Detect if Java Swing component has been hidden

Question

Assume we have the following Swing application:

    final JFrame frame = new JFrame();

    final JPanel outer = new JPanel();
    frame.add(outer);

    JComponent inner = new SomeSpecialComponent();
    outer.add(inner);

So in this example we simply have an outer panel in the frame and a special component in the panel. This special component must do something when it is hidden and shown. But the problem is that setVisible() is called on the outer panel and not on the special component. So I can't override the setVisible method in the special component and I also can't use a component listener on it. I could register the listener on the parent component but what if the outer panel is also in another panel and this outer outer panel is hidden?

Is there an easier solution than recursively adding componentlisteners to all parent components to detect a visibility change in SomeSpecialComponent?

Answer 1

Have a look at the ComponentListener (or ComponentAdapter)

http://java.sun.com/docs/books/tutorial/uiswing/events/componentlistener.html

http://java.sun.com/j2se/1.4.2/docs/api/java/awt/event/ComponentListener.html

And specifically the method:

void componentHidden(ComponentEvent e)
    Invoked when the component has been made invisible.

A complete solution would look something like:

inner.addComponentListener(new ComponentAdapter() {
    public void componentHidden(ComponentEvent ce) {
        System.out.println("Component hidden!");
    }
});

If the actions that should be carried out upon hiding is tightly coupled with the SomeSpecialCompnent, I would suggest to let SomeSpecialComponent implement ComponentListener, and add itself as a listener for the ComponentEvents in its constructor.

Another useful way (more related to add/remove operations and perhaps not suitable for your specific scenario) is to override addNotify() and removeNotify().

Answer 2

To listen for this kind of events occuring in the hierarchy, you could do the following.

class SomeSpecialComponent extends JComponent implements HierarchyListener {

    private boolean amIVisible() {
        Container c = getParent();
        while (c != null)
            if (!c.isVisible())
                return false;
            else
                c = c.getParent();
        return true;
    }

    public void addNotify() {
        super.addNotify();
        addHierarchyListener(this);
    }

    public void removeNotify() {
        removeHierarchyListener(this);
        super.removeNotify();
    }

    public void hierarchyChanged(HierarchyEvent e) {
        System.out.println("Am I visible? " + amIVisible());
    }

}

You could even be more precise in the treatement of HierarchyEvents. Have a look at

http://java.sun.com/javase/6/docs/api/java/awt/event/HierarchyEvent.html