Question

I have following class that defines attached property to set children’s margin:

public class MarginSetter
{
    public static Thickness GetMargin(DependencyObject obj)
    {
        return (Thickness)obj.GetValue(MarginProperty);
    }

    public static void SetMargin(DependencyObject obj, Thickness value)
    {
        obj.SetValue(MarginProperty, value);
    }

    public static readonly DependencyProperty MarginProperty =
        DependencyProperty.RegisterAttached("Margin", typeof(Thickness), typeof(MarginSetter), new UIPropertyMetadata(new Thickness(), CreateThicknesForChildren));

    public static void CreateThicknesForChildren(object sender, DependencyPropertyChangedEventArgs e)
    {
        var panel = sender as Panel;
        if (panel == null) return;
        foreach (var child in panel.Children)
        {
            var fe = child as FrameworkElement;
            if (fe == null) continue;
            fe.Margin = MarginSetter.GetMargin(panel);
        }
    }
}

The problem is that when CreateThicknesForChildren is invoked no child controls are added to parent yet. How to fix this class so that it will correctly set margin on all child controls?

In my project no controls are dynamically added to parent, they all are created in xaml file. By the way, designer works correctly and somehow correctly sets margin for all children elements.

Answer 1

How about registering to the Loaded event of the panel? it won’t help if you’re adding items dynamically later, but for the basic 95% it would work:

public static void CreateThicknesForChildren(object sender, DependencyPropertyChangedEventArgs e)
{
    var panel = sender as Panel;
    panel.Loaded += ...
}