Invoke Command When “ENTER” Key Is Pressed In XAML

Question

I want to invoke a command when ENTER is pressed in a TextBox. Consider the following XAML:

<UserControl
     ...
     xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
     ...>    
     ...    
     <TextBox>
          <i:Interaction.Triggers>
               <i:EventTrigger EventName="KeyUp">
                    <i:InvokeCommandAction Command="{Binding MyCommand}"
                                           CommandParameter="{Binding Text}" />
               </i:EventTrigger>
          </i:Interaction.Triggers>
     </TextBox>    
     ...    
</UserControl>

and that MyCommand is as follows:

public ICommand MyCommand {
     get { return new DelegateCommand<string>(MyCommandExecute); }
}

private void MyCommandExecute(string s) { ... }

With the above, my command is invoked for every key press. How can I restrict the command to only invoke when the ENTER key is pressed?

I understand that with Expression Blend I can use Conditions but those seem to be restricted to elements and can't consider event arguments.

I have also come across SLEX which offers its own InvokeCommandAction implementation that is built on top of the Systems.Windows.Interactivity implementation and can do what I need. Another consideration is to write my own trigger, but I'm hoping there's a way to do it without using external toolkits.

Answer 1

I like scottrudy's approach (to which I've given a +1) with the custom triggers approach as it stays true to my initial approach. I'm including a modified version of it below to use dependency properties instead of reflection info so that it's possible to bind directly to the ICommand. I'm also including an approach using attached properties to avoid using System.Windows.Interactivity if desired. The caveat to the latter approach is that you lose the feature of multiple invokations from an event, but you can apply it more generally.

---

Custom Triggers Approach

ExecuteCommandAction.cs

public class ExecuteCommandAction : TriggerAction<DependencyObject> {
    #region Properties
    public ICommand Command {
        get { return (ICommand)base.GetValue(CommandProperty); }
        set { base.SetValue(CommandProperty, value); }
    }

    public static ICommand GetCommand(DependencyObject obj) {
        return (ICommand)obj.GetValue(CommandProperty);
    }

    public static void SetCommand(DependencyObject obj, ICommand value) {
        obj.SetValue(CommandProperty, value);
    }

    // We use a DependencyProperty so we can bind commands directly rather
    // than have to use reflection info to find them
    public static readonly DependencyProperty CommandProperty =
        DependencyProperty.Register("Command", typeof(ICommand), typeof(ExecuteCommandAction), null);
    #endregion Properties

    protected override void Invoke(object parameter) {
        ICommand command = Command ?? GetCommand(AssociatedObject);
        if (command != null && command.CanExecute(parameter)) {
            command.Execute(parameter);
        }
    }
}

TextBoxEnterKeyTrigger.cs

public class TextBoxEnterKeyTrigger : TriggerBase<UIElement> {
    protected override void OnAttached() {
        base.OnAttached();
        TextBox textBox = this.AssociatedObject as TextBox;

        if (textBox != null) {
            this.AssociatedObject.KeyUp += new System.Windows.Input.KeyEventHandler(AssociatedObject_KeyUp);
        }
        else {
            throw new InvalidOperationException("This behavior only works with TextBoxes");
        }
    }

    protected override void OnDetaching() {
        base.OnDetaching();
        AssociatedObject.KeyUp -= new KeyEventHandler(AssociatedObject_KeyUp);
    }

    private void AssociatedObject_KeyUp(object sender, KeyEventArgs e) {
        if (e.Key == Key.Enter) {
            TextBox textBox = AssociatedObject as TextBox;

            //This checks for an mvvm style binding and updates the source before invoking the actions.
            BindingExpression expression = textBox.GetBindingExpression(TextBox.TextProperty);
            if (expression != null)
                expression.UpdateSource();

