我目前正在学习Java图形编程,并阅读安德鲁·戴维森(Andrew Davison)的《Java杀手级编程》。他编写了一个简单窗口应用程序,使用Image类进行离屏渲染,然后将图像复制到组件中。我觉得令人困惑的是,我们可以只创建图像,然后使用以下代码获取图形上下文:
dbg = dbImage.getGraphics();
我有点困惑,因为我们只使用dbg Graphics对象来绘制东西,但是后来我们在paintComponent方法中使用dbImage来显示我们画到dbg对象上的所有内容:
g.drawImage(dbImage, 0, 0, null);
那么,dbImage如何“知道”它包含了我们绘制到dbg Graphics对象上的所有图形内容呢?
以下是完整代码:
public class GamePanel extends JPanel implements Runnable{
private static final int PWIDTH = 500;
private static final int PHEIGHT = 400;
private Thread animator;
private volatile boolean running = false;
private volatile boolean gameOver = false;
private Graphics dbg;
private Image dbImage = null;
private Counter counter;
public GamePanel(){
setBackground(Color.white);
setPreferredSize(new Dimension(PWIDTH, PHEIGHT));
// create game components eg. counter.
counter = new Counter(10);
counter.start();
}
private void stopGame(){
running = false;
}
@Override public void addNotify(){
super.addNotify();
startGame();
}
public void startGame(){
if(animator == null || !running){
animator = new Thread(this);
animator.start();
}
}
@Override
public void run() {
running = true;
while(running){
gameUpdate();
gameRender();
repaint();
try{
Thread.sleep(20);
}
catch(InterruptedException ex){
// do something with the exception...
}
}
}
private void gameUpdate(){
if(!gameOver){
// update game state...
if(counter.getCounter() == 0)
gameOver = true;
}
}
private void gameRender(){
if(dbImage == null){
dbImage = createImage(PWIDTH, PHEIGHT);
if(dbImage == null){
System.out.println("dbImage is null");
return;
}
else
dbg = dbImage.getGraphics(); // get graphics context for drawing to off-screen images.
}
// clear the background
dbg.setColor(Color.white);
dbg.fillRect(0, 0, PWIDTH, PHEIGHT);
// draw game elements here...
dbg.setColor(Color.black);
dbg.drawString(Integer.toString(counter.getCounter()), 10, 10);
if(gameOver)
dbg.drawString("Game over...", 10, 20);
}
@Override public void paintComponent(Graphics g){
super.paintComponent(g);
if(dbImage != null)
g.drawImage(dbImage, 0, 0, null);
}
}
重要的部分只有paintComponent方法和gameRender()方法。
Graphics
实例是独立的,它属于图像而不属于其他任何东西。简单来说:Image
实例保留对它的引用。(或者根据实现方式不同,也可能是相反的。但最终结果是相同的。) - biziclop