扩展wpf动画类的奥秘

Silverlight在其动画时间线(如DoubleAnimation)上有一个名为easingFunction的属性,该属性允许您指定一个用于插值两个值的函数。即使是.net 4,我还是想把它移植到3.5。阅读后 this ,看起来很可行,但我有个奇怪的问题。

我正在扩展DoubleAnimation,如下所示:

class EasingDoubleAnimation : DoubleAnimation
{
    protected override Freezable CreateInstanceCore()
    {
        return new EasingDoubleAnimation();
    }

    protected override double GetCurrentValueCore( double defaultOriginValue, double defaultDestinationValue, AnimationClock animationClock )
    {
        Debug.WriteLine( animationClock.CurrentProgress.Value );
        return base.GetCurrentValueCore( defaultOriginValue, defaultDestinationValue, animationClock );
    }

    public EasingFunctionBase EasingFunction
    {
        get { return ( EasingFunctionBase ) GetValue( EasingFunctionProperty ); }
        set { SetValue( EasingFunctionProperty, value ); }
    }

    public static readonly DependencyProperty EasingFunctionProperty =
        DependencyProperty.Register( "EasingFunction", typeof( EasingFunctionBase ), typeof( EasingDoubleAnimation ),
            new PropertyMetadata( null ) );
}

注意,除了添加新属性之外,它没有做任何有趣的事情。

然后我可以在一些代码中使用它:

Storyboard sb = new Storyboard();
EasingDoubleAnimation ease = new EasingDoubleAnimation();
//ease.EasingFunction = new BackEase();
Storyboard.SetTarget( ease, MyTarget );
Storyboard.SetTargetProperty( ease, new PropertyPath( "(Canvas.Left)" ) );
ease.To = 100;
ease.Duration = TimeSpan.FromSeconds( 3 );
sb.Children.Add( ease );
sb.Begin();

这段代码可以很好地运行动画。

但是,如果取消注释设置easingfunction的行,动画将不再运行。调用了我的createInstanceCore方法,但从未调用getcurrentValue。奇怪的?