package org.astrogrid.ui.help;
   
   import java.util.Vector;
   import javax.swing.tree.*;
   import java.awt.event.*;
   import org.astrogrid.log.Log;
   
   /***
    * History (from the navigator package)
   *
   * Provides a way of tracking a history, such as may be used by browsers.
   * This takes & returns Object types for each instance; it's up to the
   * using class to check for typesafety.
   *
   * Is an action listener, so can listen for (eg) button events - which
   * should have the commands given as final constants below
   *
   * Similarly, it generates History events, so that other components can tell if
   * it has changed
   *
   * @author M Hill
   */
  
  public class History implements ActionListener
  {
     private Vector history = new Vector();
     private int historyCursor = -1;
     
     private Vector listeners = new Vector();
     
     //command for back/fwd actions
     public static final String HISTORY_CMD_BACK = "BackHistory";
     public static final String HISTORY_CMD_FWD = "FwdHistory";
     public static final String HISTORY_CMD_CLEAR = "Clear History";
     
     /***
      * Add item to history at current index. ie, mmove forward,
      * ie trim at current index, add new item to end, and move
      * cursor forward to that new item
      */
     public void add(Object item)
     {
        trim();
        history.add(item);
        historyCursor++;
        fireHistoryChange();
     }
     
     /***
      * Chops history so it contains only all those before and
      * <b>including</b> the historyCursor
      */
     private void trim()
     {
        if (historyCursor>-1)
           while (history.size()>(historyCursor+1))
              history.removeElementAt(historyCursor+1);
        else
           history.clear();
        
     }
  
     /***
      * Gets current history item - returns null if no history
      */
     public Object getCurrentItem()
     {
        if (historyCursor<0)
           return null;
        else
           return history.elementAt(historyCursor);
     }
     
     public Object getPreviousItem()
     {
        if (historyCursor<1)
           return null;
        else
           return history.elementAt(historyCursor-1);
     }
     
     /***
      * Clears history - resets to empty
      */
     public void clear()
     {
        historyCursor = -1;
        trim();
     }
     
     /***
      * move to previous item in history and return it.
      * Returns null if no previous
      */
     public Object moveBack()
     {
        if (historyCursor >0)
        {
           historyCursor--;
          fireHistoryChange();
          return getCurrentItem();
       }
       return null;
    }
    
    /***
     * Move to next item in history and return it
     * Returns null if no next
     */
    public Object moveForward()
    {
       if (historyCursor+1<history.size())
       {
          historyCursor++;
          fireHistoryChange();
          return getCurrentItem();
       }
       return null;
    }
    
 
    /***
     * Register listener to listen to history cursor changes
     *
    public void addHistoryListener(HistoryListener newListener)
    {
       listeners.add(newListener);
    }
    
    /**
     * Let listeners know of change
     */
    private void fireHistoryChange()
    {
       dump();//debug
       
 //      HistoryEvent event = new HistoryEvent(isFirst(), isLast());
 //      for (int i=0;i<listeners.size();i++)
 //      {
 //         ((HistoryListener) listeners.get(i)).historyChanged(event);
 //      }
    }
 
    /***
     * Returns true if cursor is on last history item
     */
    public boolean isLast()
    {
       return (historyCursor == history.size()-1);
    }
    
    /***
     * Returns true if cursor is on first history item
     */
    public boolean isFirst()
    {
       return (historyCursor == 0) || (historyCursor == -1);
    }
    
    /***
     * Returns true if there is a previous item
     */
    public boolean isPrevious()
    {
       return (historyCursor > 0);
    }
    
    /***
     * Returns true if there is a next item
     */
    public boolean isNext()
    {
       return (historyCursor < history.size()-1);
    }
    /***
     * For debug - dump history to Log
     */
    public void dump()
    {
       //dump
       String s = "History: ";
       for (int i=0;i<history.size();i++)
       {
          if (i == historyCursor)
             s = s + ">>"+history.elementAt(i)+"<<, ";
          else
             s = s + ""+history.elementAt(i)+", ";
       }
       Log.trace(s);
    }
    
    /***
     * Although the History class exposes its action methods
     * (moveBack(), etc), it can also be given instructions via
     * ActionEvents.  in other words, you can create a "Back" Button,
     * set its 'actionCommand'  to a History command (see HISTORY_CMD_*),
     * and register a History instance with it, and Robert is your
     * father's brother.
     */
    public void actionPerformed(ActionEvent anEvent)
    {
       if (anEvent.getActionCommand() == HISTORY_CMD_BACK)
          moveBack();
       else if (anEvent.getActionCommand() == HISTORY_CMD_FWD)
          moveForward();
       else if (anEvent.getActionCommand() == HISTORY_CMD_CLEAR)
          clear();
    }
    
 }