Handling Key press & release events in MVVM

Question

I am developing a wpf application using MVVM pattern. I need to separately handle Key press & release events, (e.g. in media players fwd/rev happen till user keeps key pressed & stops when he releases). After searching a lot, still I couldn't find any way to do it. Can anybody please help.

Answer 1

Thanks for your suggestions. I found a way to do this by using interactivity triggers & dependency property. Following is the dependency property for Command.

public class EventToCommand : TriggerAction<DependencyObject>
{
    public ICommand Command
    {
        get { return (ICommand)GetValue(CommandProperty); }
        set { SetValue(CommandProperty, value); }
    }

    // Using a DependencyProperty as the backing store for Command.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty CommandProperty =
        DependencyProperty.Register("Command", typeof(ICommand), typeof(EventToCommand), new PropertyMetadata(null));

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

Then just use it in the xaml as below:

    <i:Interaction.Triggers>
    <i:EventTrigger EventName="KeyUp">
        <ap:EventToCommand Command="{Binding KeyReleaseCommand}"></ap:EventToCommand>
    </i:EventTrigger>
    <i:EventTrigger EventName="KeyDown">
        <ap:EventToCommand Command="{Binding KeyDownCommand}"></ap:EventToCommand>
    </i:EventTrigger>
</i:Interaction.Triggers>

Where the KeyReleaseCommand & KeyDownCommand are RelayCommand in your ViewModel.

    public MainViewModel()
    {
        KeyDownCommand = new RelayCommand<KeyEventArgs>(OnKeyDown, null);
        KeyReleaseCommand = new RelayCommand<KeyEventArgs>(OnKeyRelease, null);
    }

    private void OnKeyRelease(KeyEventArgs args)
    {
        if (args.KeyboardDevice.Modifiers == ModifierKeys.Alt)
        {
            if (args.SystemKey == Key.Left)
            {
                Trace.WriteLine("ALT+LEFT Released");
            }
        }
    }

    public void OnKeyDown(KeyEventArgs args)
    {
        if (args.IsRepeat)
            return;

        if (args.KeyboardDevice.Modifiers == ModifierKeys.Alt)
        {
            if(args.SystemKey == Key.Left)
            {
                Trace.WriteLine("ALT+LEFT");
            }
        }
    }

Answer 2

I'm guessing, You will bind the Command to Button. If you want the Commands to fire repeatedly, you can use RepeatButton. It was designed for that purpose.You can bind your command to Command Property. It will fire your methods repeatedly until RepeatButton is released.