wpf ComboBox的SelectionBoxItem相关依赖属性

以前没有注意SelectionBoxItem相关依赖属性，这几天看wpf源码 特意研究了一番

 <Style x:Key="ComboBoxStyle1" TargetType="{x:Type ComboBox}">
             <Setter Property="FocusVisualStyle" Value="{StaticResource ComboBoxFocusVisual}"/>
             <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.WindowTextBrushKey}}"/>
             <Setter Property="Background" Value="{StaticResource ButtonNormalBackground}"/>
             <Setter Property="BorderBrush" Value="{StaticResource ButtonNormalBorder}"/>
             <Setter Property="BorderThickness" Value=""/>
             <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto"/>
             <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/>
             <Setter Property="Padding" Value="4,3"/>
             <Setter Property="ScrollViewer.CanContentScroll" Value="true"/>
             <Setter Property="ScrollViewer.PanningMode" Value="Both"/>
             <Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
             <Setter Property="Template">
                 <Setter.Value>
                     <ControlTemplate TargetType="{x:Type ComboBox}">
                         <Grid x:Name="MainGrid" SnapsToDevicePixels="true">
                             <Grid.ColumnDefinitions>
                                 <ColumnDefinition Width="*"/>
                                 <ColumnDefinition MinWidth="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}" Width=""/>
                             </Grid.ColumnDefinitions>
                             <Popup x:Name="PART_Popup" AllowsTransparency="true" Grid.ColumnSpan="" IsOpen="{Binding IsDropDownOpen, RelativeSource={RelativeSource TemplatedParent}}" Margin="" PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}" Placement="Bottom">
                                 <Microsoft_Windows_Themes:SystemDropShadowChrome x:Name="Shdw" Color="Transparent" MaxHeight="{TemplateBinding MaxDropDownHeight}" MinWidth="{Binding ActualWidth, ElementName=MainGrid}">
                                     <Border x:Name="DropDownBorder" BorderBrush="{DynamicResource {x:Static SystemColors.WindowFrameBrushKey}}" BorderThickness="" Background="{DynamicResource {x:Static SystemColors.WindowBrushKey}}">
                                         <ScrollViewer x:Name="DropDownScrollViewer">
                                             <Grid RenderOptions.ClearTypeHint="Enabled">
                                                 <Canvas HorizontalAlignment="Left" Height="" VerticalAlignment="Top" Width="">
                                                     <Rectangle x:Name="OpaqueRect" Fill="{Binding Background, ElementName=DropDownBorder}" Height="{Binding ActualHeight, ElementName=DropDownBorder}" Width="{Binding ActualWidth, ElementName=DropDownBorder}"/>
                                                 </Canvas>
                                                 <ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Contained" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
                                             </Grid>
                                         </ScrollViewer>
                                     </Border>
                                 </Microsoft_Windows_Themes:SystemDropShadowChrome>
                             </Popup>
                             <ToggleButton BorderBrush="{TemplateBinding BorderBrush}" Background="{TemplateBinding Background}" Grid.ColumnSpan="" IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" Style="{StaticResource ComboBoxReadonlyToggleButton}"/>
                             <ContentPresenter ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}" ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}" Content="{TemplateBinding SelectionBoxItem}" ContentStringFormat="{TemplateBinding SelectionBoxItemStringFormat}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" IsHitTestVisible="false" Margin="{TemplateBinding Padding}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
                         </Grid>
                         <ControlTemplate.Triggers>
                             <Trigger Property="HasDropShadow" SourceName="PART_Popup" Value="true">
                                 <Setter Property="Margin" TargetName="Shdw" Value="0,0,5,5"/>
                                 <Setter Property="Color" TargetName="Shdw" Value="#71000000"/>
                             </Trigger>
                             <Trigger Property="HasItems" Value="false">
                                 <Setter Property="Height" TargetName="DropDownBorder" Value=""/>
                             </Trigger>
                             <Trigger Property="IsEnabled" Value="false">
                                 <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
                                 <Setter Property="Background" Value="#FFF4F4F4"/>
                             </Trigger>
                             <Trigger Property="IsGrouping" Value="true">
                                 <Setter Property="ScrollViewer.CanContentScroll" Value="false"/>
                             </Trigger>
                             <Trigger Property="ScrollViewer.CanContentScroll" SourceName="DropDownScrollViewer" Value="false">
                                 <Setter Property="Canvas.Top" TargetName="OpaqueRect" Value="{Binding VerticalOffset, ElementName=DropDownScrollViewer}"/>
                                 <Setter Property="Canvas.Left" TargetName="OpaqueRect" Value="{Binding HorizontalOffset, ElementName=DropDownScrollViewer}"/>
                             </Trigger>
                         </ControlTemplate.Triggers>
                     </ControlTemplate>
                 </Setter.Value>
             </Setter>
             <Style.Triggers>
                 <Trigger Property="IsEditable" Value="true">
                     <Setter Property="BorderBrush" Value="{StaticResource TextBoxBorder}"/>
                     <Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.WindowBrushKey}}"/>
                     <Setter Property="IsTabStop" Value="false"/>
                     <Setter Property="Padding" Value=""/>
                     <Setter Property="Template" Value="{StaticResource ComboBoxEditableTemplate}"/>
                 </Trigger>
             </Style.Triggers>
         </Style>

