为什么在 Graphics 对象上调用 dispose() 会导致 JPanel 不渲染任何组件 [英] Why does calling dispose() on Graphics object cause JPanel to not render any components
问题描述
在了解到应该在使用后在 Graphics/Graphics2D 对象上调用 dispose() 之后，我开始改变我的游戏以合并这个.
After learning that dispose() should be called on Graphics/Graphics2D object after use, I went about changing my game to incorporate this.
当我在 JPanel 的重写的 paintComponent(Graphics g) 中添加 g2d.dispose() 时，我添加的组件(扩展JLabel 类)在未呈现的情况下我仍然可以点击它们等，但它们不会被绘制.
When I added g2d.dispose() in overridden paintComponent(Graphics g) of JPanel, my components which I added (extensions of JLabel class) where not rendered I was able to still click on them etc but they would not be painted.
我使用普通的 JLabel 和 JButton 进行了测试，效果相同(尽管 JButton 在鼠标悬停在其上方时呈现).
I tested with a normal JLabel and JButton with same effect (though JButton is rendered when mouse is over it).
所以我的问题是为什么会发生这种情况?
So my question is why does this happen?
这是一个 SSCCE 来演示:
Here is an SSCCE to demonstrate:
在取消对MainMenuPanel 类的paintComponent 中的dispose() 调用的注释后:
after uncommenting call to dispose() in paintComponent of MainMenuPanel class:
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Image;
import java.awt.RenderingHints;
import java.awt.event.FocusAdapter;
import java.awt.event.FocusEvent;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.image.BufferedImage;
import java.net.URL;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
public class Test {
public Test() {
try {
initComponents();
} catch (Exception ex) {
Logger.getLogger(Test.class.getName()).log(Level.SEVERE, null, ex);
}
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
@Override
public void run() {
new Test();
}
});
}
private void initComponents() throws Exception {
JFrame frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setResizable(false);
MainMenuPanel mmp = new MainMenuPanel();
frame.add(mmp);
frame.pack();
frame.setVisible(true);
}
}
class MainMenuPanel extends JPanel {
//create labels for Main Menu
private PopUpJLabel versusesModeLabel;
private PopUpJLabel singlePlayerModeLabel;
private PopUpJLabel optionsLabel;
private PopUpJLabel helpLabel;
private PopUpJLabel aboutLabel;
//create variable to hold background
private Image background;
private Dimension preferredDimensions;
public static String gameType;
public static final String SINGLE_PLAYER = "Single Player", VERSUS_MODE = "VS Mode";
/**
* Default constructor to initialize double buffered JPanel with
* GridBagLayout
*/
public MainMenuPanel() {
super(new GridBagLayout(), true);
try {
initComponents();
} catch (Exception ex) {
JOptionPane.showMessageDialog(null, "Could not load main menu background!", "Main Menu Error: 0x004", JOptionPane.ERROR_MESSAGE);
System.exit(4);
}
}
/*
* Create JPanel and its components
*/
private void initComponents() throws Exception {
//set prefered size of JPanel
preferredDimensions = new Dimension(800, 600);
background = scaleImage(800, 600, ImageIO.read(new URL("http://photos.appleinsider.com/12.08.30-Java.jpg")));
//create label instances
singlePlayerModeLabel = new PopUpJLabel("Single Player Mode");
singlePlayerModeLabel.setEnabled(false);
versusesModeLabel = new PopUpJLabel("Versus Mode");
optionsLabel = new PopUpJLabel("Options");
helpLabel = new PopUpJLabel("Help");
aboutLabel = new PopUpJLabel("About");
//create new constraints for gridbag
GridBagConstraints gc = new GridBagConstraints();
gc.fill = GridBagConstraints.HORIZONTAL;
gc.ipady = 50;//vertical spacing
//add newGameLabel to panel with constraints
gc.gridx = 0;
gc.gridy = 0;
add(singlePlayerModeLabel, gc);
gc.gridy = 1;
add(versusesModeLabel, gc);
//add optionsLabel to panel with constraints (x is the same)
gc.gridy = 2;
add(optionsLabel, gc);
//add helpLabel to panel with constraints (x is the same)
gc.gridy = 3;
add(helpLabel, gc);
//add aboutLabel to panel with constraints (x is the same)
gc.gridy = 4;
add(aboutLabel, gc);
}
public static BufferedImage scaleImage(int w, int h, BufferedImage img) throws Exception {
BufferedImage bi;
//bi = new BufferedImage(w, h, BufferedImage.TYPE_INT_RGB);
bi = new BufferedImage(w, h, img.getType());
Graphics2D g2d = (Graphics2D) bi.createGraphics();
g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
g2d.addRenderingHints(new RenderingHints(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY));
g2d.drawImage(img, 0, 0, w, h, null);
g2d.dispose();
return bi;
}
/*
* Will return the preffered size of JPanel
*/
@Override
public Dimension getPreferredSize() {
return preferredDimensions;
}
/*
* Will draw the background to JPanel with anti-aliasing on and quality rendering
*/
@Override
protected void paintComponent(Graphics grphcs) {
super.paintComponent(grphcs);
//convert graphics object to graphics2d object
Graphics2D g2d = (Graphics2D) grphcs;
//set anti-aliasing on and rendering etc
//GamePanel.applyRenderHints(g2d);
//draw the image as the background
g2d.drawImage(background, 0, 0, null);
//g2d.dispose();//if I uncomment this no LAbels will be shown
}
}
class PopUpJLabel extends JLabel {
public final static Font defaultFont = new Font("Arial", Font.PLAIN, 50);
public final static Font hoverFont = new Font("Arial", Font.BOLD, 70);
PopUpJLabel(String text) {
super(text);
setHorizontalAlignment(JLabel.CENTER);
setForeground(Color.ORANGE);
setFont(defaultFont);
//allow component to be focusable
setFocusable(true);
//add focus adapter to change fints when focus is gained or lost (used for transversing labels with keys)
addFocusListener(new FocusAdapter() {
@Override
public void focusGained(FocusEvent fe) {
super.focusGained(fe);
if (isEnabled()) {
setFont(getHoverFont());
}
}
@Override
public void focusLost(FocusEvent fe) {
super.focusLost(fe);
setFont(getDefaultFont());
}
});
addMouseListener(new MouseAdapter() {
@Override
public void mouseEntered(MouseEvent me) {
super.mouseEntered(me);
if (isEnabled()) {
setFont(getHoverFont());
}
//call for focus mouse is over this component
requestFocusInWindow();
}
});
}
Font getDefaultFont() {
return defaultFont;
}
Font getHoverFont() {
return hoverFont;
}
}
推荐答案
问题是你在 paintComponent 中使用的 Graphics 上下文是由调用者创建和提供的(框架)，它也负责处理它.
The thing is that the Graphics context you are using in paintComponent is created and provided by the caller (the framework), which is also responsible for disposing of it.
您只需要在自己实际创建Graphics 时处理它(例如通过调用Component.getGraphics()).在你的情况下，你不是在创建它，你只是在投射它，所以在这种情况下不要调用 dispose.
You only need to dispose of Graphics when you actually create it yourself (for example by calling Component.getGraphics()). In your case, you're not creating it, you're just casting it, so do not call dispose in this case.
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