WPF Scrollviewer當ScrollBar可見時，DesiredSize不會增加

Question

我有一個WPF UserControl（在ElementHost之內），ScrollViewer包含ItemsControl。 HorizontalScrollbarVisibility設置爲Auto，所以如果不需要滾動，ScrollBar會被隱藏。

我的要求是，如果ScrollBar顯示/隱藏，ElementHost會相應地調整它的高度。爲了實現這一點，我正在聽SizeChanged事件，我得到ScrollViewer的DesiredSize在EventHandler，然後我通過DesiredSize.Height到ElementHost。

1. 2.

的一種方式，其工作原理：隨着ScrollBar可見的（情況1），我擴大了我的窗前，直到ItemsControl的所有項目都可見，在ScrollBar消失， ElementHost適應高度降低（情況2）。 DesiredSize實際上在ScrollBar隱藏的時刻變小了。

另一種方式，但它不起作用：與ScrollBar不可見（情況2），我減少我的窗口大小，直到ScrollBar是必要的，並且出現。 DesiredSize保持不變，ElementHost不調整（情況3）。

任何想法？

這是Scrollviewer的XAML，一些MVVM的東西，不過不要讓她掛在這，點真的是，爲什麼DesiredSize的ScrollBar出現在不增加嗎？爲什麼它只收縮？

<ScrollViewer HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Hidden" > 
    <i:Interaction.Behaviors> 
     <beh:HeightChangedBehavior HeightChangedCommand="{Binding HeightChangedCommand}" /> 
    </i:Interaction.Behaviors> 
    <ItemsControl ItemsSource="{Binding TabHeaders}" > 
     <ItemsControl.ItemsPanel> 
      <ItemsPanelTemplate> 
       <StackPanel IsItemsHost="True" Orientation="Horizontal"/> 
      </ItemsPanelTemplate> 
     </ItemsControl.ItemsPanel> 
     <ItemsControl.ItemTemplate> 
      <DataTemplate DataType="models:TabHeaderButtonModel"> 
       <RadioButton Content="{Binding Caption}" IsChecked="{Binding IsChecked, Mode=TwoWay}" GroupName="Tabs" 
          Command="{Binding SelectionChangedCommand}" CommandParameter="{Binding}" 
          Style="{StaticResource TabHeaderToggleButtonStyle}"> 
       </RadioButton> 
      </DataTemplate> 
     </ItemsControl.ItemTemplate> 
    </ItemsControl> 
</ScrollViewer> 

的ScrollViewer風格（基本上默認WPF）：

<Style x:Key="ScrollViewerStyle1" TargetType="{x:Type ScrollViewer}"> 
    <Setter Property="Template" > 
     <Setter.Value> 
      <ControlTemplate TargetType="{x:Type ScrollViewer}"> 
       <Grid x:Name="Grid" Background="{TemplateBinding Background}"> 
        <Grid.ColumnDefinitions> 
         <ColumnDefinition Width="*"/> 
         <ColumnDefinition Width="Auto"/> 
        </Grid.ColumnDefinitions> 
        <Grid.RowDefinitions> 
         <RowDefinition Height="*"/> 
         <RowDefinition Height="Auto"/> 
        </Grid.RowDefinitions> 
        <Rectangle x:Name="Corner" Grid.Column="1" Fill="{DynamicResource {x:Static SystemColors.ControlBrushKey}}" Grid.Row="1"/> 
        <ScrollContentPresenter x:Name="PART_ScrollContentPresenter" CanContentScroll="{TemplateBinding CanContentScroll}" CanHorizontallyScroll="False" CanVerticallyScroll="False" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" Grid.Column="0" Margin="{TemplateBinding Padding}" Grid.Row="0"/> 
        <ScrollBar x:Name="PART_VerticalScrollBar" AutomationProperties.AutomationId="VerticalScrollBar" Cursor="Arrow" Grid.Column="1" Maximum="{TemplateBinding ScrollableHeight}" Minimum="0" Grid.Row="0" Visibility="{TemplateBinding ComputedVerticalScrollBarVisibility}" Value="{Binding VerticalOffset, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}}" ViewportSize="{TemplateBinding ViewportHeight}"/> 
        <ScrollBar x:Name="PART_HorizontalScrollBar" AutomationProperties.AutomationId="HorizontalScrollBar" Cursor="Arrow" Grid.Column="0" Maximum="{TemplateBinding ScrollableWidth}" Minimum="0" Orientation="Horizontal" Grid.Row="1" Visibility="{TemplateBinding ComputedHorizontalScrollBarVisibility}" Value="{Binding HorizontalOffset, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}}" ViewportSize="{TemplateBinding ViewportWidth}" Style="{DynamicResource ScrollBarStyle1}"/> 
       </Grid> 
      </ControlTemplate> 
     </Setter.Value> 
    </Setter> 
    <Style.Triggers> 
     <Trigger Property="IsEnabled" Value="false"> 
      <Setter Property="Foreground" Value="White"/> 
     </Trigger> 
    </Style.Triggers> 
</Style> 

Answer 1

我不得不通過獲取ScrollViewer中的內容所需的高度和增加了滾動條的高度，如果可見，計算所需的高度。

這對我仍然感覺有點尷尬，所以如果你有更好的解決方案，我會很樂意改變接受的答案。

public class HeightChangedBehavior : Behavior<ScrollViewer> 
{ 
    public ICommand HeightChangedCommand { get { return (ICommand)GetValue(HeightChangedCommandProperty); } set { SetValue(HeightChangedCommandProperty, value); } } 
    public static readonly DependencyProperty HeightChangedCommandProperty = DependencyProperty.Register("HeightChangedCommand", typeof(ICommand), typeof(HeightChangedBehavior), new PropertyMetadata(null)); 


    protected override void OnAttached() 
    { 
     this.AssociatedObject.ScrollChanged += AssociatedObject_ScrollChanged; 
     base.OnAttached(); 
    } 

    /// <summary> 
    /// Calculates the desired height for the scrollviewer, as the sum of its content 
    /// desired height and, if visible, the horizontal scrollbar height. 
    /// </summary> 
    void AssociatedObject_ScrollChanged(object sender, ScrollChangedEventArgs e) 
    { 
     ScrollViewer sv = (ScrollViewer)sender; 

     // get content height 
     double height = ((FrameworkElement)sv.Content).DesiredSize.Height; 

     if (sv.ComputedHorizontalScrollBarVisibility == Visibility.Visible) 
     { 
      // add scrollbar height 
      height += (double)sv.FindResource(SystemParameters.HorizontalScrollBarHeightKey); // template of scrollbar should use this key 
     } 

     int intHeight = (int)Math.Ceiling(height); // whole pixels 

     // execute the command 
     ICommand cmd = this.HeightChangedCommand; 
     if (cmd != null && intHeight != sv.ActualHeight) 
      cmd.Execute(intHeight); 
    } 
}