            InvokeActions(textBox.Text);
        }
    }
}

MyUserControl.xaml

<UserControl
    ...
    xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
    xmlns:b="clr-namespace:MyNameSpace.Interactivity"
    ...
    <TextBox>
        <i:Interaction.Triggers>
            <b:TextBoxEnterKeyTrigger>
                <b:ExecuteCommandAction Command="{Binding MyCommand}" />
            </b:TextBoxEnterKeyTrigger>
        </i:Interaction.Triggers>
    </TextBox>
    ...
</UserControl>

---

Attached Properties Approach

EnterKeyDown.cs

public sealed class EnterKeyDown {

    #region Properties

    #region Command

    public static ICommand GetCommand(DependencyObject obj) {
        return (ICommand)obj.GetValue(CommandProperty);
    }

    public static void SetCommand(DependencyObject obj, ICommand value) {
        obj.SetValue(CommandProperty, value);
    }

    public static readonly DependencyProperty CommandProperty =
        DependencyProperty.RegisterAttached("Command", typeof(ICommand), typeof(EnterKeyDown),
            new PropertyMetadata(null, OnCommandChanged));

    #endregion Command

    #region CommandArgument

    public static object GetCommandArgument(DependencyObject obj) {
        return (object)obj.GetValue(CommandArgumentProperty);
    }

    public static void SetCommandArgument(DependencyObject obj, object value) {
        obj.SetValue(CommandArgumentProperty, value);
    }

    public static readonly DependencyProperty CommandArgumentProperty =
        DependencyProperty.RegisterAttached("CommandArgument", typeof(object), typeof(EnterKeyDown),
            new PropertyMetadata(null, OnCommandArgumentChanged));

    #endregion CommandArgument

    #region HasCommandArgument


    private static bool GetHasCommandArgument(DependencyObject obj) {
        return (bool)obj.GetValue(HasCommandArgumentProperty);
    }

    private static void SetHasCommandArgument(DependencyObject obj, bool value) {
        obj.SetValue(HasCommandArgumentProperty, value);
    }

    private static readonly DependencyProperty HasCommandArgumentProperty =
        DependencyProperty.RegisterAttached("HasCommandArgument", typeof(bool), typeof(EnterKeyDown),
            new PropertyMetadata(false));


    #endregion HasCommandArgument

    #endregion Propreties

    #region Event Handling

    private static void OnCommandArgumentChanged(DependencyObject o, DependencyPropertyChangedEventArgs e) {
        SetHasCommandArgument(o, true);
    }

    private static void OnCommandChanged(DependencyObject o, DependencyPropertyChangedEventArgs e) {
        FrameworkElement element = o as FrameworkElement;
        if (element != null) {
            if (e.NewValue == null) {
                element.KeyDown -= new KeyEventHandler(FrameworkElement_KeyDown);
            }
            else if (e.OldValue == null) {
                element.KeyDown += new KeyEventHandler(FrameworkElement_KeyDown);
            }
        }
    }

    private static void FrameworkElement_KeyDown(object sender, KeyEventArgs e) {
        if (e.Key == Key.Enter) {
            DependencyObject o = sender as DependencyObject;
            ICommand command = GetCommand(sender as DependencyObject);

            FrameworkElement element = e.OriginalSource as FrameworkElement;
            if (element != null) {
                // If the command argument has been explicitly set (even to NULL)
                if (GetHasCommandArgument(o)) {
                    object commandArgument = GetCommandArgument(o);

                    // Execute the command
                    if (command.CanExecute(commandArgument)) {
                        command.Execute(commandArgument);
                    }
                }
                else if (command.CanExecute(element.DataContext)) {
                    command.Execute(element.DataContext);
                }
            }
        }
    }

    #endregion
}

MyUserControl.xaml

<UserControl
    ...
    xmlns:b="clr-namespace:MyNameSpace.Interactivity"
    ...
    <TextBox b:EnterKeyDown.Command="{Binding AddNewDetailCommand}"
             b:EnterKeyDown.CommandArgument="{Binding Path=Text,RelativeSource={RelativeSource Self}}" />
    ...
</UserControl>

