如何使WPF Expander向上扩展，同时保持标题固定

Question

我正在尝试创建一个扩展器，其中标题保持固定，内容显示在标题上方，覆盖上面的任何其他控件。在 Canvas 中设置ExpandDirection="Up"和放置 Expander 可以实现一半的效果 - 内容相对于标题向上展开，并覆盖其他控件（尽管在下面），但标题向下移动。

有什么方法可以做到这一点，但将标题保持在固定位置，以便内容最终覆盖上面的控件？

这是我到目前为止所发布的内容：

<Window x:Class="Sandbox.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="900" Width="1100">
  <StackPanel>

    <StackPanel Margin="20,0,0,0">
      <RadioButton Content="Choice One"/>
      <RadioButton Content="Choice Two"/>
      <RadioButton Content="Choice Three"/>
      <RadioButton Content="Choice Four"/>
      <RadioButton Content="Choice Five"/>
      <RadioButton Content="Choice Six"/>
    </StackPanel>

    <Canvas MinHeight="25" Panel.ZIndex="99">
      <Expander Header="This must stay fixed" ExpandDirection="Up" Width="175">
        <Grid Background="Cornsilk">
          <Grid.BitmapEffect>
            <DropShadowBitmapEffect />
          </Grid.BitmapEffect>

          <TextBlock TextWrapping="Wrap" Margin="5">
            This must expand upwards, not downwards.
            The header must remain exactly where it is.
            This TextBlock must appear above the header
            and overlay the top radio buttons instead.
          </TextBlock>
        </Grid>
      </Expander>
    </Canvas>

    <StackPanel Margin="20,0,0,0">
      <RadioButton Content="Choice One"/>
      <RadioButton Content="Choice Two"/>
      <RadioButton Content="Choice Three"/>
      <RadioButton Content="Choice Four"/>
      <RadioButton Content="Choice Five"/>
      <RadioButton Content="Choice Six"/>
    </StackPanel>

  </StackPanel>
</Window>

Answer 1

您可以使用 ToggleButton 和 Popup 代替 Expander：

<Canvas MinHeight="25" Panel.ZIndex="99">
  <ToggleButton x:Name="toggle" Content="This must stay fixed" Width="175" />
  <Popup Placement="Top" PlacementTarget="{Binding ElementName=toggle}"
         IsOpen="{Binding ElementName=toggle, Path=IsChecked}">
      <Grid Background="Cornsilk" Width="175">
          <Grid.BitmapEffect>
              <DropShadowBitmapEffect />
          </Grid.BitmapEffect>

          <TextBlock TextWrapping="Wrap" Margin="5">
            This must expand upwards, not downwards.
            The header must remain exactly where it is.
            This TextBlock must appear above the header
            and overlay the top radio buttons instead.
          </TextBlock>
      </Grid>  
  </Popup>
</Canvas>