If you look at my KeyBehavior you notice that it is doing two things: register for events so that the behavior can be triggered and handle the actual command invocation. In order to enhance reuse we can refactor this KeyBehavior in a KeyTrigger and an InvokeCommandAction. Well, we’re not going to do that, because they exist already in the silverlight sdk

A shortcoming of the InvokeCommandAction is that it can only invoke commands on the FrameworkElement itself, thus we write a custom implementation that finds commands on the DataContext instead:

public class InvokeCommandAction : TriggerAction<frameworkElement>
{
 public string CommandName { get; set; }

 protected override void Invoke(object parameter)
 {
  var viewModel = AssociatedObject.DataContext;
  GetCommandAndExecuteIt(viewModel, CommandName);
 }

 void GetCommandAndExecuteIt(object viewModel, string commandName)
 {
  var command = viewModel.GetPropertyValue<icommand>(commandName);
  if(command.CanExecute(null)) command.Execute(null);
 }
}

And now we can drag this action on our design surface in Blend and select a trigger that goes with it:

All we have to do is choose the Key and Command to invoke:

In XAML this looks like:

<interactivity:Interaction.Triggers>
 <ii:KeyTrigger Key="Right">
  <inf:InvokeCommandAction CommandName="PlayerRight"/>
 </ii:KeyTrigger>
 <ii:KeyTrigger Key="Left">
  <inf:InvokeCommandAction CommandName="PlayerLeft"/>
 </ii:KeyTrigger>
 <ii:KeyTrigger Key="Up">
  <inf:InvokeCommandAction CommandName="PlayerUp"/>
 </ii:KeyTrigger>
 <ii:KeyTrigger Key="Down">
  <inf:InvokeCommandAction CommandName="PlayerDown"/>
 </ii:KeyTrigger>
</interactivity:Interaction.Triggers>

I guess this ends our exploration of the behavior features in Silverlight.

Yesterday i demonstrated how attached properties can be used to invoke commands on specific key presses (and releases). With the aid of System.Windows.Interactivity.Behavior we can implement a true behavior and we get an extension point to do the required cleanup.

<grid>
 <interactivity:Interaction.Behaviors>
  <inf:KeyBehavior>
   <inf:KeyBehavior.DownKeyCommands>
    <inf:KeyCommandName Key="Right" CommandName="PlayerRight"  />
    <inf:KeyCommandName Key="Left" CommandName="PlayerLeft" />
    <inf:KeyCommandName Key="Up" CommandName="PlayerUp" />
    <inf:KeyCommandName Key="Down" CommandName="PlayerDown" />
   </inf:KeyBehavior.DownKeyCommands>
  </inf:KeyBehavior>
 </interactivity:Interaction.Behaviors>
 ...
</grid>

The behavior implementation is the same as yesterday, only this time we thankfully override the OnAttached and OnDetaching methods:

public class KeyBehavior : Behavior<frameworkElement>
{
 public List<keyCommandName> DownKeyCommands { get; set; }
 public List<keyCommandName> UpKeyCommands { get; set; }

 public KeyBehavior()
  : base()
 {
  DownKeyCommands = new List<keyCommandName>();
  UpKeyCommands = new List<keyCommandName>();
 }

 protected override void OnAttached()
 {
  base.OnAttached();
  SubscribeToKeyEvents();
 }

 protected override void OnDetaching()
 {
  UnsubscribeFromKeyEvents();
  base.OnDetaching();
 }

 void SubscribeToKeyEvents()
 {
  AssociatedObject.KeyDown += AssociatedObject_KeyDown;
  AssociatedObject.KeyUp += AssociatedObject_KeyUp;
 }

 void UnsubscribeFromKeyEvents()
 {
  AssociatedObject.KeyDown -= AssociatedObject_KeyDown;
  AssociatedObject.KeyUp -= AssociatedObject_KeyUp;
 }

 ...

I strongly believe that input handling is a responsability that belongs to the View. At first i simply added the following in the code-behind of my GameView:

protected override void OnKeyDown(KeyEventArgs e)
{
 base.OnKeyDown(e);

 if (e.Key == Key.Left) Model.MovePlayerLeft();
 ...
}

But i wanted to play with the cool kids so i exposed ICommand properties on my ViewModel instead and rewrote the code like this:

protected override void OnKeyDown(KeyEventArgs e)
{
 base.OnKeyDown(e);

 if (e.Key == Key.Left) Model.PlayerLeft.Execute(null);
 ...

Offcourse, designers should not have to write code at all, thus i searched for a different solution. Because there isn’t a behavior that allows me to differentiate the command based on the actual key being pressed i wrote my own KeyEvents class which allows the designer to map a key to a command. Here is an example:

 <inf:KeyEvents.Down>
  <inf:KeyCommand Key="Right" CommandName="PlayerRight"  />
  <inf:KeyCommand Key="Left" CommandName="PlayerLeft" />
  <inf:KeyCommand Key="Up" CommandName="PlayerUp" />
  <inf:KeyCommand Key="Down" CommandName="PlayerDown" />
 </inf:KeyEvents.Down>
 ...
</grid>

The down property is nothing more than an attached property:

public static readonly DependencyProperty DownProperty =
 DependencyProperty.RegisterAttached("Down", typeof(List<keyCommand>), typeof(KeyEvents), new PropertyMetadata(null, OnSetDownCallback));

A KeyCommand is a simple pair of a Key and a Command name:

public class KeyCommand
{
 public Key Key { get; set; }
 public string CommandName { get; set; }
}

The GetDown method (for the attached Down property) will instantiate a KeyBehavior class which hooks up to the element’s KeyDown and KeyUp events and uses a bit of reflection to find the commands…

public class KeyBehavior
{
 public KeyBehavior(FrameworkElement frameworkElement)
 {
  FrameworkElement = frameworkElement;
  DownKeyCommands = new List<keyCommand>();
  UpKeyCommands = new List<keyCommand>();

  frameworkElement.KeyDown += frameworkElement_KeyDown;
  frameworkElement.KeyUp += frameworkElement_KeyUp;
 }

 public FrameworkElement FrameworkElement { get; set; }
 public IList<keyCommand> DownKeyCommands { get; set; }
 public IList<keyCommand> UpKeyCommands { get; set; }

 void frameworkElement_KeyUp(object sender, KeyEventArgs e)
 {
  ExecuteCommandsForKey(e.Key, UpKeyCommands);
 }

 void frameworkElement_KeyDown(object sender, KeyEventArgs e)
 {
  ExecuteCommandsForKey(e.Key, DownKeyCommands);
 }

 void ExecuteCommandsForKey(Key key, IEnumerable<keyCommand> commands)
 {
  var commandNamesForKey = commands.Where(p => p.Key == key).Select(p => p.CommandName);
  var viewModel = FrameworkElement.DataContext;
  foreach (var command in commandNamesForKey) GetCommandAndExecuteIt(viewModel, command);
 }

 void GetCommandAndExecuteIt(object model, string name)
 {
  var command = GetCommand(model, name);
  command.Execute(null);
 }

 ICommand GetCommand(object model, string name)
 {
  return (ICommand)model.GetType().GetProperty(name).GetValue(model, null);
 }
}

The only thing that is missing is a way to unsubscribe from the events (and so you will end up with memory leaks). WeakReferences may come of use but i’ll leave that as an exercise for the reader. Many thanks go to the WPF Disciples because they inspired me to come up with this attached properties magic.