Canvas component represents a blank rectangular
* area of the screen onto which the application can draw or from
* which the application can trap input events from the user.
*
* An application must subclass the Canvas class in
* order to get useful functionality such as creating a custom
* component. The paint method must be overridden
* in order to perform custom graphics on the canvas.
*
* @author Sami Shaio
* @since 1.0
*/
public class Canvas extends Component implements Accessible {
private static final String base = "canvas";
private static int nameCounter = 0;
/*
* JDK 1.1 serialVersionUID
*/
private static final long serialVersionUID = -2284879212465893870L;
/**
* Constructs a new Canvas.
*/
public Canvas() {
}
/**
* Constructs a new Canvas given a GraphicsConfiguration object.
*
* @param config a reference to a GraphicsConfiguration object.
*
* @see GraphicsConfiguration
*/
public Canvas(GraphicsConfiguration config) {
this();
setGraphicsConfiguration(config);
}
@Override
void setGraphicsConfiguration(GraphicsConfiguration gc) {
synchronized(getTreeLock()) {
CanvasPeer peer = (CanvasPeer)getPeer();
if (peer != null) {
gc = peer.getAppropriateGraphicsConfiguration(gc);
}
super.setGraphicsConfiguration(gc);
}
}
/**
* Construct a name for this component. Called by getName() when the
* name is null.
*/
String constructComponentName() {
synchronized (Canvas.class) {
return base + nameCounter++;
}
}
/**
* Creates the peer of the canvas. This peer allows you to change the
* user interface of the canvas without changing its functionality.
* @see java.awt.Toolkit#createCanvas(java.awt.Canvas)
* @see java.awt.Component#getToolkit()
*/
public void addNotify() {
synchronized (getTreeLock()) {
if (peer == null)
peer = getToolkit().createCanvas(this);
super.addNotify();
}
}
/**
* Paints this canvas.
*
* Most applications that subclass Canvas should
* override this method in order to perform some useful operation
* (typically, custom painting of the canvas).
* The default operation is simply to clear the canvas.
* Applications that override this method need not call
* super.paint(g).
*
* @param g the specified Graphics context
* @see #update(Graphics)
* @see Component#paint(Graphics)
*/
public void paint(Graphics g) {
g.clearRect(0, 0, width, height);
}
/**
* Updates this canvas.
*
* This method is called in response to a call to repaint.
* The canvas is first cleared by filling it with the background
* color, and then completely redrawn by calling this canvas's
* paint method.
* Note: applications that override this method should either call
* super.update(g) or incorporate the functionality described
* above into their own code.
*
* @param g the specified Graphics context
* @see #paint(Graphics)
* @see Component#update(Graphics)
*/
public void update(Graphics g) {
g.clearRect(0, 0, width, height);
paint(g);
}
boolean postsOldMouseEvents() {
return true;
}
/**
* Creates a new strategy for multi-buffering on this component.
* Multi-buffering is useful for rendering performance. This method
* attempts to create the best strategy available with the number of
* buffers supplied. It will always create a BufferStrategy
* with that number of buffers.
* A page-flipping strategy is attempted first, then a blitting strategy
* using accelerated buffers. Finally, an unaccelerated blitting
* strategy is used.
*
* Each time this method is called, * the existing buffer strategy for this component is discarded. * @param numBuffers number of buffers to create, including the front buffer * @exception IllegalArgumentException if numBuffers is less than 1. * @exception IllegalStateException if the component is not displayable * @see #isDisplayable * @see #getBufferStrategy * @since 1.4 */ public void createBufferStrategy(int numBuffers) { super.createBufferStrategy(numBuffers); } /** * Creates a new strategy for multi-buffering on this component with the * required buffer capabilities. This is useful, for example, if only * accelerated memory or page flipping is desired (as specified by the * buffer capabilities). *
* Each time this method
* is called, the existing buffer strategy for this component is discarded.
* @param numBuffers number of buffers to create
* @param caps the required capabilities for creating the buffer strategy;
* cannot be null
* @exception AWTException if the capabilities supplied could not be
* supported or met; this may happen, for example, if there is not enough
* accelerated memory currently available, or if page flipping is specified
* but not possible.
* @exception IllegalArgumentException if numBuffers is less than 1, or if
* caps is null
* @see #getBufferStrategy
* @since 1.4
*/
public void createBufferStrategy(int numBuffers,
BufferCapabilities caps) throws AWTException {
super.createBufferStrategy(numBuffers, caps);
}
/**
* Returns the BufferStrategy used by this component. This
* method will return null if a BufferStrategy has not yet
* been created or has been disposed.
*
* @return the buffer strategy used by this component
* @see #createBufferStrategy
* @since 1.4
*/
public BufferStrategy getBufferStrategy() {
return super.getBufferStrategy();
}
/*
* --- Accessibility Support ---
*
*/
/**
* Gets the AccessibleContext associated with this Canvas.
* For canvases, the AccessibleContext takes the form of an
* AccessibleAWTCanvas.
* A new AccessibleAWTCanvas instance is created if necessary.
*
* @return an AccessibleAWTCanvas that serves as the
* AccessibleContext of this Canvas
* @since 1.3
*/
public AccessibleContext getAccessibleContext() {
if (accessibleContext == null) {
accessibleContext = new AccessibleAWTCanvas();
}
return accessibleContext;
}
/**
* This class implements accessibility support for the
* Canvas class. It provides an implementation of the
* Java Accessibility API appropriate to canvas user-interface elements.
* @since 1.3
*/
protected class AccessibleAWTCanvas extends AccessibleAWTComponent
{
private static final long serialVersionUID = -6325592262103146699L;
/**
* Get the role of this object.
*
* @return an instance of AccessibleRole describing the role of the
* object
* @see AccessibleRole
*/
public AccessibleRole getAccessibleRole() {
return AccessibleRole.CANVAS;
}
} // inner class AccessibleAWTCanvas
}