e674. 创建并绘制加速图像

Images in accelerated memory are much faster to draw on the screen. This example demonstrates how to take an image and make an accelerated copy of it and then use it to draw on the screen.

The problem with images in accelerated memory is that they can disappear at any time. The system is free to free accelerated memory at any time. Such images are called volatile images. To deal with this issue, it is necessary to check whether a volatile image is still valid immediately after drawing it. If it is no longer valid, it is then necessary to reconstruct the image and then attempt to draw with it again.

This example implements a convenient method for drawing volatile images. It automatically handles the task of reconstructing the volatile image when necessary.

    // This method draws a volatile image and returns it or possibly a
    // newly created volatile image object. Subsequent calls to this method
    // should always use the returned volatile image.
    // If the contents of the image is lost, it is recreated using orig.
    // img may be null, in which case a new volatile image is created.
    public VolatileImage drawVolatileImage(Graphics2D g, VolatileImage img,
                                           int x, int y, Image orig) {
        final int MAX_TRIES = 100;
        for (int i=0; i<MAX_TRIES; i++) {
            if (img != null) {
                // Draw the volatile image
                g.drawImage(img, x, y, null);

                // Check if it is still valid
                if (!img.contentsLost()) {
                    return img;
                }
            } else {
                // Create the volatile image
                img = g.getDeviceConfiguration().createCompatibleVolatileImage(
                    orig.getWidth(null), orig.getHeight(null));
            }

            // Determine how to fix the volatile image
            switch (img.validate(g.getDeviceConfiguration())) {
            case VolatileImage.IMAGE_OK:
                // This should not happen
                break;
            case VolatileImage.IMAGE_INCOMPATIBLE:
                // Create a new volatile image object;
                // this could happen if the component was moved to another device
                img.flush();
                img = g.getDeviceConfiguration().createCompatibleVolatileImage(
                    orig.getWidth(null), orig.getHeight(null));
            case VolatileImage.IMAGE_RESTORED:
                // Copy the original image to accelerated image memory
                Graphics2D gc = (Graphics2D)img.createGraphics();
                gc.drawImage(orig, 0, 0, null);
                gc.dispose();
                break;
            }
        }

        // The image failed to be drawn after MAX_TRIES;
        // draw with the non-accelerated image
        g.drawImage(orig, x, y, null);
        return img;
    }

Here's a component that uses the method:

    // Declare a component that draws a volatile image
    class MyComponent extends Canvas {
        VolatileImage volImage;
        Image origImage = null;

        MyComponent() {
            // Get image to move into accelerated image memory
            origImage = new ImageIcon("image.gif").getImage();
        }

        public void paint(Graphics g) {
            // Draw accelerated image
            int x = 0;
            int y = 0;
            volImage = drawVolatileImage((Graphics2D)g, volImage, x, y, origImage);
        }
    }

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