代理模式(Proxy Pattern)(四)：HeadFirst中CD封面虚拟代理

一、问题描述

当我们在网络上加载一些图像时，限于连接宽带和网络负载，下载可能需要一些时间，所以在等待图像加载的时候，就应该显示一些东西。我们也不希望在等待图像时，整个应用程序被挂起。一旦图像加载完成，刚才显示的东西就应该消失，图像显示出来。

二、类图

三、实现代码

1.ImageComponent

class ImageComponent extends JComponent {
private Icon icon;

public ImageComponent(Icon icon) {
this.icon = icon;
}

public void setIcon(Icon icon) {
this.icon = icon;
}

public void paintComponent(Graphics g) {
super.paintComponent(g);
int w = icon.getIconWidth();
int h = icon.getIconHeight();
int x = (800 - w)/2;
int y = (600 - h)/2;
icon.paintIcon(this, g, x, y);
}
}

2.代理类ImageProxy

class ImageProxy implements Icon {
ImageIcon imageIcon;
URL imageURL;
Thread retrievalThread;
boolean retrieving = false;

public ImageProxy(URL url) { imageURL = url; }

public int getIconWidth() {
if (imageIcon != null) {
return imageIcon.getIconWidth();
} else {
return 800;
}
}

public int getIconHeight() {
if (imageIcon != null) {
return imageIcon.getIconHeight();
} else {
return 600;
}
}

public void paintIcon(final Component c, Graphics  g, int x,  int y) {
if (imageIcon != null) {
imageIcon.paintIcon(c, g, x, y);
} else {
g.drawString("Loading CD cover, please wait...", x+300, y+190);
if (!retrieving) {
retrieving = true;

retrievalThread = new Thread(new Runnable() {
public void run() {
try {
imageIcon = new ImageIcon(imageURL, "CD Cover");
c.repaint();
} catch (Exception e) {
e.printStackTrace();
}
}
});
retrievalThread.start();
}
}
}
}

3.测试

public class ImageProxyTestDrive
{
ImageComponent imageComponent;
JFrame frame = new JFrame("CD Cover Viewer");
JMenuBar menuBar;
JMenu menu;
Hashtable cds = new Hashtable();

public static void main(String[] args) throws Exception
{
ImageProxyTestDrive testDrive = new ImageProxyTestDrive();
}

public ImageProxyTestDrive() throws Exception
{
cds.put("Ambient: Music for Airports","http://images.amazon.com/images/P/B000003S2K.01.LZZZZZZZ.jpg");
cds.put("Buddha Bar","http://images.amazon.com/images/P/B00009XBYK.01.LZZZZZZZ.jpg");
cds.put("Ima","http://images.amazon.com/images/P/B000005IRM.01.LZZZZZZZ.jpg");
cds.put("Karma","http://images.amazon.com/images/P/B000005DCB.01.LZZZZZZZ.gif");
cds.put("MCMXC A.D.","http://images.amazon.com/images/P/B000002URV.01.LZZZZZZZ.jpg");
cds.put("Northern Exposure","http://images.amazon.com/images/P/B000003SFN.01.LZZZZZZZ.jpg");
cds.put("Selected Ambient Works, Vol. 2","http://images.amazon.com/images/P/B000002MNZ.01.LZZZZZZZ.jpg");

URL initialURL = new URL((String) cds.get("Selected Ambient Works, Vol. 2"));
menuBar = new JMenuBar();
menu = new JMenu("Favorite CDs");
menuBar.add(menu);
frame.setJMenuBar(menuBar);

for (Enumeration e = cds.keys(); e.hasMoreElements();)
{
String name = (String) e.nextElement();
JMenuItem menuItem = new JMenuItem(name);
menu.add(menuItem);
menuItem.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent event)
{
imageComponent.setIcon(new ImageProxy(getCDUrl(event.getActionCommand())));
frame.repaint();
}
});
}

//建立框架和菜单

Icon icon = new ImageProxy(initialURL);
imageComponent = new ImageComponent(icon);
frame.getContentPane().add(imageComponent);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(800, 600);
frame.setVisible(true);

}

URL getCDUrl(String name)
{
try
{
return new URL((String) cds.get(name));
}
catch (MalformedURLException e)
{
e.printStackTrace();
return null;
}
}
}

运行窗口：

（加载图片时显示"Loading CD cover, please wait..."，反应太快，就不截图了）



转载请注明出处：http://blog.csdn.net/jialinqiang/article/details/8974694