可以看到在文本模式下 ContentPresenter 相关依赖属性 ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}" ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}" Content="{TemplateBinding SelectionBoxItem}" ContentStringFormat="{TemplateBinding SelectionBoxItemStringFormat}"是通过这几个SelectionBoxItem属性绑定的

以下为ComboBox相关源码

通过获取InternalSelectedItem为ComboBoxItem的content,contentTemplete,contentStringFormat属性传递给internalSelectedItem，itemTemplate ，itemStringFormat

  private void Update()
         {
             if (this.IsEditable)
             {
                 this.UpdateEditableTextBox();
             }
             else
             {
                 this.UpdateSelectionBoxItem();
             }
         }

 private void UpdateSelectionBoxItem()
         {
             object internalSelectedItem = base.InternalSelectedItem;
             DataTemplate itemTemplate = base.ItemTemplate;
             string itemStringFormat = base.ItemStringFormat;
             ContentControl control = internalSelectedItem as ContentControl;
             if (control != null)
             {
                 internalSelectedItem = control.Content;
                 itemTemplate = control.ContentTemplate;
                 itemStringFormat = control.ContentStringFormat;
             }
             if (this._clonedElement != null)
             {
                 this._clonedElement.LayoutUpdated -= new EventHandler(this.CloneLayoutUpdated);
                 this._clonedElement = null;
             }
             if (((itemTemplate == null) && (base.ItemTemplateSelector == null)) && (itemStringFormat == null))
             {
                 DependencyObject d = internalSelectedItem as DependencyObject;
                 if (d != null)
                 {
                     this._clonedElement = d as UIElement;
                     if (this._clonedElement != null)
                     {
                         VisualBrush brush = new VisualBrush(this._clonedElement) {
                             Stretch = Stretch.None,
                             ViewboxUnits = BrushMappingMode.Absolute,
                             Viewbox = new Rect(this._clonedElement.RenderSize),
                             ViewportUnits = BrushMappingMode.Absolute,
                             Viewport = new Rect(this._clonedElement.RenderSize)
                         };
                         DependencyObject parent = VisualTreeHelper.GetParent(this._clonedElement);
                         FlowDirection direction = (parent == null) ? FlowDirection.LeftToRight : ((FlowDirection) parent.GetValue(FrameworkElement.FlowDirectionProperty));
                         if (base.FlowDirection != direction)
                         {
                             brush.Transform = new MatrixTransform(new Matrix(-1.0, 0.0, 0.0, 1.0, this._clonedElement.RenderSize.Width, 0.0));
                         }
                         Rectangle rectangle = new Rectangle {
                             Fill = brush,
                             Width = this._clonedElement.RenderSize.Width,
                             Height = this._clonedElement.RenderSize.Height
                         };
                         this._clonedElement.LayoutUpdated += new EventHandler(this.CloneLayoutUpdated);
                         internalSelectedItem = rectangle;
                         itemTemplate = null;
                     }
                     else
                     {
                         internalSelectedItem = ExtractString(d);
                         itemTemplate = ContentPresenter.StringContentTemplate;
                     }
                 }
             }
             if (internalSelectedItem == null)
             {
                 internalSelectedItem = string.Empty;
                 itemTemplate = ContentPresenter.StringContentTemplate;
             }
             this.SelectionBoxItem = internalSelectedItem;
             this.SelectionBoxItemTemplate = itemTemplate;
             this.SelectionBoxItemStringFormat = itemStringFormat;
         }

         private void UpdateTextBox(string matchedText, string newText)
         {
             this.EditableTextBoxSite.Text = matchedText;
             this.EditableTextBoxSite.SelectionStart = newText.Length;
             this.EditableTextBoxSite.SelectionLength = matchedText.Length - newText.Length;
         }