1 /* 2 * Copyright (c) 1995, 2015, 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) this.peer; 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 }