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.youkuaiyun.com/jialinqiang/article/details/8974694