Question

I’m trying to listen for a window close event on the parent JFrame of a JPanel. In the WindowClosing event I’d like to de-register a listener to a different component.
Unfortunately the only code I can gaurantee to have run is the constructor for the panel. What this means is that the panel itself doesn’t have an ancestor window yet, so simply calling SwingUtilities.getWindowAncestor doesn’t work. So what I do is register a hierarchy listener, and in the hierarchyChanged event look for SHOWING_CHANGED event. When that even fires, now I can look for the window ancestor of the panel.

So basically I have the following:

class ShapeControlPanel extends JPanel{
    public ShapeControlPanel(){
        final ShapeControlPanel me = this;
        me.addHierarchyListener(new HierarchyListener() {
            @Override
            public void hierarchyChanged(HierarchyEvent e) {
                if((e.getChangeFlags() & HierarchyEvent.SHOWING_CHANGED) == HierarchyEvent.SHOWING_CHANGED){
                    SwingUtilities.getWindowAncestor(me).addWindowListener(new WindowListener() {
                        /* Snipped some empty handlers */

                        @Override
                        public void windowClosing(WindowEvent e) {
                            /* Finally get to remove the handler. */
                            me.getApparent().removeLocationSelectionListener(me.GUID(), me);
                        }
                    });
                }
            }
        });
    }
}

Is this sane? Is there a more reasonable way of getting a handle on the frame closing event?

Answer 1

It’s not the ugliest thing I’ve seen (I wouldn’t even say it’s all that bad), but you have to ask yourself: why does your panel really need to know when the window is closed? It seems to be an odd coupling that would best be removed.

I don’t know enough about your context and what you are truly trying to accomplish to suggest an alternative right now. But if a panel needs to know about the container in which it resides, there is probably some bad design with harmful coupling.