WPF ComboBox popup placement : Bottom and aligned

问题:

I'm trying to create a ComboBox with a non-standard dropdown alignment. Basically, I want the dropdown to be below the ComboBox, but aligned with the right edge of the ComboBox instead of the left edge.

What a normal ComboBox looks like, using PlacementMode="Bottom":

What I want:

I tried to play with the Popup.PlacementMode property in the template of my ComboBox, but none of the possible values seem to do what I want. Is there a simple way to do it, preferably in pure XAML?

回答1:

When I opened Expression Blend, I have come up with the solution within a few seconds:

<Popup Placement="Left" VerticalOffset="{TemplateBinding ActualHeight}" 
       HorizontalOffset="{TemplateBinding ActualWidth}"

Sometimes this application is more useful than writing xaml by hands, but not so often.

回答2:

I would use the "Custom" placementmode for the PopUp and declare a callback to place the popup control into the correct position, like it's shown here: WPF ComboBox DropDown Placement

See if an example here would work for you:

public class TestComboBox : ComboBox
{
    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();

        var popup = (Popup)Template.FindName("PART_Popup", this);
        popup.Placement = PlacementMode.Custom;
        popup.CustomPopupPlacementCallback += (Size popupSize, Size targetSize, Point offset) => 
            new[] {  new CustomPopupPlacement() { Point = new Point (targetSize.Width-popupSize.Width, targetSize.Height) } };
    }
}

hope this helps, regards

回答3:

Can someone post the complete xaml code please?

I've tried the following:

    <ComboBox Grid.Column="1" Height="24" Width="20" HorizontalAlignment="Right"
              VerticalAlignment="Top"                  
              Name="comboBox2" 
              ItemsSource="{Binding  Source={StaticResource FilterTypes}}" 
              SelectedValue="{Binding Path=SelectedType, Mode=TwoWay}" >

        <ComboBox.Template>
            <ControlTemplate>
                <Popup Placement="Left" VerticalOffset="{TemplateBinding ActualHeight}" 
                        HorizontalOffset="{TemplateBinding ActualWidth}" />
            </ControlTemplate>
        </ComboBox.Template>  
    </ComboBox>  

... after some working and testing I've found a good solution...

        <ComboBox.Style>
            <Style TargetType="ComboBox" >                                    
                <Setter Property="Popup.FlowDirection" Value="RightToLeft"/>                  
            </Style>
        </ComboBox.Style>      

回答4:

it's a little hacky, but does work. you just have to change the combobox style.

    <Grid Height="40">
        <Grid HorizontalAlignment="Center" VerticalAlignment="Center">
            <FrameworkElement Name="dummy" Visibility="Collapsed">
                <FrameworkElement.RenderTransform>
                    <TransformGroup x:Name="xformgrp">
                        <TranslateTransform X="{Binding ElementName=PopupContent, Path=ActualWidth}" />
                        <ScaleTransform ScaleX="-1" />
                        <TranslateTransform X="{Binding ElementName=chk, Path=ActualWidth}" />
                    </TransformGroup>
                </FrameworkElement.RenderTransform>
            </FrameworkElement>
            <CheckBox Name="chk" HorizontalAlignment="Center">checkthisout</CheckBox>
            <Popup IsOpen="{Binding IsChecked, ElementName=chk}" PlacementTarget="{Binding ElementName=chk}" Placement="Bottom" HorizontalOffset="{Binding ElementName=dummy, Path=RenderTransform.Value.OffsetX}">
                <TextBlock Name="PopupContent" Foreground="Yellow" Background="Blue">yeah long popupcontent</TextBlock>
            </Popup>
        </Grid>            
    </Grid>

The popups HorizontalOffset just has to get the value of PopupContent.ActualWidth-PlacementTarget.ActualWidth. To get that value I used this trick from Charles Petzold.