c# - Callback when dependency property recieves xaml change

Question

When I set the value of IsClosed during runtime, OnIsClosedChanged() is called fine. However, the Designer sets the value of the property but does not call the OnIsClosedChanged().

public static DependencyProperty IsClosedProperty = DependencyProperty.Register("IsClosed", typeof(bool), typeof(GroupBox), new FrameworkPropertyMetadata(false, FrameworkPropertyMetadataOptions.AffectsRender));

public bool IsClosed {
    get {
        return (bool)this.GetValue(IsClosedProperty);
    }
    set {
        if ((bool)this.GetValue(IsClosedProperty) == value)
            return;

        this.SetValue(IsClosedProperty, value);
        OnIsClosedChanged();
    }
}



private void OnIsClosedChanged() {
    _rowDefContent.Height = new GridLength((IsClosed ? 0 : 1), GridUnitType.Star);
}

Obviously IsClosed is not modified by the Designer and only IsClosedProperty receives the xaml change.
My question is: How can I run IsClosed after the value has been modified in the Designer. Or at least add some logic to the non-runtime changes.

Answer 1

You would have to register a PropertyChangedCallback with property metadata.

The reason is that dependency properties set in XAML or by bindings or some other source do not invoke the CLR wrapper (the setter method). The reason is explained in the XAML Loading and Dependency Properties article on MSDN:

For implementation reasons, it is computationally less expensive to identify a property as a dependency property and access the property system SetValue method to set it, rather than using the property wrapper and its setter.

...

Because the current WPF implementation of the XAML processor behavior for property setting bypasses the wrappers entirely, you should not put any additional logic into the set definitions of the wrapper for your custom dependency property. If you put such logic in the set definition, then the logic will not be executed when the property is set in XAML rather than in code.

Your code should look like this:

public static readonly DependencyProperty IsClosedProperty =
    DependencyProperty.Register(
        "IsClosed", typeof(bool), typeof(GroupBox),
        new FrameworkPropertyMetadata(false,
            FrameworkPropertyMetadataOptions.AffectsRender,
            (o, e) => ((GroupBox)o).OnIsClosedChanged()));

public bool IsClosed
{
    get { return (bool)GetValue(IsClosedProperty); }
    set { SetValue(IsClosedProperty, value); }
}

private void OnIsClosedChanged()
{
    _rowDefContent.Height = new GridLength((IsClosed ? 0 : 1), GridUnitType.Star);
}