Here is a complete example
Xaml
<Window x:Class="Playground.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:Playground"
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
Title="MainWindow" Height="350" Width="525">
<Window.InputBindings>
<KeyBinding Modifiers="Alt" Key="LeftAlt" Command="{Binding AltPressedCommand}" />
</Window.InputBindings>
<Window.Resources>
<BooleanToVisibilityConverter x:Key="boolToVisibilityConverter"/>
</Window.Resources>
<i:Interaction.Triggers>
<i:EventTrigger EventName="PreviewKeyUp">
<i:InvokeCommandAction Command="{Binding AltUnpressedCommand}"/>
</i:EventTrigger>
</i:Interaction.Triggers>
<Grid>
<Button Content="My Button" Visibility="{Binding IsAltPressed, Converter={StaticResource boolToVisibilityConverter}}"/>
</Grid>
</Window>
ViewModel
public class MainWindowViewModel : NotificationObject
{
public MainWindowViewModel()
{
AltPressedCommand = new DelegateCommand(() => IsAltPressed = true);
AltUnpressedCommand = new DelegateCommand(() => IsAltPressed = false);
}
public DelegateCommand AltPressedCommand { get; set; }
public DelegateCommand AltUnpressedCommand { get; set; }
private bool _IsAltPressed;
public bool IsAltPressed
{
get { return _IsAltPressed; }
set
{
if (value != _IsAltPressed)
{
_IsAltPressed = value;
RaisePropertyChanged("IsAltPressed");
}
}
}
}
Explanation
The visibility of the control is binded to a boolean property via BooleanToVisibilityConverter
.
Then I use two commands. One fired when the Alt key is being pressed using KeyBinding, and the second is fired when the key up occurs. I left out a check of the Alt key when the key up occurs that you should add. If you want to purely use MVVM this could get tricky because you need to send a parameter to the command stating the key being pressed up.
Edit
I use the following behavior to pass the key parameter from the PreviewKeyUp
event
public class PreviewKeyUpBehavior : Behavior<UIElement>
{
#region Properties
public ICommand Command
{
get { return (ICommand)GetValue(CommandProperty); }
set { SetValue(CommandProperty, value); }
}
public static readonly DependencyProperty CommandProperty =
DependencyProperty.Register("Command", typeof(ICommand), typeof(PeviewKeyUpBehavior));
#endregion
#region Methods
protected override void OnAttached()
{
AssociatedObject.PreviewKeyUp += OnPreviewKeyUp;
base.OnAttached();
}
private void OnPreviewKeyUp(object sender, System.Windows.Input.KeyEventArgs e)
{
if (Command == null) return;
// Execute command and send the key as the command parameter
Command.Execute(e.Key == Key.System ? e.SystemKey : e.Key);
}
#endregion
}
This will raise the binded command when the PreviewKeyUp
is fired and send the key as the commands parameter. I then altered the code in the View and ViewModel as follows:
<!-- Used behaviors instead of triggers -->
<i:Interaction.Behaviors>
<local:PreviewKeyUpBehavior Command="{Binding KeyUnpressedCommand}"/>
</i:Interaction.Behaviors>
Changed the command to take a nullable key parameter
public DelegateCommand<Key?> KeyUnpressedCommand { get; set; }
And implemented it
KeyUnpressedCommand = new DelegateCommand<Key?>(key =>
{
if (key == Key.LeftAlt)
IsAltPressed = false;
});
Hope this helps