【Win10】让 AppBarButton 支持更复杂的 Icon 内容

最近有一个需求，需要制作这么一个 AppBarButton：





这个 AppBarButton 的 Icon 是一个评论框图标里面再显示评论数（大于 99 条则显示 99+）。其中评论数是通过数据绑定得来的。

要单独显示这个评论框图标或者单独显示评论数都不是难题，然而要同时显示这两者就成了一个大问题了，因为 AppBarButton 的 Icon 属性只能设置一个。再看看 Icon 属性的类型，是 IconElement 这个类。但是，这个类的构造函数是内部的，我们也没法继承它。那么，从 Icon 属性上入手是不可能了。

但是，要注意到，绝大部分的控件都是有模板的，然后可以修改的。对于 AppBarButton，是不是也这样呢？答案是肯定的。

在 MSDN 上，我们可以查阅到 AppBarButton 的默认样式和模板：https://msdn.microsoft.com/zh-cn/library/windows/apps/mt299105.aspx

<!-- Default style for Windows.UI.Xaml.Controls.AppBarButton -->
<Style TargetType="AppBarButton">
<Setter Property="Background" Value="Transparent"/>
<Setter Property="Foreground" Value="{ThemeResource SystemControlForegroundBaseHighBrush}"/>
<Setter Property="HorizontalAlignment" Value="Left"/>
<Setter Property="VerticalAlignment" Value="Top"/>
<Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}"/>
<Setter Property="FontWeight" Value="Normal"/>
<Setter Property="Width" Value="68"/>
<Setter Property="UseSystemFocusVisuals" Value="True" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="AppBarButton">
<Grid
x:Name="Root"
MinWidth="{TemplateBinding MinWidth}"
MaxWidth="{TemplateBinding MaxWidth}"
Background="{TemplateBinding Background}">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="ApplicationViewStates">
<VisualState x:Name="FullSize"/>
<VisualState x:Name="Compact">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="TextLabel" Storyboard.TargetProperty="Visibility">
<DiscreteObjectKeyFrame KeyTime="0" Value="Collapsed"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Overflow">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentRoot" Storyboard.TargetProperty="Visibility">
<DiscreteObjectKeyFrame KeyTime="0" Value="Collapsed"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="OverflowTextLabel" Storyboard.TargetProperty="Visibility">
<DiscreteObjectKeyFrame KeyTime="0" Value="Visible"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="OverflowWithToggleButtons">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentRoot" Storyboard.TargetProperty="Visibility">
<DiscreteObjectKeyFrame KeyTime="0" Value="Collapsed"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="OverflowTextLabel" Storyboard.TargetProperty="Visibility">
<DiscreteObjectKeyFrame KeyTime="0" Value="Visible"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="OverflowTextLabel" Storyboard.TargetProperty="Margin">
<DiscreteObjectKeyFrame KeyTime="0" Value="38,11,0,13"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal">
<Storyboard>
<PointerUpThemeAnimation Storyboard.TargetName="OverflowTextLabel" />
</Storyboard>
</VisualState>
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Root" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightListLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Content" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltBaseHighBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="TextLabel" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltBaseHighBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="OverflowTextLabel" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltBaseHighBrush}"/>
</ObjectAnimationUsingKeyFrames>
<PointerUpThemeAnimation Storyboard.TargetName="OverflowTextLabel" />
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Root" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightListMediumBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Content" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltBaseHighBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="TextLabel" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltBaseHighBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="OverflowTextLabel" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltBaseHighBrush}"/>
</ObjectAnimationUsingKeyFrames>
<PointerDownThemeAnimation Storyboard.TargetName="OverflowTextLabel" />
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Content" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledBaseLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="TextLabel" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledBaseLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="OverflowTextLabel" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledBaseLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>

<StackPanel x:Name="ContentRoot" MinHeight="{ThemeResource AppBarThemeCompactHeight}">
<ContentPresenter
x:Name="Content"
Height="20"
Margin="0,14,0,4"
Content="{TemplateBinding Icon}"
Foreground="{TemplateBinding Foreground}"
HorizontalAlignment="Stretch"
AutomationProperties.AccessibilityView="Raw"/>
<TextBlock
x:Name="TextLabel"
Text="{TemplateBinding Label}"
Foreground="{TemplateBinding Foreground}"
FontSize="12"
FontFamily="{TemplateBinding FontFamily}"
TextAlignment="Center"
TextWrapping="Wrap"
Margin="0,0,0,6"/>
</StackPanel>

<TextBlock
x:Name="OverflowTextLabel"
Text="{TemplateBinding Label}"
Foreground="{TemplateBinding Foreground}"
FontSize="15"
FontFamily="{TemplateBinding FontFamily}"
TextAlignment="Left"
TextTrimming="Clip"
TextWrapping="NoWrap"
HorizontalAlignment="Stretch"
VerticalAlignment="Center"
Margin="12,11,0,13"
Visibility="Collapsed"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

仔细看一下这个模板，我们可以发现这么一句：

Content="{TemplateBinding Icon}"

我们尝试一下将 Icon 修改为 Content。（Content 属性是 AppBarButton 从 Button 类继承得来的，但是 AppBarButton 它自身并没有使用到这个属性。）

然后编写 XAML 测试下：

<CommandBar>
<AppBarButton Style="{StaticResource MyAppBarButtonStyle}"
Label="评论">
<AppBarButton.Content>
<Grid>
<FontIcon Glyph=""
FontFamily="Segoe MDL2 Assets"
HorizontalAlignment="Center"
VerticalAlignment="Center" />
<TextBlock Text="99+"
HorizontalAlignment="Center"
VerticalAlignment="Center"
FontSize="9"
RenderTransformOrigin="0.5,0.5">
<TextBlock.RenderTransform>
<TranslateTransform Y="-2.5" />
</TextBlock.RenderTransform>
</TextBlock>
</Grid>
</AppBarButton.Content>
</AppBarButton>
</CommandBar>

运行后，你应该可以看到文章开头的效果了。

最后，当然是稍微封装下了，因为 AppBarButton 是能够继承的。由于没啥技术含量-_-|||，这里就大家自己弄了。