
 g2d.drawImage（img，x，y，this）; 
 
 if（selection！= null）{
 
 Color color = UIManager.getColor（Table.selectionBackground）; 
 g2d.setColor（color）; 
 Composite comp = g2d.getComposite（）; 
 g2d.setComposite（AlphaComposite.SrcOver.derive（0.5f））; 
 g2d.fill（选择）; 
 g2d.setComposite（comp）; 
 g2d.draw（选择）; 
 
} 
 g2d.dispose（）; 
} 
 
保护无效clearSelection（）{
选择=空; 
 repaint（）; 
} 
 
保护无效setSelection（矩形矩形）{
选择=矩形; 
 repaint（）; 
} 
} 
 
} 
  

If I'm working with a JPanel, what's the difference between the Graphics object returned by getGraphics, and the Graphics object that is passed in paintComponent method?

解决方案

getGraphics

• Can be null
• Is a "snap shot" of the last paint process
• Anything painted to it will be lost on the next paint cycle

You should avoid using getGraphics and simply use what is past to the paintComponent method.

In theory, there is no difference between them, but if you want what you have painted to survive between repaints, then you should be using paintComponent

Updated with example

Without an example code to go by I'm guessing...but...

Basically, this has a JPanel that acts as the primary "canvas" that draws an image and, when set, a Rectangle which acts as the selection.

It uses a second class as the MouseListener to actually make the decisions about what to paint

import java.awt.AlphaComposite;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Composite;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class MouseSelection {

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

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

                ImagePane imgPane = new ImagePane();
                new MouseHandler(imgPane);

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

    public class MouseHandler    extends MouseAdapter {

        private ImagePane imgPane;

        private Point clickPoint;

        public MouseHandler(ImagePane imgPane) {
            this.imgPane = imgPane;
            imgPane.addMouseMotionListener(this);
            imgPane.addMouseListener(this);
        }

        @Override
        public void mousePressed(MouseEvent e) {
            imgPane.clearSelection();
            clickPoint = e.getPoint();
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            clickPoint = null;
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            if (clickPoint != null) {
                Point dragPoint = e.getPoint();

                int x = Math.min(clickPoint.x, dragPoint.x);
                int y = Math.min(clickPoint.y, dragPoint.y);
                int width = Math.max(clickPoint.x, dragPoint.x) - x;
                int height = Math.max(clickPoint.y, dragPoint.y) - y;

                imgPane.setSelection(new Rectangle(x, y, width, height));

            }
        }

    }

    public class ImagePane extends JPanel {

        private BufferedImage img;
        private Rectangle selection;

        public ImagePane() {
            try {
                img = ImageIO.read(new File("C:\\hold\\thumbnails\\issue459.jpg"));
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }

        @Override
        public Dimension getPreferredSize() {
            return img == null ? new Dimension(200, 200) : new Dimension(img.getWidth(), img.getHeight());
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            if (img != null) {
                int x = (getWidth() - img.getWidth()) / 2;
                int y = (getHeight() - img.getHeight()) / 2;
                g2d.drawImage(img, x, y, this);
            }
            if (selection != null) {

                Color color = UIManager.getColor("Table.selectionBackground");
                g2d.setColor(color);
                Composite comp = g2d.getComposite();
                g2d.setComposite(AlphaComposite.SrcOver.derive(0.5f));
                g2d.fill(selection);
                g2d.setComposite(comp);
                g2d.draw(selection);

            }
            g2d.dispose();
        }

        protected void clearSelection() {
            selection = null;
            repaint();
        }

        protected void setSelection(Rectangle rectangle) {
            selection = rectangle;
            repaint();
        }
    }

}

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