dndlist.java

北大Java 语言程序设计 ppt课件及源码

/**
 * This is an example of a component, which serves as a DragSource as 
 * well as Drop Target.
 * To illustrate the concept, JList has been used as a droppable target
 * and a draggable source.
 * Any component can be used instead of a JList.
 * The code also contains debugging messages which can be used for 
 * diagnostics and understanding the flow of events.
 * 
 * @version 1.0
 */

import java.awt.*;
import java.awt.dnd.*;
import java.awt.datatransfer.*;

import java.util.Hashtable;
import java.util.List;
import java.util.Iterator;

import java.io.*;
import java.io.IOException;

import javax.swing.JList;
import javax.swing.DefaultListModel;




public class DNDList extends JList
    implements DNDComponentInterface, DropTargetListener,DragSourceListener, DragGestureListener    {

  /**
   * enables this component to be a dropTarget
   */

  DropTarget dropTarget = null;

  /**
   * enables this component to be a Drag Source
   */
  DragSource dragSource = null;


  /**
   * constructor - initializes the DropTarget and DragSource.
   */

  public DNDList() {
    
    dropTarget = new DropTarget (this, this);
    dragSource = new DragSource();
    dragSource.createDefaultDragGestureRecognizer( this, DnDConstants.ACTION_MOVE, this);
  }

  /**
   * is invoked when you are dragging over the DropSite
   * 
   */

  public void dragEnter (DropTargetDragEvent event) {
    
    // debug messages for diagnostics 
    System.out.println( "dragEnter");
    event.acceptDrag (DnDConstants.ACTION_MOVE);
  }

  /**
   * is invoked when you are exit the DropSite without dropping
   *
   */

  public void dragExit (DropTargetEvent event) {
    System.out.println( "dragExit");
    
  }

  /**
   * is invoked when a drag operation is going on
   * 
   */

  public void dragOver (DropTargetDragEvent event) {
    System.out.println( "dragOver");
  }

  /**
   * a drop has occurred
   * 
   */

 
  public void drop (DropTargetDropEvent event) {
    
    try {
        Transferable transferable = event.getTransferable();
                   
        // we accept only Strings      
        if (transferable.isDataFlavorSupported (DataFlavor.stringFlavor)){
        
            event.acceptDrop(DnDConstants.ACTION_MOVE);
            String s = (String)transferable.getTransferData ( DataFlavor.stringFlavor);
            addElement( s );
            event.getDropTargetContext().dropComplete(true);
        } 
        else{
            event.rejectDrop();
        }
    }
    catch (IOException exception) {
        exception.printStackTrace();
        System.err.println( "Exception" + exception.getMessage());
        event.rejectDrop();
    } 
    catch (UnsupportedFlavorException ufException ) {
      ufException.printStackTrace();
      System.err.println( "Exception" + ufException.getMessage());
      event.rejectDrop();
    }
  }

  /**
   * is invoked if the use modifies the current drop gesture
   * 
   */
    

  public void dropActionChanged ( DropTargetDragEvent event ) {
  }

  /**
   * a drag gesture has been initiated
   * 
   */
  
  public void dragGestureRecognized( DragGestureEvent event) {
    
    Object selected = getSelectedValue();
    if ( selected != null ){
        StringSelection text = new StringSelection( selected.toString()); 
        
        // as the name suggests, starts the dragging
        dragSource.startDrag (event, DragSource.DefaultMoveDrop, text, this);
    } else {
        System.out.println( "nothing was selected");   
    }
  }

  /**
   * this message goes to DragSourceListener, informing it that the dragging 
   * has ended
   * 
   */

  public void dragDropEnd (DragSourceDropEvent event) {   
    if ( event.getDropSuccess()){
        removeElement();
    }
  }

  /**
   * this message goes to DragSourceListener, informing it that the dragging 
   * has entered the DropSite
   * 
   */

  public void dragEnter (DragSourceDragEvent event) {
    System.out.println( " dragEnter");
  }

  /**
   * this message goes to DragSourceListener, informing it that the dragging 
   * has exited the DropSite
   * 
   */

  public void dragExit (DragSourceEvent event) {
    System.out.println( "dragExit");
    
  }

  /**
   * this message goes to DragSourceListener, informing it that the dragging is currently 
   * ocurring over the DropSite
   * 
   */

  public void dragOver (DragSourceDragEvent event) {
    System.out.println( "dragExit");
    
  }

  /**
   * is invoked when the user changes the dropAction
   * 
   */
   
  public void dropActionChanged ( DragSourceDragEvent event) {
    System.out.println( "dropActionChanged"); 
  }

  /**
   * adds elements to itself
   * 
   */
   
   public void addElement( Object s ){
        (( DefaultListModel )getModel()).addElement (s.toString());
  }

  /**
   * removes an element from itself
   */
   
  public void removeElement(){
    (( DefaultListModel)getModel()).removeElement( getSelectedValue());
  }
  
}