wpf Change the text of Textbox on KeyBinding Gesture

Question

how can i solve this using MVVM pattern, and I am using Devexpress MVVM. I have many textbox in form.

And i need to set the textbox text into "[blank]" when the user press the Ctrl+B and the current text of the textbox is null or ""

But i am looking for a way to use the IValueConverter if possible

I have a class similar to this

public class BlankText : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return value;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (string.IsNullOrEmpty(value.ToString()))
                return "[blank]";
            else
                return value;
        }
    }

And i have this code in the resources

    <UserControl.Resources>
        <c:BlankText x:Key="BlankText"/>
    </UserControl.Resources>

This is my TextBox

           <TextBox Text="{Binding District}"  >
                <TextBox.InputBindings>
                    <KeyBinding Gesture="Ctrl+B">
                    </KeyBinding>
                </TextBox.InputBindings>
            </TextBox>

But my problem is how can i call it on keypress? Am i doing it right?

Answer 1

In order to perform actions using KeyBinding, you can't use IValueConverter. IValueConverters are for converting values, not performing actions. What you need is to define a class that implements ICommand, and then assign an instance of that class to KeyBinding.Command.

public class BlankCommand : ICommand 
{
    public MyViewModel ViewModel { get; }

    public BlankCommand(MyViewModel vm)
    {
        this.ViewModel = vm;
    }

    public void Execute(object parameter) 
    {
        // parameter is the name of the property to modify

        var type = ViewModel.GetType();
        var prop = type.GetProperty(parameter as string);
        var value = prop.GetValue(ViewModel);

        if(string.IsNullOrEmpty(value))
            prop.SetValue(ViewModel, "[blank]");
    }

    public boolean CanExecute(object parameter) => true;

    public event EventHandler CanExecuteChanged;
}

Then create an instance of this class and attach it to your ViewModel so that the KeyBinding can access it:

<TextBox Text="{Binding District}">
    <TextBox.InputBindings>
        <KeyBinding Gesture="Ctrl+B" Command="{Binding MyBlankCommand}" CommandParameter="District"/>
    </TextBox.InputBindings>
</TextBox>

Changing the text to say "[blank]" when the user presses a keyboard shortcut is a weird UX pattern, however. I would suggest adding a placeholder to the text box instead.