Be notified when visual/logical child added/removed

Question

I am currently looking for a way to be notified when a child is added to the visual or logical children.

I am aware of the Visual::OnVisualChildrenChanged method, but it does not apply to me since I can't always inherit and override this function. I am looking for an event.

So, is there a way for the owner of a FrameworkElement/Visual to be notified when a child is added?

Answer 1

I believe that FrameworkElement.Loaded and FrameworkElement.Unloaded are fired when the control is added to and removed from the Visual Tree, respectively. However, the few times that I was trying to do anything with them I wasn't able to get them to fire consistently (I was using Class Event Handlers at the time, so that might have something to do with it).

Answer 2

Maybe it would be worth to add to child field keeping reference to parent and send notification after change back to it?

Answer 3

EDIT:

I just thought of something. If you are able to set a Loaded handler on each element which appears in your visual tree, perhaps it could be done with a technique like this (consider this a VERY basic idea, just as a start towards a possible solution):

public class ElementChildrenChangedEventArgs
{
    public ElementChildrenChangedEventArgs(FrameworkElement parent, FrameworkElement child)
    {
        Parent = parent;
        Child = child;
    }

    public FrameworkElement Parent { get; private set;}
    public FrameworkElement Child { get; private set;}
}

public delegate void ChildrenChanged(ElementChildrenChangedEventArgs args);

public static class ElementLoadedManager
{
    private static readonly HashSet<FrameworkElement> elements = new HashSet<FrameworkElement>();

    public static void Process_Loaded(FrameworkElement fe)
    {
        FrameworkElement feParent = VisualTreeHelper.GetParent(fe) as FrameworkElement;

        if (feParent != null)
        {
            if (elements.Contains(feParent))
            {
                InvokeChildrenChanged(feParent, fe);
            }
        }

        if (!elements.Contains(fe))
        {
            elements.Add(fe);
        }

        fe.Unloaded += Element_Unloaded;           
    }

    static void Element_Unloaded(object sender, RoutedEventArgs e)
    {
        FrameworkElement fe = sender as FrameworkElement;
        elements.Remove(fe);
        ClearUnloaded(fe);
    }

    static void ClearUnloaded(FrameworkElement fe)
    {
        fe.Unloaded -= Element_Unloaded;
    }

    public static event ChildrenChanged ChildrenChanged;

    private static void InvokeChildrenChanged(FrameworkElement parent, FrameworkElement child)
    {
        ElementChildrenChangedEventArgs args = new ElementChildrenChangedEventArgs(parent, child);

        ChildrenChanged changed = ChildrenChanged;

        if (changed != null) changed(args);
    }
}

The idea (and requirement) would be to invoke the Process_Loaded(FrameworkElement) method in each element's Loaded handler, which is possible to do with some style/template setting.

And since Loaded is a routed event, you could set the handler only on the parent window and check for e.OriginalSource.