Change attached property from CustomPanel doesn't refresh panel

Question

I created dummy custom panel ShelfPanel with attached property Exact influence panel arrange:

class ShelfPanel : Panel
{
    #region Start attached property
    public static DependencyProperty ExactProperty = DependencyProperty.RegisterAttached("Exact", typeof(int), typeof(ShelfPanel),
        new FrameworkPropertyMetadata(0, 
            FrameworkPropertyMetadataOptions.AffectsArrange | // Does this options have auto action?
            FrameworkPropertyMetadataOptions.AffectsRender)); // Does this options have auto action?

    public static void SetExact(UIElement element, int value)
    {
        element.SetValue(ExactProperty, value);
    }

    public static int GetExact(UIElement element)
    {
        return (int)element.GetValue(ExactProperty);
    }
    #endregion

    protected override Size MeasureOverride(Size availableSize)
    {
        Size final = new Size();
        foreach (UIElement child in InternalChildren)
        {
            child.Measure(availableSize);
            final.Height += child.DesiredSize.Height;
        }
        return final;
    }

    protected override Size ArrangeOverride(Size finalSize)
    {
        foreach (UIElement child in InternalChildren)
        {
            Point position = new Point();
            int exact = ShelfPanel.GetExact(child);
            // Calculate position based on attached Exact
            position.X = exact * 100;
            position.Y = exact * 100;
            child.Arrange(new Rect(position, child.DesiredSize));
        }
        return finalSize;
    }
}

<local:ShelfPanel>
     <local:Box local:ShelfPanel.Exact="0" MouseDown="Box_MouseDown"/>
     <local:Box local:ShelfPanel.Exact="1" />
     <local:Box local:ShelfPanel.Exact="2" />
</local:ShelfPanel>

public partial class MainWindow : Window // Codebehind for previous xaml
{
    public MainWindow()
    {
        InitializeComponent();
    }

    private void Box_MouseDown(object sender, MouseButtonEventArgs e)
    {
        // I certainly sure this code get triggered after click.
        ShelfPanel.SetExact(sender as UIElement, 3); 
    }
}

This works perfect <local:Box> are arranged as planned. As you can deduce from code, after click on first <local:Box> it should change it position to 3, just after others 2. But surprisingly nothing happen.

Doesn't FrameworkPropertyMetadataOptions.Affects Arrange or FrameworkPropertyMetadataOptions.AffectsRender automatically repaint panel? To make this works I need to add PropertyChangedCallbackand call there InvalidateVisual()?

Answer 1

You are setting the attached property on a Box, but want the parent element of the Box, i.e. the ShelfPanel to be arranged.

You should therefore set FrameworkPropertyMetadataOptions.AffectsParentArrange:

public static readonly DependencyProperty ExactProperty =
    DependencyProperty.RegisterAttached("Exact", typeof(int), typeof(ShelfPanel),
        new FrameworkPropertyMetadata(0,
            FrameworkPropertyMetadataOptions.AffectsParentArrange));