Question

How can I set focus on a TextBox when F3 key pressed.

In other words, do some thing like bellow:

private void Window_PreviewKeyDown(object sender, KeyEventArgs )
{
       switch (e.Key)
       {
           case Key.F3:
               myTextBox1.Focus();
               break;
           case Key.F4:
               myTextBox2.Focus();
               break;
           default:
               break;
      }
}

note:I want to do it in xaml.

Answer 1

You could do this by created an attached property that takes a shortcut key then create an input binding on the window hosting that control.. it’s a bit of a complex class but very easy to use.

start by adding a new class to your project below.

 public class TextBoxHelper : DependencyObject
    {
        public class MvvmCommand : DependencyObject, ICommand
        {
            readonly Action<object> _execute;
            readonly Func<object, bool> _canExecute;
            public event EventHandler CanExecuteChanged;
            public MvvmCommand(Action<object> execute, Func<object, bool> canExecute = null)
            {
                if (execute == null) throw new ArgumentNullException("command");
                _canExecute = canExecute == null ? parmeter => MvvmCommand.AlwaysCanExecute() : canExecute;
                _execute = execute;
            }
            public object Tag
            {
                get { return (object)GetValue(TagProperty); }
                set { SetValue(TagProperty, value); }
            }
            public static readonly DependencyProperty TagProperty = DependencyProperty.Register("Tag", typeof(object), typeof(MvvmCommand), new PropertyMetadata(null));
            static bool AlwaysCanExecute()
            {
                return true;
            }
            public void EvaluateCanExecute()
            {
                EventHandler temp = CanExecuteChanged;
                if (temp != null)
                    temp(this, EventArgs.Empty);
            }
            public virtual void Execute(object parameter)
            {
                _execute(parameter == null ? this : parameter);
            }
            public virtual bool CanExecute(object parameter)
            {
                return _canExecute == null ? true : _canExecute(parameter);
            }
        }

        public static Key GetFocusKey(DependencyObject obj)
        {
            return (Key)obj.GetValue(FocusKeyProperty);
        }

        public static void SetFocusKey(DependencyObject obj, Key value)
        {
            obj.SetValue(FocusKeyProperty, value);
        }

        public static readonly DependencyProperty FocusKeyProperty =
            DependencyProperty.RegisterAttached("FocusKey", typeof(Key), typeof(TextBoxHelper), new PropertyMetadata(Key.None, new PropertyChangedCallback((s, e) =>
                {
                    UIElement targetElement = s as UIElement;
                    if (targetElement != null)
                    {
                        MvvmCommand command = new MvvmCommand(parameter => TextBoxHelper.FocusCommand(parameter))
                            {
                                Tag = targetElement, 
                            };
                        InputGesture inputg = new KeyGesture((Key)e.NewValue);
                        (Window.GetWindow(targetElement)).InputBindings.Add(new InputBinding(command, inputg));
                    }
                })));

        public static void FocusCommand(object parameter)
        {
            MvvmCommand targetCommand = parameter as MvvmCommand;
            if (targetCommand != null)
            {
                UIElement targetElement = targetCommand.Tag as UIElement;
                if (targetElement != null)
                {
                    targetElement.Focus();
                }
            }
        }
    }

now in XAML all you need to do to set your focus keys is assign that FocusKey property, an example below has 2 textboxes, one gets focus when F1 is pressed, the other when F7 is pressed.

<Window x:Class="WpfApplication5.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApplication5"
        DataContext="{Binding RelativeSource={RelativeSource Self}}"
        Title="MainWindow" Height="131" Width="460">

    <Grid Margin="10">
        <TextBox Margin="0,0,0,60" local:TextBoxHelper.FocusKey="F1" />
        <TextBox Margin="0,35,0,0" local:TextBoxHelper.FocusKey="F7" />
    </Grid>
</Window>