1 /*
2 * Copyright (c) 1995, 2010, Oracle and/or its affiliates. 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. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25 package java.awt;
26
27 import java.awt.image.BufferStrategy;
28 import java.awt.peer.CanvasPeer;
29 import javax.accessibility.*;
30
31 /**
32 * A <code>Canvas</code> component represents a blank rectangular
33 * area of the screen onto which the application can draw or from
34 * which the application can trap input events from the user.
35 * <p>
36 * An application must subclass the <code>Canvas</code> class in
37 * order to get useful functionality such as creating a custom
38 * component. The <code>paint</code> method must be overridden
39 * in order to perform custom graphics on the canvas.
40 *
41 * @author Sami Shaio
42 * @since 1.0
43 */
44 public class Canvas extends Component implements Accessible {
45
46 private static final String base = "canvas";
47 private static int nameCounter = 0;
48
49 /*
50 * JDK 1.1 serialVersionUID
51 */
52 private static final long serialVersionUID = -2284879212465893870L;
53
54 /**
55 * Constructs a new Canvas.
56 */
57 public Canvas() {
58 }
59
60 /**
61 * Constructs a new Canvas given a GraphicsConfiguration object.
62 *
63 * @param config a reference to a GraphicsConfiguration object.
64 *
65 * @see GraphicsConfiguration
66 */
67 public Canvas(GraphicsConfiguration config) {
68 this();
69 setGraphicsConfiguration(config);
70 }
71
72 @Override
73 void setGraphicsConfiguration(GraphicsConfiguration gc) {
74 synchronized(getTreeLock()) {
75 CanvasPeer peer = (CanvasPeer)getPeer();
76 if (peer != null) {
77 gc = peer.getAppropriateGraphicsConfiguration(gc);
78 }
79 super.setGraphicsConfiguration(gc);
80 }
81 }
82
83 /**
84 * Construct a name for this component. Called by getName() when the
85 * name is null.
86 */
87 String constructComponentName() {
88 synchronized (Canvas.class) {
89 return base + nameCounter++;
90 }
91 }
92
93 /**
94 * Creates the peer of the canvas. This peer allows you to change the
95 * user interface of the canvas without changing its functionality.
96 * @see java.awt.Toolkit#createCanvas(java.awt.Canvas)
97 * @see java.awt.Component#getToolkit()
98 */
99 public void addNotify() {
100 synchronized (getTreeLock()) {
101 if (peer == null)
102 peer = getToolkit().createCanvas(this);
103 super.addNotify();
104 }
105 }
106
107 /**
108 * Paints this canvas.
109 * <p>
110 * Most applications that subclass <code>Canvas</code> should
111 * override this method in order to perform some useful operation
112 * (typically, custom painting of the canvas).
113 * The default operation is simply to clear the canvas.
114 * Applications that override this method need not call
115 * super.paint(g).
116 *
117 * @param g the specified Graphics context
118 * @see #update(Graphics)
119 * @see Component#paint(Graphics)
120 */
121 public void paint(Graphics g) {
122 g.clearRect(0, 0, width, height);
123 }
124
125 /**
126 * Updates this canvas.
127 * <p>
128 * This method is called in response to a call to <code>repaint</code>.
129 * The canvas is first cleared by filling it with the background
130 * color, and then completely redrawn by calling this canvas's
131 * <code>paint</code> method.
132 * Note: applications that override this method should either call
133 * super.update(g) or incorporate the functionality described
134 * above into their own code.
135 *
136 * @param g the specified Graphics context
137 * @see #paint(Graphics)
138 * @see Component#update(Graphics)
139 */
140 public void update(Graphics g) {
141 g.clearRect(0, 0, width, height);
142 paint(g);
143 }
144
145 boolean postsOldMouseEvents() {
146 return true;
147 }
148
149 /**
150 * Creates a new strategy for multi-buffering on this component.
151 * Multi-buffering is useful for rendering performance. This method
152 * attempts to create the best strategy available with the number of
153 * buffers supplied. It will always create a <code>BufferStrategy</code>
154 * with that number of buffers.
155 * A page-flipping strategy is attempted first, then a blitting strategy
156 * using accelerated buffers. Finally, an unaccelerated blitting
157 * strategy is used.
158 * <p>
159 * Each time this method is called,
160 * the existing buffer strategy for this component is discarded.
161 * @param numBuffers number of buffers to create, including the front buffer
162 * @exception IllegalArgumentException if numBuffers is less than 1.
163 * @exception IllegalStateException if the component is not displayable
164 * @see #isDisplayable
165 * @see #getBufferStrategy
166 * @since 1.4
167 */
168 public void createBufferStrategy(int numBuffers) {
169 super.createBufferStrategy(numBuffers);
170 }
171
172 /**
173 * Creates a new strategy for multi-buffering on this component with the
174 * required buffer capabilities. This is useful, for example, if only
175 * accelerated memory or page flipping is desired (as specified by the
176 * buffer capabilities).
177 * <p>
178 * Each time this method
179 * is called, the existing buffer strategy for this component is discarded.
180 * @param numBuffers number of buffers to create
181 * @param caps the required capabilities for creating the buffer strategy;
182 * cannot be <code>null</code>
183 * @exception AWTException if the capabilities supplied could not be
184 * supported or met; this may happen, for example, if there is not enough
185 * accelerated memory currently available, or if page flipping is specified
186 * but not possible.
187 * @exception IllegalArgumentException if numBuffers is less than 1, or if
188 * caps is <code>null</code>
189 * @see #getBufferStrategy
190 * @since 1.4
191 */
192 public void createBufferStrategy(int numBuffers,
193 BufferCapabilities caps) throws AWTException {
194 super.createBufferStrategy(numBuffers, caps);
195 }
196
197 /**
198 * Returns the <code>BufferStrategy</code> used by this component. This
199 * method will return null if a <code>BufferStrategy</code> has not yet
200 * been created or has been disposed.
201 *
202 * @return the buffer strategy used by this component
203 * @see #createBufferStrategy
204 * @since 1.4
205 */
206 public BufferStrategy getBufferStrategy() {
207 return super.getBufferStrategy();
208 }
209
210 /*
211 * --- Accessibility Support ---
212 *
213 */
214
215 /**
216 * Gets the AccessibleContext associated with this Canvas.
217 * For canvases, the AccessibleContext takes the form of an
218 * AccessibleAWTCanvas.
219 * A new AccessibleAWTCanvas instance is created if necessary.
220 *
221 * @return an AccessibleAWTCanvas that serves as the
222 * AccessibleContext of this Canvas
223 * @since 1.3
224 */
225 public AccessibleContext getAccessibleContext() {
226 if (accessibleContext == null) {
227 accessibleContext = new AccessibleAWTCanvas();
228 }
229 return accessibleContext;
230 }
231
232 /**
233 * This class implements accessibility support for the
234 * <code>Canvas</code> class. It provides an implementation of the
235 * Java Accessibility API appropriate to canvas user-interface elements.
236 * @since 1.3
237 */
238 protected class AccessibleAWTCanvas extends AccessibleAWTComponent
239 {
240 private static final long serialVersionUID = -6325592262103146699L;
241
242 /**
243 * Get the role of this object.
244 *
245 * @return an instance of AccessibleRole describing the role of the
246 * object
247 * @see AccessibleRole
248 */
249 public AccessibleRole getAccessibleRole() {
250 return AccessibleRole.CANVAS;
251 }
252
253 } // inner class AccessibleAWTCanvas
254 }