Java基础之扩展GUI——显示About对话框（Sketcher 2 displaying an About dialog）

控制台程序。

最简单的对话框仅仅显示一些信息。为了说明这一点，可以为Sketcher添加Help菜单项和About菜单项，之后再显示About对话框来提供有关应用程序的信息。

要新建的对话框类从JDialog中派生以便创建About对话框，这个新类的名称是AboutDialog。

把AboutDialog类作为OK按钮的动作监听器，就可以使这个类变成自包含的。另外，因为所有操作都只在SketcherFrame类中进行，所以可以将之定义为SketcherFrame的嵌套类。

import javax.swing.JComponent;
import java.util.*;
import java.awt.*;
import java.awt.event.MouseEvent;
import javax.swing.event.MouseInputAdapter;

@SuppressWarnings("serial")
public class SketcherView extends JComponent implements Observer {
  public SketcherView(Sketcher theApp) {
    this.theApp = theApp;
    MouseHandler handler = new MouseHandler();                         // create the mouse listener
    addMouseListener(handler);                                         // Listen for button events
    addMouseMotionListener(handler);                                   // Listen for motion events
  }

  // Method called by Observable object when it changes
  public void update(Observable o, Object rectangle) {
    if(rectangle != null) {
      repaint((java.awt.Rectangle)rectangle);
    } else {
      repaint();
    }
  }

  // Method to draw on the view
  @Override
  public void paint(Graphics g) {
    Graphics2D g2D = (Graphics2D)g;                                    // Get a 2D device context
    for(Element element: theApp.getModel()) {                          // For each element in the model
      element.draw(g2D);                                               // ...draw the element
    }
  }

  class MouseHandler extends MouseInputAdapter {
    @Override
    public void mousePressed(MouseEvent e)  {
      start = e.getPoint();                                            // Save the cursor position in start
      buttonState = e.getButton();                                     // Record which button was pressed
      if(buttonState == MouseEvent.BUTTON1) {
        g2D = (Graphics2D)getGraphics();                               // Get graphics context
        g2D.setXORMode(getBackground());                               // Set XOR mode
      }
    }

    @Override
    public void mouseDragged(MouseEvent e) {
      last = e.getPoint();                                             // Save cursor position

      if(buttonState == MouseEvent.BUTTON1) {
        if(tempElement == null) {                                      // Is there an element?
          tempElement = Element.createElement(                         // No, so create one
                                 theApp.getWindow().getElementType(),
                                 theApp.getWindow().getElementColor(),
                                 start, last);
        } else {
          tempElement.draw(g2D);                                       // Yes draw to erase it
          tempElement.modify(start, last);                             // Now modify it
        }
        tempElement.draw(g2D);                                         // and draw it
     }
    }

    @Override
    public void mouseReleased(MouseEvent e) {
      if(e.getButton() == MouseEvent.BUTTON1) {
        buttonState = MouseEvent.NOBUTTON;                             // Reset the button state

        if(tempElement != null) {                                      // If there is an element...
          theApp.getModel().add(tempElement);                          // ...add it to the model...
          tempElement = null;                                          // ...and reset the field
        }
        if(g2D != null) {                                              // If there抯 a graphics context
          g2D.dispose();                                               // ...release the resource...
          g2D = null;                                                  // ...and reset field to null
        }
        start = last = null;                                           // Remove any points
      }
    }

    @Override
    public void mouseEntered(MouseEvent e) {
      setCursor(Cursor.getPredefinedCursor(Cursor.CROSSHAIR_CURSOR));
    }

    @Override
    public void mouseExited(MouseEvent e) {
      setCursor(Cursor.getDefaultCursor());
    }

    private Point start;                                               // Stores cursor position on press
    private Point last;                                                // Stores cursor position on drag
    private Element tempElement = null;                                // Stores a temporary element
    private int buttonState = MouseEvent.NOBUTTON;                     // Records button state
    private Graphics2D g2D = null;                                     // Temporary graphics context
   }

  private Sketcher theApp;                                             // The application object
}

在AboutDialog构造函数中，AboutDialog对象应是OK按钮的监听器，所以把this变量作为参数传送给按钮的addActionListener()方法。
pack()方法继承自Window类。这个方法封装了窗口中的组件，把窗口设置为相对其中的组件而言最优的尺寸，之后布置组件。注意，如果这里没有调用pack()方法，对话框的尺寸就不会设置，对话框也不能显示出来。

选择OK按钮时调用actionPerformed()方法。这个方法通过调用AboutDialog对象的dispose()方法来销毁对话框，这样对话框窗口就会从屏幕上消失并释放占用的资源。

特别声明：以上只是为了演示，实际上javax.swing包中的JOptionPane类定义了一些静态方法，用来创建和显示标准的模态对话框。