wpf ComboBox的SelectionBoxItem相关依赖属性

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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

 1 <Style x:Key="ComboBoxStyle1" TargetType="{x:Type ComboBox}">
 2             <Setter Property="FocusVisualStyle" Value="{StaticResource ComboBoxFocusVisual}"/>
 3             <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.WindowTextBrushKey}}"/>
 4             <Setter Property="Background" Value="{StaticResource ButtonNormalBackground}"/>
 5             <Setter Property="BorderBrush" Value="{StaticResource ButtonNormalBorder}"/>
 6             <Setter Property="BorderThickness" Value="1"/>
 7             <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto"/>
 8             <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/>
 9             <Setter Property="Padding" Value="4,3"/>
10             <Setter Property="ScrollViewer.CanContentScroll" Value="true"/>
11             <Setter Property="ScrollViewer.PanningMode" Value="Both"/>
12             <Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
13             <Setter Property="Template">
14                 <Setter.Value>
15                     <ControlTemplate TargetType="{x:Type ComboBox}">
16                         <Grid x:Name="MainGrid" SnapsToDevicePixels="true">
17                             <Grid.ColumnDefinitions>
18                                 <ColumnDefinition Width="*"/>
19                                 <ColumnDefinition MinWidth="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}" Width="0"/>
20                             </Grid.ColumnDefinitions>
21                             <Popup x:Name="PART_Popup" AllowsTransparency="true" Grid.ColumnSpan="2" IsOpen="{Binding IsDropDownOpen, RelativeSource={RelativeSource TemplatedParent}}" Margin="1" PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}" Placement="Bottom">
22                                 <Microsoft_Windows_Themes:SystemDropShadowChrome x:Name="Shdw" Color="Transparent" MaxHeight="{TemplateBinding MaxDropDownHeight}" MinWidth="{Binding ActualWidth, ElementName=MainGrid}">
23                                     <Border x:Name="DropDownBorder" BorderBrush="{DynamicResource {x:Static SystemColors.WindowFrameBrushKey}}" BorderThickness="1" Background="{DynamicResource {x:Static SystemColors.WindowBrushKey}}">
24                                         <ScrollViewer x:Name="DropDownScrollViewer">
25                                             <Grid RenderOptions.ClearTypeHint="Enabled">
26                                                 <Canvas HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
27                                                     <Rectangle x:Name="OpaqueRect" Fill="{Binding Background, ElementName=DropDownBorder}" Height="{Binding ActualHeight, ElementName=DropDownBorder}" Width="{Binding ActualWidth, ElementName=DropDownBorder}"/>
28                                                 </Canvas>
29                                                 <ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Contained" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
30                                             </Grid>
31                                         </ScrollViewer>
32                                     </Border>
33                                 </Microsoft_Windows_Themes:SystemDropShadowChrome>
34                             </Popup>
35                             <ToggleButton BorderBrush="{TemplateBinding BorderBrush}" Background="{TemplateBinding Background}" Grid.ColumnSpan="2" IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" Style="{StaticResource ComboBoxReadonlyToggleButton}"/>
36                             <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}"/>
37                         </Grid>
38                         <ControlTemplate.Triggers>
39                             <Trigger Property="HasDropShadow" SourceName="PART_Popup" Value="true">
40                                 <Setter Property="Margin" TargetName="Shdw" Value="0,0,5,5"/>
41                                 <Setter Property="Color" TargetName="Shdw" Value="#71000000"/>
42                             </Trigger>
43                             <Trigger Property="HasItems" Value="false">
44                                 <Setter Property="Height" TargetName="DropDownBorder" Value="95"/>
45                             </Trigger>
46                             <Trigger Property="IsEnabled" Value="false">
47                                 <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
48                                 <Setter Property="Background" Value="#FFF4F4F4"/>
49                             </Trigger>
50                             <Trigger Property="IsGrouping" Value="true">
51                                 <Setter Property="ScrollViewer.CanContentScroll" Value="false"/>
52                             </Trigger>
53                             <Trigger Property="ScrollViewer.CanContentScroll" SourceName="DropDownScrollViewer" Value="false">
54                                 <Setter Property="Canvas.Top" TargetName="OpaqueRect" Value="{Binding VerticalOffset, ElementName=DropDownScrollViewer}"/>
55                                 <Setter Property="Canvas.Left" TargetName="OpaqueRect" Value="{Binding HorizontalOffset, ElementName=DropDownScrollViewer}"/>
56                             </Trigger>
57                         </ControlTemplate.Triggers>
58                     </ControlTemplate>
59                 </Setter.Value>
60             </Setter>
61             <Style.Triggers>
62                 <Trigger Property="IsEditable" Value="true">
63                     <Setter Property="BorderBrush" Value="{StaticResource TextBoxBorder}"/>
64                     <Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.WindowBrushKey}}"/>
65                     <Setter Property="IsTabStop" Value="false"/>
66                     <Setter Property="Padding" Value="3"/>
67                     <Setter Property="Template" Value="{StaticResource ComboBoxEditableTemplate}"/>
68                 </Trigger>
69             </Style.Triggers>
70         </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

