UWP Button添加圆角阴影（三）

Composition

DropShadow是CompositionAPI中的东西，使用Storyboard设置某个属性，就是频繁的触发put_xxx()方法，效率远远不如使用CompositionAnimation。
Composition对象的基类CompositionObject拥有一个属性叫ImplicitAnimations，可以通过他实现累死css的transition的效果，也就是对应属性修改的时候，平滑的过渡过去。
可以从DropShadowPanel的源代码中看到，DropShadow是设置在ShadowElement上的ChildVisual。
相关内容可以查阅将可视化层与 XAML 结合使用 - ElementCompositionPreview.SetElementChildVisual 方法。
而我们要做的，是把整个构造过程倒过来，通过VisualTreeHelper，从DropShadow中拿到ShadowElement，然后获取他的ChildVisual和Shadow，将ImplicitAnimations设置到Shadow上。
下面贴代码：

public static class DropShadowPanelHelper
{
    public static bool GetIsTransitionEnable(DropShadowPanel obj)
    {
        return (bool)obj.GetValue(IsTransitionEnableProperty);
    }

    public static void SetIsTransitionEnable(DropShadowPanel obj, bool value)
    {
        obj.SetValue(IsTransitionEnableProperty, value);
    }

    public static readonly DependencyProperty IsTransitionEnableProperty =
        DependencyProperty.RegisterAttached("IsTransitionEnable", typeof(bool), typeof(DropShadowPanelHelper), new PropertyMetadata(false, IsTransitionEnablePropertyChanged));


    private static void IsTransitionEnablePropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (e.NewValue != e.OldValue)
        {
            if (d is DropShadowPanel sender)
            {
                //尝试获取ShadowElement，如果为空，可能是DropShadowPanel还没有ApplyTemplate，注册DropShadowPanel的Loaded事件，在Loaded中再获取一次。
                var shadowElement = sender.FindDescendantByName("ShadowElement") as Border;
                if (shadowElement != null)
                {
                    SetImplicitAnimation(shadowElement, (bool)e.NewValue);
                }
                else
                {
                    sender.Loaded += DropShadowPanel_Loaded;
                }
            }
        }
    }

    private static void DropShadowPanel_Loaded(object sender, RoutedEventArgs e)
    {
        var dropShadowPanel = (DropShadowPanel)sender;
        dropShadowPanel.Loaded -= DropShadowPanel_Loaded;
        var shadowElement = dropShadowPanel.FindDescendantByName("ShadowElement") as Border;
        if (shadowElement != null)
        {
            SetImplicitAnimation(shadowElement, GetIsTransitionEnable(dropShadowPanel));
        }
    }

    private static void SetImplicitAnimation(FrameworkElement element, bool IsEnable)
    {
        if (ElementCompositionPreview.GetElementChildVisual(element) is SpriteVisual shadowVisual &&
            shadowVisual.Shadow is DropShadow shadow)
        {
            if (IsEnable)
            {
                //获取合成器
                var compositor = shadowVisual.Compositor;
              
                //创建ImplicitAnimationCollection
                var imp = compositor.CreateImplicitAnimationCollection();

                //创建BlurRadius动画，注意不要忘记设置Duration和Target
                var bluran = compositor.CreateScalarKeyFrameAnimation();
                //插入一个表达式关键帧，帧在进度为1的时候，值是最终值
                bluran.InsertExpressionKeyFrame(1f, "this.FinalValue");
                bluran.Duration = TimeSpan.FromSeconds(0.2d);
                bluran.Target = "BlurRadius";

                //创建Offset动画
                var offsetan = compositor.CreateVector3KeyFrameAnimation();
                offsetan.InsertExpressionKeyFrame(1f, "this.FinalValue");
                offsetan.Duration = TimeSpan.FromSeconds(0.2d);
                offsetan.Target = "Offset";

                //创建Opacity动画
                var opacityan = compositor.CreateScalarKeyFrameAnimation();
                opacityan.InsertExpressionKeyFrame(1f, "this.FinalValue");
                opacityan.Duration = TimeSpan.FromSeconds(0.2d);
                opacityan.Target = "Opacity";

                //ImplicitAnimationCollection是IDictionary，每个子项都要是KeyFrame动画，子项的Key和动画的Target要一样。
                ImplictAnimationCollection
                imp[bluran.Target] = bluran;
                imp[offsetan.Target] = offsetan;
                imp[opacityan.Target] = opacityan;