Answer 2

I ran into this same issue yesterday and solved it using custom triggers. It may seem a bit much at first, but I found this general pattern is usable for doing a lot of the things that I used to accomplish using event handlers directly in a view (like double click events). The first step is to create a trigger action that can accept a parameter since we will need it later.

public class ExecuteCommandAction : TriggerAction<FrameworkElement>
{
    public string Command { get; set; }

    protected override void Invoke(object o)
    {
        if (Command != null)
        {
            object ctx = AssociatedObject.DataContext;
            if (ctx != null)
            {
                var cmd = ctx.GetType().GetProperty(Command)
                    .GetValue(ctx, null) as ICommand;
                if (cmd != null && cmd.CanExecute(o))
                {
                    cmd.Execute(o);
                }
            }
        }
    }
}

The next step is to create the trigger. You could do some interesting things with base classes to make it more generic for capturing different types of key presses, but we'll keep it simple.

public class TextBoxEnterKeyTrigger: TriggerBase<UIElement>
{
    protected override void OnAttached()
    {
        base.OnAttached();
        AssociatedObject.KeyUp += AssociatedObject_KeyUp;
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();
        AssociatedObject.KeyUp -= AssociatedObject_KeyUp;
    }

    void AssociatedObject_KeyUp(object sender, System.Windows.Input.KeyEventArgs e)
    {
        if (e.Key == Key.Enter)
        {
            TextBox textBox = AssociatedObject as TextBox;
            object o = textBox == null ? null : textBox.Text;
            if (o != null)
            {
                InvokeActions(o);
            }
        }
    }
}

Keep in mind that even though you may have a data binding in place to your TextBox value, the property changed event won't fire because your textbox hasn't lost focus. For this reason I am passing the value of the TextBox.Text property to the command. The last step is to use this feature in your XAML. You need to be sure to include the Interactivity namespace as well as the namespace that contains your code from above.

<UserControl
...
xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
xmlns:common="clr-namespace:My.UI;assembly=My.UI">
    <TextBox Text="{Binding Path=MyText, Mode=TwoWay}" IsEnabled="{Binding CanMyCommand}">
        <i:Interaction.Triggers>
            <common:TextBoxEnterKeyTrigger>
                <common:ExecuteCommandAction Command=MyCommand" />
            </common:TextBoxEnterKeyTrigger>
        </i:Interaction.Triggers>
    </TextBox>
</UserControl>

Answer 3

On top of my mind.. You can pass event args to command and than in ViewModel check if e.KeyPress = Keys.Enter.. this is not really code :) i dont have my VS on this computer.. this is rather an idea :)

Answer 4

I used scottrudy's code in my application however, my textbox text is bound to some property in viewmodel class and this property is not getting updated by the time command is invoked after pressiong ENTER key because my textbox hasn't lost focus yet. So, to resolved this, i added the following code snippets just above InvokeActions(o) in AssociatedObject_KeyUp method and updated text property is getting updated in viewmodel class.

                    BindingExpression bindingExpression = (textBox).GetBindingExpression(TextBox.TextProperty);
                bindingExpression.UpdateSource();

Answer 5

There is KeyTrigger in expression blend.

<UserControl
xmlns:i="clr-namespace:System.Windows.Interactivity;
         assembly=System.Windows.Interactivity"
xmlns:iex="clr-namespace:Microsoft.Expression.Interactivity.Input;
           assembly=Microsoft.Expression.Interactions" ...>    
     <TextBox>
         <i:Interaction.Triggers>
            <iex:KeyTrigger Key="Enter">
               <i:InvokeCommandAction Command="{Binding PasswordLoginCommand}" />
            </iex:KeyTrigger>
         </i:Interaction.Triggers>
     </TextBox>    
</UserControl>

System.Windows.Interactivity and Microsoft.Expression.Interactions assemblies are available for WPF in the official Nuget package.