Question

I have class that is derived from FrameworkElement, and I want WPF to update its Location property by using DoubleAnimation. I register the property as DependendencyProperty:

public class TimeCursor : FrameworkElement
{
    public static readonly DependencyProperty LocationProperty;

    public double Location
    {
        get { return (double)GetValue(LocationProperty); }
        set
        {
            SetValue(LocationProperty, value);
        }
    }

    static TimeCursor()
    {
        LocationProperty = DependencyProperty.Register("Location", typeof(double), typeof(TimeCursor));
    }
}

Following code sets up the storyboard.

TimeCursor timeCursor;
private void SetCursorAnimation()
{
    timeCursor = new TimeCursor();
    NameScope.SetNameScope(this, new NameScope());
    RegisterName("TimeCursor", timeCursor);

    storyboard.Children.Clear();

    DoubleAnimation animation = new DoubleAnimation(LeftOffset, LeftOffset + (VerticalLineCount - 1) * HorizontalGap + VerticalLineThickness, 
                new Duration(TimeSpan.FromMilliseconds(musicDuration)), FillBehavior.HoldEnd);

    Storyboard.SetTargetName(animation, "TimeCursor");
    Storyboard.SetTargetProperty(animation, new PropertyPath(TimeCursor.LocationProperty));

    storyboard.Children.Add(animation);
}

Then I call storyboard.Begin(this) from another method of the object which contains the above SetCursorAnimation() method and this object is derived from Canvas. However the Location property is never updated(set accessor of Location is never called) and no exception is thrown. What am I doing wrong?

Answer 1

When a dependency property is animated (or set in XAML, or set by a Style Setter, etc.), WPF does not call the CLR wrapper, but instead directly accesses the underlying DependencyObject and DependencyProperty objects. See the Implementing the “Wrapper” section in Checklist for Defining a Dependency Property and also Implications for Custom Dependency Properties.

In order to get notified about property changes, you have to register a PropertyChangedCallback:

public class TimeCursor : FrameworkElement
{
    public static readonly DependencyProperty LocationProperty =
        DependencyProperty.Register(
            "Location", typeof(double), typeof(TimeCursor),
            new FrameworkPropertyMetadata(LocationPropertyChanged)); // register callback here

    public double Location
    {
        get { return (double)GetValue(LocationProperty); }
        set { SetValue(LocationProperty, value); }
    }

    private static void LocationPropertyChanged(
        DependencyObject obj, DependencyPropertyChangedEventArgs e)
    {
        var timeCursor = obj as TimeCursor;

        // handle Location property changes here
        ...
    }
}

---

And note also that animating a dependency property does not necessarily require a Storyboard. You could simply call the BeginAnimation method on your TimeCursor instance:

var animation = new DoubleAnimation(LeftOffset,
    LeftOffset + (VerticalLineCount - 1) * HorizontalGap + VerticalLineThickness, 
    new Duration(TimeSpan.FromMilliseconds(musicDuration)),
    FillBehavior.HoldEnd);

timeCursor.BeginAnimation(TimeCursor.LocationProperty, animation);