1 /*
2 * Copyright 1995-2007 Sun Microsystems, Inc. All Rights Reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Sun designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Sun in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
22 * CA 95054 USA or visit www.sun.com if you need additional information or
23 * have any questions.
24 */
25 package java.awt;
26
27 import java.awt.image.BufferStrategy;
28 import javax.accessibility;
29
30 /**
31 * A <code>Canvas</code> component represents a blank rectangular
32 * area of the screen onto which the application can draw or from
33 * which the application can trap input events from the user.
34 * <p>
35 * An application must subclass the <code>Canvas</code> class in
36 * order to get useful functionality such as creating a custom
37 * component. The <code>paint</code> method must be overridden
38 * in order to perform custom graphics on the canvas.
39 *
40 * @author Sami Shaio
41 * @since JDK1.0
42 */
43 public class Canvas extends Component implements Accessible {
44
45 private static final String base = "canvas";
46 private static int nameCounter = 0;
47
48 /*
49 * JDK 1.1 serialVersionUID
50 */
51 private static final long serialVersionUID = -2284879212465893870L;
52
53 /**
54 * Constructs a new Canvas.
55 */
56 public Canvas() {
57 }
58
59 /**
60 * Constructs a new Canvas given a GraphicsConfiguration object.
61 *
62 * @param config a reference to a GraphicsConfiguration object.
63 *
64 * @see GraphicsConfiguration
65 */
66 public Canvas(GraphicsConfiguration config) {
67 this();
68 graphicsConfig = config;
69 }
70
71 /**
72 * Construct a name for this component. Called by getName() when the
73 * name is null.
74 */
75 String constructComponentName() {
76 synchronized (Canvas.class) {
77 return base + nameCounter++;
78 }
79 }
80
81 /**
82 * Creates the peer of the canvas. This peer allows you to change the
83 * user interface of the canvas without changing its functionality.
84 * @see java.awt.Toolkit#createCanvas(java.awt.Canvas)
85 * @see java.awt.Component#getToolkit()
86 */
87 public void addNotify() {
88 synchronized (getTreeLock()) {
89 if (peer == null)
90 peer = getToolkit().createCanvas(this);
91 super.addNotify();
92 }
93 }
94
95 /**
96 * Paints this canvas.
97 * <p>
98 * Most applications that subclass <code>Canvas</code> should
99 * override this method in order to perform some useful operation
100 * (typically, custom painting of the canvas).
101 * The default operation is simply to clear the canvas.
102 * Applications that override this method need not call
103 * super.paint(g).
104 *
105 * @param g the specified Graphics context
106 * @see #update(Graphics)
107 * @see Component#paint(Graphics)
108 */
109 public void paint(Graphics g) {
110 g.clearRect(0, 0, width, height);
111 }
112
113 /**
114 * Updates this canvas.
115 * <p>
116 * This method is called in response to a call to <code>repaint</code>.
117 * The canvas is first cleared by filling it with the background
118 * color, and then completely redrawn by calling this canvas's
119 * <code>paint</code> method.
120 * Note: applications that override this method should either call
121 * super.update(g) or incorporate the functionality described
122 * above into their own code.
123 *
124 * @param g the specified Graphics context
125 * @see #paint(Graphics)
126 * @see Component#update(Graphics)
127 */
128 public void update(Graphics g) {
129 g.clearRect(0, 0, width, height);
130 paint(g);
131 }
132
133 boolean postsOldMouseEvents() {
134 return true;
135 }
136
137 /**
138 * Creates a new strategy for multi-buffering on this component.
139 * Multi-buffering is useful for rendering performance. This method
140 * attempts to create the best strategy available with the number of
141 * buffers supplied. It will always create a <code>BufferStrategy</code>
142 * with that number of buffers.
143 * A page-flipping strategy is attempted first, then a blitting strategy
144 * using accelerated buffers. Finally, an unaccelerated blitting
145 * strategy is used.
146 * <p>
147 * Each time this method is called,
148 * the existing buffer strategy for this component is discarded.
149 * @param numBuffers number of buffers to create, including the front buffer
150 * @exception IllegalArgumentException if numBuffers is less than 1.
151 * @exception IllegalStateException if the component is not displayable
152 * @see #isDisplayable
153 * @see #getBufferStrategy
154 * @since 1.4
155 */
156 public void createBufferStrategy(int numBuffers) {
157 super.createBufferStrategy(numBuffers);
158 }
159
160 /**
161 * Creates a new strategy for multi-buffering on this component with the
162 * required buffer capabilities. This is useful, for example, if only
163 * accelerated memory or page flipping is desired (as specified by the
164 * buffer capabilities).
165 * <p>
166 * Each time this method
167 * is called, the existing buffer strategy for this component is discarded.
168 * @param numBuffers number of buffers to create
169 * @param caps the required capabilities for creating the buffer strategy;
170 * cannot be <code>null</code>
171 * @exception AWTException if the capabilities supplied could not be
172 * supported or met; this may happen, for example, if there is not enough
173 * accelerated memory currently available, or if page flipping is specified
174 * but not possible.
175 * @exception IllegalArgumentException if numBuffers is less than 1, or if
176 * caps is <code>null</code>
177 * @see #getBufferStrategy
178 * @since 1.4
179 */
180 public void createBufferStrategy(int numBuffers,
181 BufferCapabilities caps) throws AWTException {
182 super.createBufferStrategy(numBuffers, caps);
183 }
184
185 /**
186 * Returns the <code>BufferStrategy</code> used by this component. This
187 * method will return null if a <code>BufferStrategy</code> has not yet
188 * been created or has been disposed.
189 *
190 * @return the buffer strategy used by this component
191 * @see #createBufferStrategy
192 * @since 1.4
193 */
194 public BufferStrategy getBufferStrategy() {
195 return super.getBufferStrategy();
196 }
197
198 /*
199 * --- Accessibility Support ---
200 *
201 */
202
203 /**
204 * Gets the AccessibleContext associated with this Canvas.
205 * For canvases, the AccessibleContext takes the form of an
206 * AccessibleAWTCanvas.
207 * A new AccessibleAWTCanvas instance is created if necessary.
208 *
209 * @return an AccessibleAWTCanvas that serves as the
210 * AccessibleContext of this Canvas
211 * @since 1.3
212 */
213 public AccessibleContext getAccessibleContext() {
214 if (accessibleContext == null) {
215 accessibleContext = new AccessibleAWTCanvas();
216 }
217 return accessibleContext;
218 }
219
220 /**
221 * This class implements accessibility support for the
222 * <code>Canvas</code> class. It provides an implementation of the
223 * Java Accessibility API appropriate to canvas user-interface elements.
224 * @since 1.3
225 */
226 protected class AccessibleAWTCanvas extends AccessibleAWTComponent
227 {
228 private static final long serialVersionUID = -6325592262103146699L;
229
230 /**
231 * Get the role of this object.
232 *
233 * @return an instance of AccessibleRole describing the role of the
234 * object
235 * @see AccessibleRole
236 */
237 public AccessibleRole getAccessibleRole() {
238 return AccessibleRole.CANVAS;
239 }
240
241 } // inner class AccessibleAWTCanvas
242 }
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