/********************************************************************
*
*  This library is free software; you can redistribute it and/or
*  modify it under the terms of the GNU Library General Public
*  License as published by the Free Software Foundation; either
*  version 2 of the License, or (at your option) any later version.
* 
*  This library is distributed in the hope that it will be useful,
*  but WITHOUT ANY WARRANTY; without even the implied warranty of
*  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
*  Library General Public License for more details.
* 
*  You should have received a copy of the GNU Library General Public
*  License along with this library; if not, write to the
*  Free Software Foundation, Inc., 59 Temple Place - Suite 330,
*  Boston, MA  02111-1307, USA.
********************************************************************/

package uk.ac.sanger.artemis.components;

import javax.swing.SwingUtilities;

/**
* This is the 3rd version of SwingWorker (also known as
* SwingWorker 3), an abstract class that you subclass to
* perform GUI-related work in a dedicated thread.  For
* instructions on using this class, see:
* 
* http://java.sun.com/docs/books/tutorial/uiswing/misc/threads.html
*
* Note that the API changed slightly in the 3rd version:
* You must now invoke start() on the SwingWorker after
* creating it.
*/
public abstract class SwingWorker {
    private Object value;  // see getValue(), setValue()
    private Thread thread;

    /** 
    * Class to maintain reference to current worker thread
    * under separate synchronization control.
    */
    private static class ThreadVar {
        private Thread thread;
        ThreadVar(Thread t) { thread = t; }
        synchronized Thread get() { return thread; }
        synchronized void clear() { thread = null; }
    }

    private ThreadVar threadVar;

    /** 
    * Get the value produced by the worker thread, or null if it 
    * hasn't been constructed yet.
    */
    protected synchronized Object getValue() { 
        return value; 
    }

    /** 
    * Set the value produced by worker thread 
    */
    private synchronized void setValue(Object x) { 
        value = x; 
    }

    /** 
    * Compute the value to be returned by the <code>get</code> method. 
    */
    public abstract Object construct();

    /**
    * Called on the event dispatching thread (not on the worker thread)
    * after the <code>construct</code> method has returned.
    */
    public void finished() {
    }

    /**
    * A new method that interrupts the worker thread.  Call this method
    * to force the worker to stop what it's doing.
    */
    public void interrupt() {
        Thread t = threadVar.get();
        if (t != null) {
            t.interrupt();
        }
        threadVar.clear();
    }

    /**
    * Return the value created by the <code>construct</code> method.  
    * Returns null if either the constructing thread or the current
    * thread was interrupted before a value was produced.
    * 
    * @return the value created by the <code>construct</code> method
    */
    public Object get() {
        while (true) {  
            Thread t = threadVar.get();
            if (t == null) {
                return getValue();
            }
            try {
                t.join();
            }
            catch (InterruptedException e) {
                Thread.currentThread().interrupt(); // propagate
                return null;
            }
        }
    }


    /**
    * Start a thread that will call the <code>construct</code> method
    * and then exit.
    */
    public SwingWorker() {
        final Runnable doFinished = new Runnable() {
           public void run() { finished(); }
        };

        Runnable doConstruct = new Runnable() { 
            public void run() {
                try {
                    setValue(construct());
                }
                finally {
                    threadVar.clear();
                }

                SwingUtilities.invokeLater(doFinished);
            }
        };

        Thread t = new Thread(doConstruct);
        threadVar = new ThreadVar(t);
    }

    /**
    * Start the worker thread.
    */
    public void start() {
        Thread t = threadVar.get();
        if (t != null) {
            t.start();
        }
    }
}