用Shape做动画的实例详解

上一篇几乎都在说DoubleAnimation的应用，这篇说说PointAnimation。

1. 使用PointAnimation

使用PointAnimation可以让Shape变形，但实际上没看到多少人会这么用，毕竟WPF做的软件多数不需要这么花俏。

1.1 在XAML上使用PointAnimation

<Storyboard x:Name="Storyboard2"            RepeatBehavior="Forever"            AutoReverse="True"            Duration="0:0:4"><PointAnimation Storyboard.TargetProperty="(Path.Data).(PathGeometry.Figures)[0].(PathFigure.StartPoint)"                    Storyboard.TargetName="Path2"                    To="0,0"                    EnableDependentAnimation="True" /><PointAnimation Storyboard.TargetProperty="(Path.Data).(PathGeometry.Figures)[0].(PathFigure.Segments)[0].(LineSegment.Point)"                    Storyboard.TargetName="Path2"                    To="100,0"                    EnableDependentAnimation="True" /><ColorAnimation To="#FF85C82E"                    Storyboard.TargetProperty="(Shape.Fill).(SolidColorBrush.Color)"                    Storyboard.TargetName="Path2" /></Storyboard>…<Path Margin="0,20,0,0"      x:Name="Path2"      Fill="GreenYellow"><Path.Data><PathGeometry><PathFigure StartPoint="50,0"><LineSegment Point="50,0" /><LineSegment Point="0,100" /><LineSegment Point="0,100" /><LineSegment Point="100,100" /><LineSegment Point="100,100" /></PathFigure></PathGeometry></Path.Data></Path>

在这个例子里最头痛的地方是Property-path 语法，如果不能熟记的话最好依赖Blend生成。

1.2 在代码中使用PointAnimation

如果Point数量很多，例如图表，通常会在C#代码中使用PointAnimation：

_storyboard = new Storyboard();Random random = new Random();for (int i = 0; i < _pathFigure.Segments.Count; i++){var animation = new PointAnimation { Duration = TimeSpan.FromSeconds(3) };    Storyboard.SetTarget(animation, _pathFigure.Segments[i]);    Storyboard.SetTargetProperty(animation, "(LineSegment.Point)");    animation.EnableDependentAnimation = true;    animation.EasingFunction = new QuarticEase { EasingMode = EasingMode.EaseOut };    animation.To = new Point((_pathFigure.Segments[i] as LineSegment).Point.X, (i % 2 == 0 ? 1 : -1) * i * 1.2 + 60);    _storyboard.Children.Add(animation);}_storyboard.Begin();

因为可以直接SetTarget，所以Property-path语法就可以很简单。

2. 扩展PointAnimation

上面两个例子的动画都还算简单，如果更复杂些，XAML或C#代码都需要写到很复杂。我参考了这个网页想做出类似的动画，但发现需要写很多XAML所以放弃用PointAnimation实现。这个页面的动画核心是这段HTML：

<polygon fill="#FFD41D" points="97.3,0 127.4,60.9 194.6,70.7 145.9,118.1 157.4,185.1 97.3,153.5 37.2,185.1 48.6,118.1 0,70.7     67.2,60.9">        <animate id="animation-to-check" begin="indefinite" fill="freeze" attributeName="points" dur="500ms" to="110,58.2 147.3,0 192.1,29 141.7,105.1 118.7,139.8 88.8,185.1 46.1,156.5 0,125 23.5,86.6     71.1,116.7"/>      <animate id="animation-to-star" begin="indefinite" fill="freeze" attributeName="points" dur="500ms" to="97.3,0 127.4,60.9 194.6,70.7 145.9,118.1 157.4,185.1 97.3,153.5 37.2,185.1 48.6,118.1 0,70.7     67.2,60.9"/>  </polygon>

只需一组Point的集合就可以控制所有Point的动画，确实比PointAnimation高效很多。在WPF中可以通过继承Timeline实现一个PointCollectionAnimamtion，具体可以参考这个项目。可惜的是虽然UWP的Timeline类并不封闭，但完全不知道如何继承并派生一个自定义的Animation。

这时候需要稍微变通一下思维。可以将DoubleAnimation理解成这样：Storyboard将TimeSpan传递给DoubleAnimation，DoubleAnimation通过这个TimeSpan（有时还需要结合EasingFunction）计算出目标属性的当前值最后传递给目标属性，如下图所示：

