Question

How can I animate the width of an element from 0 to its actual width in WPF?

I tried this:

<ControlTemplate.Triggers>
    <EventTrigger RoutedEvent="Loaded">
        <BeginStoryboard>
            <Storyboard>
                <DoubleAnimation Duration="0:0:0.3" To="{Binding ElementName=MyElement, Path=ActualWidth}" From="0" Storyboard.TargetProperty="Width" Storyboard.TargetName="MyElement" />
            </Storyboard>
        </BeginStoryboard>
    </EventTrigger>
</ControlTemplate.Triggers>

If I change the binding to a hard coded value, like 100, then the width is properly animated, except that I want to bind to the actual width of the element.

If it matters, MyElement is a border, and I’m animating a tab item.

For the record, this does not work either:

To="{Binding RelativeSource={RelativeSource AncestorType={x:Type Border}}, Path=ActualWidth}"

Answer 1

I’m sure this is wrong for SO many reasons and please feel free to tell me how many WPF laws I’ve violated but.. I solved the same problem by creating my own BindableDoubleAnimation.

public class BindableDoubleAnimation : DoubleAnimationBase
{
    DoubleAnimation internalAnimation;

    public DoubleAnimation InternalAnimation { get { return internalAnimation; } }

    public double To
    {
        get { return (double)GetValue(ToProperty); }
        set { SetValue(ToProperty, value); }
    }

    /// <summary>
    /// Dependency backing property for the <see cref="To"/> property.
    /// </summary>
    public static readonly DependencyProperty ToProperty =
        DependencyProperty.Register("To", typeof(double), typeof(BindableDoubleAnimation), new UIPropertyMetadata(0d, new PropertyChangedCallback((s, e) =>
            {
                BindableDoubleAnimation sender = (BindableDoubleAnimation)s;
                sender.internalAnimation.To = (double)e.NewValue;
            })));


    public double From
    {
        get { return (double)GetValue(FromProperty); }
        set { SetValue(FromProperty, value); }
    }

    /// <summary>
    /// Dependency backing property for the <see cref="From"/> property.
    /// </summary>
    public static readonly DependencyProperty FromProperty =
        DependencyProperty.Register("From", typeof(double), typeof(BindableDoubleAnimation), new UIPropertyMetadata(0d, new PropertyChangedCallback((s, e) =>
        {
            BindableDoubleAnimation sender = (BindableDoubleAnimation)s;
            sender.internalAnimation.From = (double)e.NewValue;
        })));


    public BindableDoubleAnimation()
    {
        internalAnimation = new DoubleAnimation();
    }

    protected override double GetCurrentValueCore(double defaultOriginValue, double defaultDestinationValue, AnimationClock animationClock)
    {
        return internalAnimation.GetCurrentValue(defaultOriginValue, defaultDestinationValue, animationClock);
    }

    protected override Freezable CreateInstanceCore()
    {
        return internalAnimation.Clone();;
    }
}

I’m now free to use bindings for the To From properties.

<local:BindableDoubleAnimation Storyboard.TargetProperty="Width" From="0" To="{Binding ActualWidth, RelativeSource={RelativeSource AncestorType=Window}}"/>