Setting IsExpanded on a WPF TreeViewItem from a DataTrigger

Question

I\'m trying to set the IsExpanded property of my TreeView items using a conditional template, in the XAML:

Answer 1

I had to do something similar, and I resolved it this way:

<TreeView ItemsSource="{Binding source}"
          SnapsToDevicePixels="{Binding Path=myStatusToBool}"
          >
  <TreeView.ItemContainerStyle>
    <Style>
      <Setter Property="TreeViewItem.IsExpanded"
              Value="False"
              />
      <Style.Triggers>
        <DataTrigger Binding="{Binding Path=SnapsToDevicePixels,RelativeSource={RelativeSource AncestorType=TreeView}}"
                     Value="True">
            <Setter Property="TreeViewItem.IsExpanded"
                    Value="True"
                    />
        </DataTrigger>
      </Style.Triggers>
    </Style>
  </TreeView.ItemContainerStyle>

  <TreeView.Resources>
  .....
  .....
  </TreeView.Resources>
</TreeView>

Answer 2

There are a few things wrong here. The first is that you are setting the property TreeViewItem.IsSelected on a HierarchicalDataTemplate. This won't work. Instead, you're going to need to set an ItemContainerStyle on the TreeView:

<TreeView>
    <TreeView.ItemContainerStyle>
        <Style TargetType="{x:Type TreeViewItem}">
        <!-- put logic for handling expansion here -->
        </Style>
    </TreeView.ItemContainerStyle>
</TreeView>

You can't just put the Trigger in here, however. Because of DependencyProperty value precedence, if your user clicks on the nodes to expand or collapse them, your triggers won't be #1 on the precedence list (that is a local value). Therefore, your'e best bet is to create a new IValueConverter to convert from MyStatus to a bool. And then setup a TwoWay binding in a Setter in the Style:

<Style TargetType="{x:Type TreeViewItem}">
    <Setter Property="IsExpanded" 
            Value="{Binding MyStatus, Converter={StaticResource statusToBool}}" />
</Style>

And your converter:

public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
    return ((MyStatus)value) == MyStatus.Opened;
}

public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
    return ((bool)value) ? MyStatus.Opened : MyStatus.Closed;
}