How can I filter keyboard number keys

Question

I wish to focus a specific Textbox and accept the key press only when the user presses a number key (1,2,3 etc.) otherwise I don't want to focus that Textbox...

Senario:

I have ListView (having a custom view) in a view. below the list view i have a Textbox.

Lets say ListView contains items numbered as 1, 2, 3, 33, 373 etc.

Now when I press a number key, lets say Key 3, the following action should occur:

1. Focus that Specific TextBox
2. Append the TextBox text with the number input
3. Select the item in the ListView with the same number as TextBox.Text has

My Xaml for what I tried

<ListView Name="lv"
              Grid.Row="1"
              ItemsSource="{Binding}"
              View="{Binding Path=SelectedItem,
                             ElementName=viewComboBox}" DisplayMemberPath="Name" IsTextSearchEnabled="True" TextSearch.TextPath="{Binding Path=Person.Name}"/>
    <TextBox  Grid.Row="2" Text="{Binding Path=TextSearch.Text,ElementName=lv}"></TextBox>  

Nothing is displayed in the TextBox and I don't know how to handle numeric key press.

I need this and its bewildering using MVVM...

Any help in this regard would be great. And some guidance using code would be even better... Thanks...

Answer 1

This logic is specific to view. Its fine if you put it in code behind. MVVM doesn't stop you from writing view specific code in code behind.

However if your religiously follow the 'no code behind' approach then you can create a Behavior and put all the code in that. Your behavior will be attached to listbox and will take textbox reference as property. It will listen to keydown event on listbox and add keys to textbox's text property.

You shouldn't really be having logic like this in ViewModel

Answer 2

If you are using MVVM then attached behavior is the way to go ... http://eladm.wordpress.com/2009/04/02/attached-behavior/

1. Name you key press source element like ListView ...

   <ListView x:Name="MyListView">
      ....
   </ListView>
   

2. Declare and define an attached property of type ListView say NumericKeyPressBehavior.FocusTarget

3. The Source for this FocusTarget will be you MyListView and the behavior will be attached to your TextBox

    <TextBox local:NumericKeyPressBehavior.FocusTarget="{Binding ElementName=MyListView}" 
             .... />
   

4. In NumericKeyPressBehavior.FocusTarget's dependency property changed event handler, handle the key press on the ListView and then based on if a numeric key was pressed, focus the target TextBox and also append the pressed key character to it.

   private static void  OnFocusTargetChanged(
       DependencyObject o, 
       DependencyPropertyChangedEventArgs e)
   {
       var textBox = o as TextBox;
       var listView = e.NewValue as ListView;
       if (textBox != null && listView != null)
       {
           listView.KeyUp += 
            (o1, e1) =>
               {
                   var keyChar = GetKeyCharFromKeyCode(e1.Key);
                   if ("0123456789".Contains(keyChar.ToString()))
                   {
                       textBox.Focus();
                       textBox.Text += keyChar.ToString();
                   }
               }
       }
   }
   

GetKeyCharFromKeyCode is found here.... C# How to translate virtual keycode to char?

Does this help?