既然这样，也可以接收到这个计算出来的Double，再通过Converter计算出目标的PointCollection值：

假设告诉这个Converter当传入的Double值（命名为Progress）为0的时候，PointCollection是{0,0 1,1 …}，Progress为100时PointCollection是{1,1 2,2 …}，当Progress处于其中任何值时的计算方法则是：

private PointCollection GetCurrentPoints(PointCollection fromPoints, PointCollection toPoints, double percentage){var result = new PointCollection();for (var i = 0;        i < Math.Min(fromPoints.Count, toPoints.Count);        i++)    {        var x = (1 - percentage / 100d) * fromPoints[i].X + percentage / 100d * toPoints[i].X;        var y = (1 - percentage / 100d) * fromPoints[i].Y + percentage / 100d * toPoints[i].Y;        result.Add(new Point(x, y));    }return result;}

这样就完成了从TimeSpan到PointCollection的转换过程。然后就是定义在XAML上的使用方式。参考上面PointCollectionAnimation，虽然多了个Converter，但XAML也应该足够简洁：

<local:ProgressToPointCollectionBridge x:Name="ProgressToPointCollectionBridge"><PointCollection>97.3,0 127.4,60.9 194.6,70.7 145.9,118.1 157.4,185.1 97.3,153.5 37.2,185.1 48.6,118.1 0,70.7 67.2,60.9</PointCollection><PointCollection>110,58.2 147.3,0 192.1,29 141.7,105.1 118.7,139.8 88.8,185.1 46.1,156.5 0,125 23.5,86.6 71.1,116.7</PointCollection></local:ProgressToPointCollectionBridge><Storyboard x:Name="Storyboard1"            FillBehavior="HoldEnd"><DoubleAnimation Duration="0:0:2"                        To="100"                        FillBehavior="HoldEnd"                        Storyboard.TargetProperty="(local:ProgressToPointCollectionBridge.Progress)"                        Storyboard.TargetName="ProgressToPointCollectionBridge"                        EnableDependentAnimation="True"/></Storyboard>…<Polygon x:Name="polygon"            Points="{Binding Source={StaticResource ProgressToPointCollectionBridge},Path=Points}"            Stroke="DarkOliveGreen"            StrokeThickness="2"            Height="250"            Width="250"            Stretch="Fill" />

最终我选择了将这个Converter命名为ProgressToPointCollectionBridge。可以看出Polygon 将Points绑定到ProgressToPointCollectionBridge，DoubleAnimation 改变ProgressToPointCollectionBridge.Progress，从而改变Points。XAML的简洁程度还算令人满意，如果需要操作多个点的话相对于PointAnimation的优势就很大。

运行结果如下：

完整的XAML：

<UserControl.Resources><local:ProgressToPointCollectionBridge x:Name="ProgressToPointCollectionBridge"><PointCollection>97.3,0 127.4,60.9 194.6,70.7 145.9,118.1 157.4,185.1 97.3,153.5 37.2,185.1 48.6,118.1 0,70.7 67.2,60.9</PointCollection><PointCollection>110,58.2 147.3,0 192.1,29 141.7,105.1 118.7,139.8 88.8,185.1 46.1,156.5 0,125 23.5,86.6 71.1,116.7</PointCollection></local:ProgressToPointCollectionBridge><Storyboard x:Name="Storyboard1"                FillBehavior="HoldEnd"><DoubleAnimation Duration="0:0:2"                            To="100"                            FillBehavior="HoldEnd"                            Storyboard.TargetProperty="(local:ProgressToPointCollectionBridge.Progress)"                            Storyboard.TargetName="ProgressToPointCollectionBridge"                            EnableDependentAnimation="True"><DoubleAnimation.EasingFunction><ElasticEase EasingMode="EaseInOut" /></DoubleAnimation.EasingFunction></DoubleAnimation><ColorAnimation Duration="0:0:2"                        To="#FF48F412"                        Storyboard.TargetProperty="(Shape.Fill).(SolidColorBrush.Color)"                        Storyboard.TargetName="polygon"                        d:IsOptimized="True"><ColorAnimation.EasingFunction><ElasticEase EasingMode="EaseInOut" /></ColorAnimation.EasingFunction></ColorAnimation></Storyboard></UserControl.Resources><Grid x:Name="LayoutRoot"        Background="White"><Polygon x:Name="polygon"                Points="{Binding Source={StaticResource ProgressToPointCollectionBridge},Path=Points}"                Stroke="DarkOliveGreen"                StrokeThickness="2"                Height="250"                Width="250"                Stretch="Fill"                Fill="#FFEBF412" /></Grid>

