WPF中自定义GridLengthAnimation

需求

我们想在编辑一个列表中某一个条目时，将编辑的详情内容也放置当前面，比如右侧。

可以通过将一个Grid,分成两个Cloumn,动态调整两个Cloumn的Width，就可以实现这个需求。

我们知道，Clomun的Width是个

，而默认的动画没有这样子的。我们就需要自己实现这样一人动画。

设计

我们从Animation的类图上看到

我们可以从需求

我们想在编辑一个列表中某一个条目时，将编辑的详情内容也放置当前面，比如右侧。

可以通过将一个Grid,分成两个Cloumn,动态调整两个Cloumn的Width，就可以实现这个需求。

我们知道，Clomun的Width是个GridLength，而默认的动画没有这样子的。我们就需要自己实现这样一人动画。

设计

我们从Animation的类图上看到AnimationTimeline继承，重写其GetCurrentValue

public class GridLengthAnimation : AnimationTimeline
    {
        /// <summary>
        /// Returns the type of object to animate
        /// </summary>
        public override Type TargetPropertyType => typeof(GridLength);

        /// <summary>
        /// Creates an instance of the animation object
        /// </summary>
        /// <returns>Returns the instance of the GridLengthAnimation</returns>
        protected override System.Windows.Freezable CreateInstanceCore()
        {
            return new GridLengthAnimation();
        }

        /// <summary>
        /// Dependency property for the From property
        /// </summary>
        public static readonly DependencyProperty FromProperty = DependencyProperty.Register("From", typeof(GridLength),
            typeof(GridLengthAnimation));

        /// <summary>
        /// CLR Wrapper for the From depenendency property
        /// </summary>
        public GridLength From
        {
            get
            {
                return (GridLength)GetValue(GridLengthAnimation.FromProperty);
            }
            set
            {
                SetValue(GridLengthAnimation.FromProperty, value);
            }
        }

        /// <summary>
        /// Dependency property for the To property
        /// </summary>
        public static readonly DependencyProperty ToProperty = DependencyProperty.Register("To", typeof(GridLength),
            typeof(GridLengthAnimation));

        /// <summary>
        /// CLR Wrapper for the To property
        /// </summary>
        public GridLength To
        {
            get
            {
                return (GridLength)GetValue(GridLengthAnimation.ToProperty);
            }
            set
            {
                SetValue(GridLengthAnimation.ToProperty, value);
            }
        }

        /// <summary>
        /// Animates the grid let set
        /// </summary>
        /// <param name="defaultOriginValue">The original value to animate</param>
        /// <param name="defaultDestinationValue">The final value</param>
        /// <param name="animationClock">The animation clock (timer)</param>
        /// <returns>Returns the new grid length to set</returns>
        public override object GetCurrentValue(object defaultOriginValue,
            object defaultDestinationValue, AnimationClock animationClock)
        {
            double fromVal = ((GridLength)GetValue(GridLengthAnimation.FromProperty)).Value;

            double toVal = ((GridLength)GetValue(GridLengthAnimation.ToProperty)).Value;

            if (fromVal > toVal)
                return new GridLength((1 - animationClock.CurrentProgress.Value) * (fromVal - toVal) + toVal, GridUnitType.Star);
            else
                return new GridLength(animationClock.CurrentProgress.Value * (toVal - fromVal) + fromVal, GridUnitType.Star);
        }

如上所示，我们仿着默认动画实现了From,To,同时将其属性定义为GridLength,当动画执行时，我们重写了GetCurrentValue,使其根据From/To属性相关联。

优化

通过以上代码，我们实现了在GridLength变化时，实现动画。但是，试用后我们发现，动画，有点太线性。这个时候，怎么办？

可以通过引入EasingFunction来实现。我们知道EasingFunction其实就是一个与时间t有关的时间函数f(t).通过时间函数的处理，我们使动画过渡不要那么线性。

  /// <summary>
        /// The <see cref="EasingFunction" /> dependency property's name.
        /// </summary>
        public const string EasingFunctionPropertyName = "EasingFunction";

        /// <summary>
        /// Gets or sets the value of the <see cref="EasingFunction" />
        /// property. This is a dependency property.
        /// </summary>
        public IEasingFunction EasingFunction
        {
            get
            {
                return (IEasingFunction)GetValue(EasingFunctionProperty);
            }
            set
            {
                SetValue(EasingFunctionProperty, value);
            }
        }

        /// <summary>
        /// Identifies the <see cref="EasingFunction" /> dependency property.
        /// </summary>
        public static readonly DependencyProperty EasingFunctionProperty = DependencyProperty.Register(
            EasingFunctionPropertyName,
            typeof(IEasingFunction),
            typeof(GridLengthAnimation),
            new UIPropertyMetadata(null));

对应的，还要重写GetCurrentValue函数。

public override object GetCurrentValue(object defaultOriginValue,
            object defaultDestinationValue, AnimationClock animationClock)
        {
            double fromVal = ((GridLength)GetValue(FromProperty)).Value;

            double toVal = ((GridLength)GetValue(ToProperty)).Value;

            //check that from was set from the caller
            //if (fromVal == 1)
            //    //set the from as the actual value
            //    fromVal = ((GridLength)defaultDestinationValue).Value;

            double progress = animationClock.CurrentProgress.Value;

            IEasingFunction easingFunction = EasingFunction;
            if (easingFunction != null)
            {
                progress = easingFunction.Ease(progress);
            }

            if (fromVal > toVal)
                return new GridLength((1 - progress) * (fromVal - toVal) + toVal, GridUnitType.Star);

                return new GridLength(progress * (toVal - fromVal) + fromVal, GridUnitType.Star);
        }

使用

   <anim:GridLengthAnimation Storyboard.TargetProperty="Width" From="0" To="*" Duration="0:0:0.5"/>