Question

I have an application which shows a Tree where can select nodes of the tree and add them to a list. To keep the code clean I have moved the TreeView into it’s own UserControl(I use this tree in several places) and xaml file. To add a node to the list I have an ‘add’ button. However I want to gray-out this ‘add’ button when none of the treeviewitems are selected. What is the wisest way to do this. I can bind to the complete usercontrol and write a more complicated converter, but this seems inelegant. Are there any simple solutions?

I would have hoped something along the lines of “ElementName=xamlFile.TargetElement” would have worked…

Answer 1

Commands provide automatic disabling of buttons using them when the command cannot execute. If you create a command and add handlers for it in your UserControl the external Add Button can use the command.

<local:TreeViewControl x:Name="Tree"/>
<Button Content="Add" Command="{x:Static local:TreeViewControl.AddCommand}" CommandTarget="{Binding ElementName=Tree}"/>

Command creation and hookup:

public partial class TreeViewControl : UserControl
{
    public static RoutedCommand AddCommand { get; private set; }

    static TreeViewControl()
    {
        AddCommand = new RoutedCommand("AddCommand", typeof(TreeViewControl));
    }

    public TreeViewControl()
    {
        InitializeComponent();

        CommandBindings.Add(new CommandBinding(AddCommand, AddExecuted, AddCanExecute));
    }

    public void AddExecuted(object sender, ExecutedRoutedEventArgs e)
    {
    }

    public void AddCanExecute(object sender, CanExecuteRoutedEventArgs e)
    {
        e.CanExecute = false; // your logic here
    }
}

If using MVVM you can use DelegateCommand or RelayCommand to do similar things from a ViewModel.