WPF附加属性在设计时没有影响（xaml编辑器）

Question

I have a Attached Property that works great at runtime, and I would like to get it working at design time as well. I've read a bit of the docs and followed some tutorials, but I they assume more familiarity than I have.

The purpose of this DP (from source blog post (microsoft.co.il) ) is:

• Attached to any Panel type
• Set the margin for each item child item
• (future) Custom margin for final child

How can I get this property to have the same effect in the designer as it does at runtime? It affects the layout, so I think its important to see what is going on there too.

Simplified XAML :

<StackPanel Orientation="Horizontal" local:PanelItems.Margin="0,0,10,0">
    <TextBox Width="120" Text="{Binding Email}"/>
    <Button Content="Search" Command="{Binding SearchCommand}"/>
</StackPanel>

The current class (works fine at runtime).

I've remove inheriting from DependencyObject since that didn't help (I was following msdn examples) for this this case. Those examples don't handle attached properties, I suppose.

public static class PanelItems : DependencyObject
{
    public static readonly DependencyProperty MarginProperty = 
        DependencyProperty.RegisterAttached(
            "Margin", typeof(Thickness), typeof(PanelItems),
            new UIPropertyMetadata(new Thickness(), MarginChangedCallback)
        );

    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 void MarginChangedCallback(object sender, DependencyPropertyChangedEventArgs e)
    {
        // requires a panel control (e.g. a StackPanel)
        var panel = sender as Panel;
        if (panel == null) return;
        panel.Loaded += new RoutedEventHandler(OnPanelLoaded);
    }

    static void OnPanelLoaded(object sender, RoutedEventArgs e)
    {
        var panel = sender as Panel;
        foreach (var child in panel.Children.OfType<FrameworkElement>())
            child.Margin = PanelItems.GetMargin(panel);
    }        
}

Solution Notes

I added an extension method to check if the value has been set for a given object, which allows targeting only direct children. There is no other way that I can see, since there is no context back to the original change that caused the cascade.

public static class DependencyExtensions
{
    /// <summary>
    /// Checks if a DependencObject has a value set for the Dependency Property.
    /// Element is an object for conveninence purpose, and the return value is 
    /// always false if it is not a DependencyObject (e.g. a UI element)
    /// </summary>
    /// <param name="property">The DependencyProperty to check</param>
    /// <param name="element">The element (a DependencyObject) to check against</param>
    /// <returns></returns>
    public static bool IsSetFor(this DependencyProperty property, object element)
    {
        if (element is DependencyObject d)
        {
            return d.ReadLocalValue(property) != DependencyProperty.UnsetValue;
        }
        return false;
    }
}

After that I modified the event a little to check both. Nested panels work fine as well; the inner panel is according to the outer panels rule, and the inner children get the correct margin.

if (sender is FrameworkElement el && MarginProperty.IsSetFor(el.Parent))
{
    el.Margin = (Thickness)args.NewValue;
}

Note: Its also possible to call GetMargin(el as DependencyObject) though the values are the same as NewValue for children.

One final note: I had project code switched off on the designer, so no matter what I did I never could see the changes there. Here is the tiny unlabeled button with huge effects:

Answer 1

Here's a quick draft of a Margin property with value inheritance, which is thus automatically applied to all child elements.

Besides all child elements, a Margin is also applied to the Panel itself, which you may somehow try to avoid (perhaps by simply checking if the target element is a Panel).

public static class PanelItems
{
    public static readonly DependencyProperty MarginProperty =
        DependencyProperty.RegisterAttached(
            "Margin", typeof(Thickness), typeof(PanelItems),
            new FrameworkPropertyMetadata(new Thickness(),
                FrameworkPropertyMetadataOptions.Inherits, MarginChangedCallback));

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

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

    private static void MarginChangedCallback(
        object sender, DependencyPropertyChangedEventArgs e)
    {
        var element = sender as FrameworkElement;

        if (element != null && !(element is Panel))
        {
            element.Margin = (Thickness)e.NewValue;
        }
    }
}