How to intercept a bound command?

[sam-wheat]

I wrote a simple control that displays a popup with a ListBox. Implementer of my control can bind a command to SelectionChangedCommand, which will execute when user selects an item from the ListBox. However, before SelectionChangedCommand executes, I want to set IsDropDownOpen property on my control to false which will close the popup.

Basically I want to intercept SelectionChangedCommand, set IsDropDownOpen to false, then allow SelectionChangedCommand to execute normally. How do I do this?

StaysOpen property of Popup does not achive the desired effect and in fact prevents the popup from closing when the mouse leaves.

Example project attached.

Thank you
CommandProblem.zip
.

[ThomasGoulet73]

@sam-wheat The easiest way would be to add a new Action to change the property value of IsDropDownOpen before the InvokeCommandAction.

Something like this:

Content of Generic.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:Behaviors="http://schemas.microsoft.com/xaml/behaviors"
                    xmlns:local="clr-namespace:CommandProblem" >
    <Style TargetType="local:DropDownButton">
        <Setter Property="ButtonStyle">
            <Setter.Value>
                <Style TargetType="{x:Type Button}">
                    <Setter Property="Padding" Value="3"/>
                </Style>
            </Setter.Value>
        </Setter>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type local:DropDownButton }">
                    <StackPanel Orientation="Vertical" DataContext="{Binding RelativeSource={RelativeSource AncestorType=local:DropDownButton}}">
                        <Behaviors:Interaction.Triggers>
                            <Behaviors:EventTrigger EventName="MouseLeave">
                                <Behaviors:InvokeCommandAction Command="{Binding MouseLeaveCommand}"/>
                            </Behaviors:EventTrigger>
                        </Behaviors:Interaction.Triggers>
                        <Button x:Name="MartyMcButton" Content="{Binding Content}" Command="{Binding ToggleDropDownCommand}" Style="{TemplateBinding ButtonStyle}"></Button>
                        <Popup IsOpen="{Binding IsDropDownOpen}" PlacementTarget="{Binding ElementName=MartyMcButton}" Placement="Bottom" AllowsTransparency="True" >
                            <Border  Margin="0,0,8,8" >
                                <Border.Effect>
                                    <DropShadowEffect BlurRadius="5" Opacity="0.4"/>
                                </Border.Effect>
                                <ListBox ItemsSource="{Binding ItemsSource}" SelectedItem="{Binding SelectedItem}" ItemTemplate="{Binding ItemTemplate}" Style="{TemplateBinding ListBoxStyle}" >
                                    <Behaviors:Interaction.Triggers>
                                        <Behaviors:EventTrigger EventName="SelectionChanged">
                                            <Behaviors:ChangePropertyAction PropertyName="IsDropDownOpen" TargetObject="{Binding RelativeSource={RelativeSource AncestorType=local:DropDownButton}}" Value="False"/>
                                            <Behaviors:InvokeCommandAction Command="{Binding SelectionChangedCommand}" CommandParameter="{Binding SelectedItem}"/>
                                        </Behaviors:EventTrigger>
                                    </Behaviors:Interaction.Triggers>
                                </ListBox>
                            </Border>
                        </Popup>
                    </StackPanel>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>

Another easy way would be to call SelectionChangedCommand from a PropertyChangedCallback on the SelectedItem property instead of from a EventTrigger (Though this would also call SelectionChangedCommand when SelectedItem is changed from code, not just when a user select an item).

Something like this:

public static readonly DependencyProperty SelectedItemProperty =
    DependencyProperty.Register("SelectedItem", typeof(object), typeof(DropDownButton), new FrameworkPropertyMetadata(OnSelectedItemChanged) { BindsTwoWayByDefault = true });

private static void OnSelectedItemChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    DropDownButton dropDownButton = (DropDownButton)d;
    dropDownButton.IsDropDownOpen = false;
    dropDownButton.SelectionChangedCommand?.Execute(e.NewValue);
}