JDoubleBufferedComponent


1   package JSci.swing;
2   
3   import java.awt.*;
4   import javax.swing.*;
5   
6   /**
7   * The JDoubleBufferedComponent class provides double buffering functionality.
8   * Painting events simply cause the offscreen buffer to be painted.
9   * It is the responsibility of sub-classes to explicitly update the offscreen buffer.
10  * The offscreen buffer can be updated in two ways.
11  * <ol>
12  * <li>Override the {@link #offscreenPaint(Graphics) offscreenPaint} method and use the {@link #redraw() redraw} method. Passive rendering.</li>
13  * <li>Draw to the graphics context returned by the {@link #getOffscreenGraphics() getOffscreenGraphics} method and use the {@link java.awt.Component#repaint() repaint} method. Active rendering.</li>
14  * </ol>
15  * The first way alone should be sufficient for most purposes.
16  * @version 1.3
17  * @author Mark Hale
18  */
19  public abstract class JDoubleBufferedComponent extends JComponent {
20          private Image buffer = null;
21          private boolean doRedraw = true;
22          /**
23          * Constructs a double buffered canvas.
24          */
25          public JDoubleBufferedComponent() {
26                  super.setDoubleBuffered(false);
27          }
28          /**
29          * Paints the canvas using double buffering.
30          * @see #offscreenPaint
31          */
32          public final void paintComponent(Graphics g) {
33                  Insets insets = getInsets();
34                  if(doRedraw) {
35                          doRedraw = false;
36                          final int width = getWidth()-insets.left-insets.right;
37                          final int height = getHeight()-insets.top-insets.bottom;
38                          buffer = createImage(width, height);
39                          if(buffer == null)
40                                  return;
41                          final Graphics graphics=buffer.getGraphics();
42                          /* save original color */
43                          Color oldColor = graphics.getColor();
44                          graphics.setColor(getBackground());
45                          graphics.fillRect(insets.left, insets.top, width, height);
46                          /* restore original color */
47                          graphics.setColor(oldColor);
48                          offscreenPaint(graphics);
49                  }
50                  g.drawImage(buffer, insets.left, insets.top, null);
51          }
52          /**
53          * Updates the canvas.
54          */
55          public final void update(Graphics g) {
56                  paint(g);
57          }
58          /**
59          * Prints the canvas.
60          */
61          public final void printComponent(Graphics g) {
62                  offscreenPaint(g);
63          }
64          /**
65          * Double buffering cannot be controlled for this component.
66          * This method always throws an exception.
67          */
68          public final void setDoubleBuffered(boolean flag) {
69                  throw new IllegalArgumentException  ();
70          }
71          /**
72          * Double buffering is always enabled.
73          * @return true.
74          */
75          public final boolean isDoubleBuffered() {
76                  return true;
77          }
78          /**
79          * Redraws the canvas.
80          * This method may safely be called from outside the event-dispatching thread.
81          */
82          public final void redraw() {
83                  doRedraw = true;
84                  repaint();
85          }
86          /**
87          * Returns the offscreen graphics context or <code>null</code> if not available.
88          */
89          protected final Graphics getOffscreenGraphics() {
90                  return (buffer != null) ? buffer.getGraphics() : null;
91          }
92          /**
93          * Paints the canvas off-screen.
94          * Override this method instead of paint(Graphics g).
95          */
96          protected abstract void offscreenPaint(Graphics g);
97  }
98  
99
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