C＃XAML设置器值设置为True，具体取决于字符串属性值

Question

I am having trouble with what seems like a fairly straightforward task. I have a treeview with nodes, and I would like to set the TreeViewItem 'IsExpanded' property to True for the whole treeview if a specific string property 'SearchTerm' of my View Model is not empty. In other words, if string property is not null, IsExpanded value should be True. I have already done this in codebehind but I prefer to do this in XAML for cleanness.

To describe the code below, I created a converter which will convert a null string to 'False' and non-null to 'True'. In my XAML I call this converter when I attempt to bind the string value from the viewmodel in the TreeView ItemContainerStyle. It appears that the converter is never even fired.

My XAML (simplified):

<UserControl.Resources>
    <cv:ExpandNodesIfSearchConverter x:Key="ExpandAll">
    </cv:ExpandNodesIfSearchConverter>
</UserControl.Resources>

<TreeView Grid.Row="2" x:Name="myTreeView"
      ItemsSource="{Binding Sponsors}"
      SelectedItemChanged="TreeView_SelectedItemChanged" >

        <TreeView.ItemContainerStyle>
            <Style TargetType="{x:Type TreeViewItem}">
                 <!-- if SearchTerm is not null, use converter to set value to true and expand all nodes -->
                <Setter Property="IsExpanded" Value="{Binding Path=SearchTerm, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource ExpandAll}}" />
            </Style>
        </TreeView.ItemContainerStyle>

        <!-- TreeView data -->

    </TreeView>

My View Model:

public class TreeViewVM : INotifyPropertyChanged
{
    private string _searchterm;
    public string SearchTerm
    {
        get
        {
            return _searchterm;
        }
        set
        {
            _searchterm = value;
            OnPropertyChanged("SearchTerm");
        }
    }
}

My Converter:

class ExpandNodesIfSearchConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        //if searchterm is not null, return true to expand all items, otherwise return false
        string searchterm = value.ToString();
        if (string.IsNullOrEmpty(searchterm))
            return false;
        else
            return true;
    }

    public object ConvertBack(object value, Type targetTypes, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Answer 1

I solved my issue by using the ElementName in the Setter tag rather than the viewmodel property. searchTxt being the name of the TextBox which SeachTerm was bound to.

<Setter Property="IsExpanded" Value="{Binding ElementName=searchTxt, Path=Text, Mode=OneWay, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource ExpandAll}}" />