上一篇幾乎都在說DoubleAnimation的應用，這篇說說PointAnimation。
使用PointAnimation可以讓Shape變形，但其實沒看到多少人會這麼用，畢竟WPF做的軟體多數不需要這麼花俏。
<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>
在這個範例裡最頭痛的地方是Property-path 語法，如果不能熟記的話最好依賴Blend生成。
如果Point數量很多，例如圖表，通常會在C#程式碼中使用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();
#因為可以直接SetTarget，所以Property-path語法就可以很簡單。
上面兩個範例的動畫都還算簡單，如果更複雜些，XAML或C#程式碼都需要寫到很複雜。我參考了這個網頁 想做類似的動畫，但發現需要寫很多XAML所以放棄用PointAnimation實作。這個頁面的動畫核心是這段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>只需一組Point的集合就可以控制所有Point的動畫，確實比PointAnimation高效很多。 在WPF中可以透過繼承Timeline實作一個PointCollectionAnimamtion，具體可以參考這個項目。可惜的是雖然UWP的Timeline類別並不封閉，但完全不知道如何繼承並衍生出一個自訂的Animation。
這時候需要稍微變通一下思維。可以將DoubleAnimation理解成這樣：Storyboard將TimeSpan傳遞給DoubleAnimation，DoubleAnimation透過這個TimeSpan（有時還需要結合EasingFunction）計算出目標屬性的目前值最後傳遞給目標屬性，如下圖所示：
既然這樣，也可以接收到這個計算出來的Double，再透過Converter計算出目標的PointCollection值：
假設告訴這個Converter當傳入的Double值（命名為Progress）為0的時候，PointCollection是{0,0 1,1 …}，Progress為100時PointCollection是{1,1 2,2 …}，當Progress處於其中任何值時的計算方法則是：
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;
}這樣就完成了從TimeSpan到PointCollection的轉換過程。然後就是定義在XAML上的使用方式。參考上面PointCollectionAnimation，雖然多了個Converter，但XAML也應該夠簡潔：
<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" />最後我選擇了將這個Converter命名為ProgressToPointCollectionBridge。可以看出Polygon 將Points綁定到ProgressToPointCollectionBridge，DoubleAnimation 改變ProgressToPointCollectionBridge.Progress，進而改變Points。 XAML的簡潔程度還算令人滿意，如果需要操作多點的話相對於PointAnimation的優勢就很大。
運行結果如下：
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}如果將DoubleAnimation說成“對目標的Double屬性做動畫”，那PointAnimation可以說成“對目標的Point.X和Point.Y兩個Double屬性同時做動畫”，ColorAnimation則是“對目標的Color.A、R、G、B四個Int屬性同時做動畫”。這樣理解的話PointAnimation和ColorAnimation只不過是DoubleAnimation的延伸而已，進一步的說，透過DoubleAnimation應該可以延伸出所有類型屬性的動畫。不過我並不清楚怎麼在UWP上自訂動畫，只能透過本文的折衷方式擴充。雖然XAML需要寫複雜些，但這樣也有它的好處：
不需要了解太多Animation相關類別的知識，只需要有依賴屬性、綁定等基礎知識就夠了。
不會因為動畫API的改變而更改，可以相容於WPF、Silverlight和UWP（大概吧，我沒有真的在WPF上測試這些程式碼）。
程式碼夠簡單，省略了計算TimeSpan及EasingFunction的步驟。 稍微修改下還可以做成泛型的AnimationBridge < T >，提供PointCollection以外資料型別的支援。
結合上一篇文章再發散一下，總覺得將來遇到什麼UWP沒有提供的功能都可以透過變通的方法實現，Binding和DependencyProperty真是UWP開發者最好的朋友。
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