!7
  
cmodule NestedDesktopPane {
  transient JDesktopPane desktopPane;
  L<Win> windows = syncL();

  sclass Win {
    Rect rect;
    S title;
    transient JInternalFrame frame;
  }

  visualize {
    desktopPane = jDesktopPane();
    restoreFrames();
    ret withCenteredButtons(desktopPane,
      "Add frame", rThreadEnter addAFrame);
  }
 
  void unvisualize {
    super.unvisualize();
    for (Win win : windows) win.frame = null;
  }
  
  void addAFrame {
    JInternalFrame f = showInternalFrame(desktopPane, "Hello world");
    new Win win;
    win.rect = boundsAsRect(f);
    win.title = internalFrameTitle(f);
    setWinFrame(win, f);
    windows.add(win);
    change();
  }

  void setWinFrame(Win win, JInternalFrame f) {
    win.frame = f;
    onBoundsChange(f, r { grabRect(win) });
    onInternalFrameClosing(f, r { win.frame = null; removeWin(win) });
  }

  void restoreFrames {
    for (Win win : windows)
      setWinFrame(win, setBounds(win.rect, showInternalFrame(desktopPane, win.title)));
  }
  
  void grabRect(Win win) {
    if (win != null && win.frame != null && set_trueIfChanged(win, rect := boundsAsRect(win.frame)))
      change();
  }

  void removeWin(Win win) {
    if (windows.remove(win)) {
      disposeInternalFrame(win.frame);
      change();
    }
  }
}