


JTextFields on Top of Active Drawing on JPanel: Threading Issues
Question:
Can Swing construct a multi-buffered rendering environment where Swing user interface elements can be added on top of it?
In this scenario, a red rectangle animates on a green background. The background remains static, so it's rendered onto a BufferedImage and redrawn only for the necessary portions to clear the rectangle's previous location.
However, adding a JTextField leads to repaint fails on cursor blinks.
Answer:
The key to this issue lies in JPanel's paintComponent() method.
Solution:
To fix this, invoke the superclass's method and erase the old drawing:
@Override protected void paintComponent(Graphics g) { super.paintComponent(g); int width = this.getWidth(); int height = this.getHeight(); g.setColor(Color.black); g.fillRect(0, 0, width, height); ... }
Additional Notes:
- Setting the background color in the constructor eliminates the need for filling the panel in paintComponent().
- Using super.paintComponent() ensures that text fields function correctly.
- Note that super.paintComponent() updates the delegate's background color, which can be avoided by setting setOpaque(false).
Improved Code:
import java.awt.Color; import java.awt.Dimension; import java.awt.EventQueue; import java.awt.Graphics; import java.awt.Graphics2D; import java.awt.GraphicsConfiguration; import java.awt.GraphicsEnvironment; import java.awt.Rectangle; import java.awt.Transparency; import java.awt.event.ActionEvent; import java.awt.event.ActionListener; import java.awt.event.ComponentAdapter; import java.awt.event.ComponentEvent; import java.awt.event.MouseAdapter; import java.awt.event.MouseEvent; import java.awt.image.BufferedImage; import java.util.Random; import javax.swing.JFrame; import javax.swing.JPanel; import javax.swing.JTextField; import javax.swing.Timer; /** @see http://stackoverflow.com/questions/3256941 */ public class AnimationTest extends JPanel implements ActionListener { private static final int WIDE = 640; private static final int HIGH = 480; private static final int RADIUS = 25; private static final int FRAMES = 24; private final Timer timer = new Timer(20, this); private final Rectangle rect = new Rectangle(); private BufferedImage background; private int index; private long totalTime; private long averageTime; private int frameCount; public static void main(String[] args) { EventQueue.invokeLater(new Runnable() { @Override public void run() { new AnimationTest().create(); } }); } private void create() { JFrame f = new JFrame("AnimationTest"); f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); f.add(this); f.pack(); f.setLocationRelativeTo(null); f.setVisible(true); timer.start(); } public AnimationTest() { super(true); this.setOpaque(false); this.setPreferredSize(new Dimension(WIDE, HIGH)); this.addMouseListener(new MouseHandler()); this.addComponentListener(new ComponentHandler()); } @Override protected void paintComponent(Graphics g) { long start = System.nanoTime(); int w = this.getWidth(); int h = this.getHeight(); g.drawImage(background, 0, 0, this); double theta = 2 * Math.PI * index++ / 64; g.setColor(Color.blue); rect.setRect( (int) (Math.sin(theta) * w / 3 + w / 2 - RADIUS), (int) (Math.cos(theta) * h / 3 + h / 2 - RADIUS), 2 * RADIUS, 2 * RADIUS); g.fillOval(rect.x, rect.y, rect.width, rect.height); g.setColor(Color.white); if (frameCount == FRAMES) { averageTime = totalTime / FRAMES; totalTime = 0; frameCount = 0; } else { totalTime += System.nanoTime() - start; frameCount++; } String s = String.format("%1.3f", averageTime / 1000000d); g.drawString(s, 5, 16); } @Override public void actionPerformed(ActionEvent e) { this.repaint(); } private class MouseHandler extends MouseAdapter { @Override public void mousePressed(MouseEvent e) { super.mousePressed(e); JTextField field = new JTextField("test"); Dimension d = field.getPreferredSize(); field.setBounds(e.getX(), e.getY(), d.width, d.height); add(field); } } private class ComponentHandler extends ComponentAdapter { private final GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment(); private final GraphicsConfiguration gc = ge.getDefaultScreenDevice().getDefaultConfiguration(); private final Random r = new Random(); @Override public void componentResized(ComponentEvent e) { super.componentResized(e); int w = getWidth(); int h = getHeight(); background = gc.createCompatibleImage(w, h, Transparency.OPAQUE); Graphics2D g = background.createGraphics(); g.clearRect(0, 0, w, h); g.setColor(Color.green.darker()); for (int i = 0; i
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