 1  private void Update()
 3             if (this.IsEditable)
 5                 this.UpdateEditableTextBox();
 7             else
 9                 this.UpdateSelectionBoxItem();
10             }
 1 private void UpdateSelectionBoxItem()
 3             object internalSelectedItem = base.InternalSelectedItem;
 4             DataTemplate itemTemplate = base.ItemTemplate;
 5             string itemStringFormat = base.ItemStringFormat;
 6             ContentControl control = internalSelectedItem as ContentControl;
 7             if (control != null)
 9                 internalSelectedItem = control.Content;
10                 itemTemplate = control.ContentTemplate;
11                 itemStringFormat = control.ContentStringFormat;
12             }
13             if (this._clonedElement != null)
14             {
15                 this._clonedElement.LayoutUpdated -= new EventHandler(this.CloneLayoutUpdated);
16                 this._clonedElement = null;
17             }
18             if (((itemTemplate == null) && (base.ItemTemplateSelector == null)) && (itemStringFormat == null))
19             {
20                 DependencyObject d = internalSelectedItem as DependencyObject;
21                 if (d != null)
22                 {
23                     this._clonedElement = d as UIElement;
24                     if (this._clonedElement != null)
25                     {
26                         VisualBrush brush = new VisualBrush(this._clonedElement) {
27                             Stretch = Stretch.None,
28                             ViewboxUnits = BrushMappingMode.Absolute,
29                             Viewbox = new Rect(this._clonedElement.RenderSize),
30                             ViewportUnits = BrushMappingMode.Absolute,
31                             Viewport = new Rect(this._clonedElement.RenderSize)
32                         };
33                         DependencyObject parent = VisualTreeHelper.GetParent(this._clonedElement);
34                         FlowDirection direction = (parent == null) ? FlowDirection.LeftToRight : ((FlowDirection) parent.GetValue(FrameworkElement.FlowDirectionProperty));
35                         if (base.FlowDirection != direction)
36                         {
37                             brush.Transform = new MatrixTransform(new Matrix(-1.0, 0.0, 0.0, 1.0, this._clonedElement.RenderSize.Width, 0.0));
38                         }
39                         Rectangle rectangle = new Rectangle {
40                             Fill = brush,
41                             Width = this._clonedElement.RenderSize.Width,
42                             Height = this._clonedElement.RenderSize.Height
43                         };
44                         this._clonedElement.LayoutUpdated += new EventHandler(this.CloneLayoutUpdated);
45                         internalSelectedItem = rectangle;
46                         itemTemplate = null;
47                     }
48                     else
49                     {
50                         internalSelectedItem = ExtractString(d);
51                         itemTemplate = ContentPresenter.StringContentTemplate;
52                     }
53                 }
54             }
55             if (internalSelectedItem == null)
56             {
57                 internalSelectedItem = string.Empty;
58                 itemTemplate = ContentPresenter.StringContentTemplate;
59             }
60             this.SelectionBoxItem = internalSelectedItem;
61             this.SelectionBoxItemTemplate = itemTemplate;
62             this.SelectionBoxItemStringFormat = itemStringFormat;
63         }
65         private void UpdateTextBox(string matchedText, string newText)
66         {
67             this.EditableTextBoxSite.Text = matchedText;
68             this.EditableTextBoxSite.SelectionStart = newText.Length;
69             this.EditableTextBoxSite.SelectionLength = matchedText.Length - newText.Length;