                //给shadow设置ImplicitAnimations
                shadow.ImplicitAnimations = imp;
            }
            else
            {
                var imp = shadow.ImplicitAnimations;
                shadow.ImplicitAnimations = null;
                if (imp != null)
                {
                    imp.Dispose();
                    imp = null;
                }
            }
        }
    }

}

表达式关键帧的关键字相关的内容可以查阅：ExpressionAnimation Class - Expression Keywords
最后的Xaml是这样的：

<Style TargetType="Button" x:Key="CornerRadiusShadowButtonStyle">
    <Setter Property="Background" Value="#007acc" />
    <Setter Property="Foreground" Value="White" />
    <Setter Property="BorderBrush" Value="Transparent" />
    <Setter Property="BorderThickness" Value="0" />
    <Setter Property="Padding" Value="20,10,20,10" />
    <Setter Property="HorizontalAlignment" Value="Left" />
    <Setter Property="VerticalAlignment" Value="Center" />
    <Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}" />
    <Setter Property="FontWeight" Value="Normal" />
    <Setter Property="FontSize" Value="{ThemeResource ControlContentThemeFontSize}" />
    <Setter Property="UseSystemFocusVisuals" Value="True" />
    <Setter Property="FocusVisualMargin" Value="-3" />
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="Button">
                <Grid x:Name="RootGrid">
                    <VisualStateManager.VisualStateGroups>
                        <VisualStateGroup x:Name="CommonStates">
                            <VisualState x:Name="Normal" />
                            <VisualState x:Name="PointerOver">
                                <VisualState.Setters>
                                    <Setter Target="Shadow.OffsetY" Value="2" />
                                    <Setter Target="Shadow.BlurRadius" Value="8" />
                                </VisualState.Setters>
                            </VisualState>
                            <VisualState x:Name="Pressed">
                                <Storyboard>
                                    <DoubleAnimation Storyboard.TargetName="Shadow" Storyboard.TargetProperty="OffsetY" To="3" Duration="0" />
                                    <DoubleAnimation Storyboard.TargetName="Shadow" Storyboard.TargetProperty="BlurRadius" To="12" Duration="0" />
                                </Storyboard>
                            </VisualState>
                            <VisualState x:Name="Disabled">
                                <Storyboard>
                                    <ObjectAnimationUsingKeyFrames Storyboard.TargetName="Background" Storyboard.TargetProperty="Fill">
                                        <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource ButtonBackgroundDisabled}" />
                                    </ObjectAnimationUsingKeyFrames>
                                </Storyboard>
                            </VisualState>
                        </VisualStateGroup>
                    </VisualStateManager.VisualStateGroups>
                    <control:DropShadowPanel x:Name="Shadow" 
                                             xmlns:control="using:Microsoft.Toolkit.Uwp.UI.Controls" 
                                             xmlns:helper="using:TestApp1.Helpers"
                                             HorizontalContentAlignment="Stretch" 
                                             helper:DropShadowPanelHelper.IsTransitionEnable="True"
                                             BlurRadius="5" OffsetX="1" OffsetY="1" Color="Black">
                        <Rectangle x:Name="Background" Fill="{TemplateBinding Background}" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" RadiusX="5" RadiusY="5" />
                    </control:DropShadowPanel>
                    <ContentPresenter x:Name="ContentPresenter"
                        BorderBrush="{TemplateBinding BorderBrush}"
                        BorderThickness="{TemplateBinding BorderThickness}"
                        Content="{TemplateBinding Content}"
                        ContentTransitions="{TemplateBinding ContentTransitions}"
                        ContentTemplate="{TemplateBinding ContentTemplate}"
                        Padding="{TemplateBinding Padding}"
                        HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                        VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                        AutomationProperties.AccessibilityView="Raw" />
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

启用方式就是中的xmlns:helper="using:TestApp1.Helpers" helper:DropShadowPanelHelper.IsTransitionEnable="True"
VisualState中有两种写法，第一种是写在Setter中，优点是写的少，缺点是不能控制起始时间。
第二种是写在Storyboard中，Duration一定要是0，因为真正的动画我们定义在ImplicitAnimation中了，这里的Animation只是用来触发值修改，不需要插值。
最终效果有点小掉帧，是DropShadow的问题，17763中的ThemeShadow应该会有改善。。。

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