src/java.desktop/share/classes/java/awt/image/BufferStrategy.java

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package java.awt.image;

import java.awt.BufferCapabilities;
import java.awt.Graphics;
import java.awt.Image;

/**
 * The {@code BufferStrategy} class represents the mechanism with which
 * to organize complex memory on a particular {@code Canvas} or
 * {@code Window}.  Hardware and software limitations determine whether and
 * how a particular buffer strategy can be implemented.  These limitations
 * are detectable through the capabilities of the
 * {@code GraphicsConfiguration} used when creating the
 * {@code Canvas} or {@code Window}.
 * <p>
 * It is worth noting that the terms <i>buffer</i> and <i>surface</i> are meant
 * to be synonymous: an area of contiguous memory, either in video device
 * memory or in system memory.
 * <p>
 * There are several types of complex buffer strategies, including
 * sequential ring buffering and blit buffering.
 * Sequential ring buffering (i.e., double or triple
 * buffering) is the most common; an application draws to a single <i>back
 * buffer</i> and then moves the contents to the front (display) in a single
 * step, either by copying the data or moving the video pointer.
 * Moving the video pointer exchanges the buffers so that the first buffer
 * drawn becomes the <i>front buffer</i>, or what is currently displayed on the
 * device; this is called <i>page flipping</i>.
 * <p>
 * Alternatively, the contents of the back buffer can be copied, or
 * <i>blitted</i> forward in a chain instead of moving the video pointer.
 * <pre>{@code
 * Double buffering:
 *
 *                    ***********         ***********
 *                    *         * ------> *         *
 * [To display] <---- * Front B *   Show  * Back B. * <---- Rendering
 *                    *         * <------ *         *
 *                    ***********         ***********
 *
 * Triple buffering:
 *
 * [To      ***********         ***********        ***********
 * display] *         * --------+---------+------> *         *
 *    <---- * Front B *   Show  * Mid. B. *        * Back B. * <---- Rendering
 *          *         * <------ *         * <----- *         *
 *          ***********         ***********        ***********
 *
 * }</pre>
 * <p>
 * Here is an example of how buffer strategies can be created and used:
 * <pre><code>
 *
 * // Check the capabilities of the GraphicsConfiguration
 * ...
 *
 * // Create our component
 * Window w = new Window(gc);
 *
 * // Show our window
 * w.setVisible(true);
 *
 * // Create a general double-buffering strategy
 * w.createBufferStrategy(2);
 * BufferStrategy strategy = w.getBufferStrategy();
 *
 * // Main loop
 * while (!done) {
 *     // Prepare for rendering the next frame
 *     // ...
 *
 *     // Render single frame
 *     do {
 *         // The following loop ensures that the contents of the drawing buffer
 *         // are consistent in case the underlying surface was recreated
 *         do {
 *             // Get a new graphics context every time through the loop
 *             // to make sure the strategy is validated
 *             Graphics graphics = strategy.getDrawGraphics();
 *
 *             // Render to graphics
 *             // ...
 *
 *             // Dispose the graphics
 *             graphics.dispose();
 *
 *             // Repeat the rendering if the drawing buffer contents
 *             // were restored
 *         } while (strategy.contentsRestored());
 *
 *         // Display the buffer
 *         strategy.show();
 *
 *         // Repeat the rendering if the drawing buffer was lost
 *     } while (strategy.contentsLost());
 * }
 *
 * // Dispose the window
 * w.setVisible(false);
 * w.dispose();
 * </code></pre>
 *
 * @see java.awt.Window
 * @see java.awt.Canvas
 * @see java.awt.GraphicsConfiguration
 * @see VolatileImage
 * @author Michael Martak
 * @since 1.4
 */
public abstract class BufferStrategy {

    /**
     * Constructor for subclasses to call.
     */
    protected BufferStrategy() {}

    /**
     * Returns the {@code BufferCapabilities} for this
     * {@code BufferStrategy}.
     *
     * @return the buffering capabilities of this strategy
     */
    public abstract BufferCapabilities getCapabilities();

    /**
     * Creates a graphics context for the drawing buffer.  This method may not
     * be synchronized for performance reasons; use of this method by multiple
     * threads should be handled at the application level.  Disposal of the
     * graphics object obtained must be handled by the application.
     *
     * @return a graphics context for the drawing buffer
     */
    public abstract Graphics getDrawGraphics();

    /**
     * Returns whether the drawing buffer was lost since the last call to
     * {@code getDrawGraphics}.  Since the buffers in a buffer strategy
     * are usually type {@code VolatileImage}, they may become lost.
     * For a discussion on lost buffers, see {@code VolatileImage}.
     *
     * @return Whether or not the drawing buffer was lost since the last call
     * to {@code getDrawGraphics}.
     * @see java.awt.image.VolatileImage
     */
    public abstract boolean contentsLost();

    /**
     * Returns whether the drawing buffer was recently restored from a lost
     * state and reinitialized to the default background color (white).
     * Since the buffers in a buffer strategy are usually type
     * {@code VolatileImage}, they may become lost.  If a surface has
     * been recently restored from a lost state since the last call to
     * {@code getDrawGraphics}, it may require repainting.
     * For a discussion on lost buffers, see {@code VolatileImage}.
     *
     * @return Whether or not the drawing buffer was restored since the last
     *         call to {@code getDrawGraphics}.
     * @see java.awt.image.VolatileImage
     */
    public abstract boolean contentsRestored();

    /**
     * Makes the next available buffer visible by either copying the memory
     * (blitting) or changing the display pointer (flipping).
     */
    public abstract void show();

    /**
     * Releases system resources currently consumed by this
     * {@code BufferStrategy} and
     * removes it from the associated Component.  After invoking this
     * method, {@code getBufferStrategy} will return null.  Trying
     * to use a {@code BufferStrategy} after it has been disposed will
     * result in undefined behavior.
     *
     * @see java.awt.Window#createBufferStrategy
     * @see java.awt.Canvas#createBufferStrategy
     * @see java.awt.Window#getBufferStrategy
     * @see java.awt.Canvas#getBufferStrategy
     * @since 1.6
     */
    public void dispose() {
    }
}