How to be notified that a composite's child received/lost focus?

Question

I have an SWT Composite that I need to pass to some other code which will add children to it at will. Is there any way to be notified that a child of the composite

Answer 1

As noted by Favonius, you can hook layout events like SWT.Resize to determine when you're being painted and recompute your child hierarchy, adding listeners as appropriate. Another option is simply to listen for all focus events and only pay attention to those that are for controls that you're interested in.

Displays have filters which, like listeners, are notified of events, however filters differ in that they are run before listeners, they have the opportunity to cancel events, and they are notified for all of a type of event on the entire Display.

You could thus use a Filter to examine all focus events and determine if it's one that you're interested in. For example:

public class MyControl extends Composite
{
    private final Listener focusListener;

    public MyControl(final Composite parent, final int style)
    {
        /* initialize the control... */

        focusListener = new Listener()
        {
            public void handleEvent(Event event)
            {
                if (!(event.widget instanceof Control))
                {
                    return;
                }

                boolean isOurChild = false;
                for (Control c = (Control) event.widget; c != null; c = c.getParent())
                {
                    if (c == container)
                    {
                        isOurChild = true;
                        break;
                    }
                }

                if (isOurChild)
                {
                    System.out.println("Our child is " + (event.type == SWT.FocusIn ? "focused" : "unfocused"));
                }
            }
        };

        getDisplay().addFilter(SWT.FocusIn, focusListener);
        getDisplay().addFilter(SWT.FocusOut, focusListener);

        addDisposeListener(new DisposeListener()
        {
            public void widgetDisposed(DisposeEvent e)
            {
                getDisplay().removeFilter(SWT.FocusIn, focusListener);
                getDisplay().removeFilter(SWT.FocusOut, focusListener);
            }
        });
    }
}

Do note the javadoc for Display's warnings about using filters:

They should generally be avoided for performance, debugging and code maintenance reasons.

Obviously you're looking at performance trade-offs in either solution - depending on what type of application you're delivering and your users' workflow, it may make more sense to add focus listeners when you resize, or it may make more sense to simply listen to all focus events and ignore the ones you're not interested in.