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 and examples of
 * using this class, see:
 *
 * <p>http://java.sun.com/docs/books/tutorial/uiswing/misc/threads.html
 *
 * <p>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()

  /**
   * 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();
    }
  }
}
