Animation ImageBrush.ImageSource情节提要动画

我把按钮画成椭圆形

        <Style x:Key="CircleImageButton" TargetType="Button">
    <Setter Property="Background" Value="Transparent" />
    <Setter Property="BorderBrush" Value="{ThemeResource PhoneForegroundBrush}" />
    <Setter Property="Foreground" Value="{ThemeResource PhoneForegroundBrush}" />
    <Setter Property="BorderThickness" Value="{ThemeResource PhoneBorderThickness}" />
    <Setter Property="FontFamily" Value="{ThemeResource PhoneFontFamilyNormal}" />
    <Setter Property="FontWeight" Value="{ThemeResource PhoneButtonFontWeight}" />
    <Setter Property="FontSize" Value="{ThemeResource TextStyleLargeFontSize}" />
    <Setter Property="Padding" Value="{ThemeResource PhoneButtonContentPadding}" />
    <Setter Property="MinHeight" Value="{ThemeResource PhoneButtonMinHeight}" />
    <Setter Property="MinWidth" Value="{ThemeResource PhoneButtonMinWidth}" />
    <Setter Property="HorizontalAlignment" Value="Left" />
    <Setter Property="VerticalAlignment" Value="Center" />
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="Button">
                <Grid x:Name="Grid" Background="Transparent">
                    <VisualStateManager.VisualStateGroups>
                        <VisualStateGroup x:Name="CommonStates">
                            <VisualStateGroup.Transitions>
                                <VisualTransition From="Pressed" To="PointerOver">
                                    <Storyboard>
                                        <PointerUpThemeAnimation Storyboard.TargetName="ImageEllipse" />
                                    </Storyboard>
                                </VisualTransition>
                                <VisualTransition From="PointerOver" To="Normal">
                                    <Storyboard>
                                        <PointerUpThemeAnimation Storyboard.TargetName="ImageEllipse" />
                                    </Storyboard>
                                </VisualTransition>
                                <VisualTransition From="Pressed" To="Normal">
                                    <Storyboard>
                                        <PointerUpThemeAnimation Storyboard.TargetName="ImageEllipse" />
                                    </Storyboard>
                                </VisualTransition>
                            </VisualStateGroup.Transitions>
                            <VisualState x:Name="Normal" />
                            <VisualState x:Name="PointerOver">
                            </VisualState>
                            <VisualState x:Name="Pressed">
                                <Storyboard>
                                    <PointerDownThemeAnimation Storyboard.TargetName="ImageEllipse" />
                                    <ObjectAnimationUsingKeyFrames Storyboard.TargetName="ImageEllipse" Storyboard.TargetProperty="(Shape.Fill).(ImageBrush.ImageSource)">
                                        <DiscreteObjectKeyFrame KeyTime="0" Value="ms-appx:///Assets/Buttons/photo_icon_tap.png" />
                                    </ObjectAnimationUsingKeyFrames>
                                </Storyboard>
                            </VisualState>
                        </VisualStateGroup>
                    </VisualStateManager.VisualStateGroups>
                    <Ellipse x:Name="ImageEllipse"
                             MinWidth="127"
                             MinHeight="127">
                        <Ellipse.Fill>
                            <ImageBrush ImageSource="ms-appx:///Assets/Buttons/photo_icon_pas.png" />
                        </Ellipse.Fill>
                    </Ellipse>
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

当我按下按钮时，没有错误信息，但没有图片

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我假设属性Storyboard.TargetProperty=Shape.Fill.ImageBrush.ImageSource

确定ImageBrush中的url吗？问题已更改为相对uri