ProgressToPointCollectionBridge:

[ContentProperty(Name = nameof(Children))]public class ProgressToPointCollectionBridge : DependencyObject{public ProgressToPointCollectionBridge()    {        Children = new ObservableCollection<PointCollection>();    }/// <summary>///     获取或设置Points的值/// </summary>public PointCollection Points    {get { return (PointCollection) GetValue(PointsProperty); }set { SetValue(PointsProperty, value); }    }/// <summary>///     获取或设置Progress的值/// </summary>public double Progress    {get { return (double) GetValue(ProgressProperty); }set { SetValue(ProgressProperty, value); }    }/// <summary>///     获取或设置Children的值/// </summary>public Collection<PointCollection> Children    {get { return (Collection<PointCollection>) GetValue(ChildrenProperty); }set { SetValue(ChildrenProperty, value); }    }protected virtual void OnProgressChanged(double oldValue, double newValue)    {UpdatePoints();    }protected virtual void OnChildrenChanged(Collection<PointCollection> oldValue, Collection<PointCollection> newValue)    {var oldCollection = oldValue as INotifyCollectionChanged;if (oldCollection != null)            oldCollection.CollectionChanged -= OnChildrenCollectionChanged;var newCollection = newValue as INotifyCollectionChanged;if (newCollection != null)            newCollection.CollectionChanged += OnChildrenCollectionChanged;UpdatePoints();    }private void OnChildrenCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)    {UpdatePoints();    }private void UpdatePoints()    {if (Children == null || Children.Any() == false)        {            Points = null;        }else if (Children.Count == 1)        {var fromPoints = new PointCollection();for (var i = 0; i < Children[0].Count; i++)                fromPoints.Add(new Point(0, 0));var toPoints = Children[0];            Points = GetCurrentPoints(fromPoints, toPoints, Progress);        }else{var rangePerSection = 100d / (Children.Count - 1);var fromIndex = Math.Min(Children.Count - 2, Convert.ToInt32(Math.Floor(Progress / rangePerSection)));            fromIndex = Math.Max(fromIndex, 0);var toIndex = fromIndex + 1;            PointCollection fromPoints;if (fromIndex == toIndex)            {                fromPoints = new PointCollection();for (var i = 0; i < Children[0].Count; i++)                    fromPoints.Add(new Point(0, 0));            }else{                fromPoints = Children.ElementAt(fromIndex);            }var toPoints = Children.ElementAt(toIndex);            var percentage = (Progress / rangePerSection - fromIndex) * 100;            Points = GetCurrentPoints(fromPoints, toPoints, percentage);        }    }private PointCollection GetCurrentPoints(PointCollection fromPoints, PointCollection toPoints, double percentage)    {var result = new PointCollection();for (var i = 0;            i < Math.Min(fromPoints.Count, toPoints.Count);            i++)        {            var x = (1 - percentage / 100d) * fromPoints[i].X + percentage / 100d * toPoints[i].X;            var y = (1 - percentage / 100d) * fromPoints[i].Y + percentage / 100d * toPoints[i].Y;            result.Add(new Point(x, y));        }return result;    }#region DependencyProperties#endregion}

3. 结语

如果将DoubleAnimation说成“对目标的Double属性做动画”，那PointAnimation可以说成“对目标的Point.X和Point.Y两个Double属性同时做动画”，ColorAnimation则是“对目标的Color.A、R、G、B四个Int属性同时做动画”。这样理解的话PointAnimation和ColorAnimation只不过是DoubleAnimation的延伸而已，进一步的说，通过DoubleAnimation应该可以延伸出所有类型属性的动画。不过我并不清楚怎么在UWP上自定义动画，只能通过本文的折衷方式扩展。虽然XAML需要写复杂些，但这样也有它的好处：

不需要了解太多Animation相关类的知识，只需要有依赖属性、绑定等基础知识就够了。
不会因为动画API的改变而更改，可以兼容WPF、Silverlight和UWP（大概吧，我没有真的在WPF上测试这些代码）。
代码足够简单，省去了计算TimeSpan及EasingFunction的步骤。稍微修改下还可以做成泛型的AnimationBridge < T >，提供PointCollection以外数据类型的支持。

结合上一篇文章再发散一下，总觉得将来遇到什么UWP没有提供的功能都可以通过变通的方法实现，Binding和DependencyProperty真是UWP开发者最好的朋友。

4. 参考

How SVG Shape Morphing Works
Gadal MetaSyllabus