如何绑定ComboBox子样式的依赖属性？

可以使用样式模板中的属性绑定来实现。以下是一个示例，其中ItemsSource是ComboBox依赖属性，可以在模板中用Binding语法进行绑定：

<Style x:Key="SubComboBoxStyle" TargetType="ComboBox">
    <Setter Property="Padding" Value="0"/>
    <Setter Property="Margin" Value="0"/>
    <Setter Property="MinWidth" Value="120"/>
    <Setter Property="Background" Value="White"/>
    <Setter Property="BorderThickness" Value="0"/>
    <Setter Property="BorderBrush" Value="Transparent"/>
    <Setter Property="ItemContainerStyle" Value="{StaticResource ComboBoxItemStyle}"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="ComboBox">
                <Grid>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="*" />
                        <ColumnDefinition Width="Auto" />
                    </Grid.ColumnDefinitions>

                    <Border x:Name="Border" Grid.ColumnSpan="2" CornerRadius="2"
                    Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}"
                    BorderBrush="{TemplateBinding BorderBrush}">
                        <Grid>
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition Width="*" />
                                <ColumnDefinition Width="Auto" />
                            </Grid.ColumnDefinitions>
                            
                            <ContentPresenter x:Name="ContentSite" IsHitTestVisible="False" Visibility="Collapsed"
                                HorizontalAlignment="Stretch" VerticalAlignment="Center"
                                ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
                                Content="{TemplateBinding SelectionBoxItem}" />
                            <Button x:Name="DropDownButton" Grid.Column="1" Width="20" Height="20" Margin="0,0,4,0" 
                                    Focusable="False" Foreground="{StaticResource GrayBrush}" Background="Transparent"
                                    BorderThickness="0" Content="▾"
                                    Style="{StaticResource ExpanderButtonStyle}"
                                    />
                        </Grid>
                    </Border>

                    <Popup x:Name="Popup" IsOpen="{TemplateBinding IsDropDownOpen}" Placement="Bottom"
                            Width="{TemplateBinding ActualWidth}" StaysOpen="False" 
                            PlacementTarget="{Binding ElementName=Border}"
                            PopupAnimation="{DynamicResource