Programmatically create ItemsPanelTemplate for Silverlight ComboBox?

Question

I am trying to create a Blend behavior related to ComboBoxes. In order to get the effect I want, the ItemsPanel of the ComboBox has to have a certain element added to it. I don't want to do this in every ComboBox that uses the behavior, so I want the Behavior to be able to inject the ItemsPanelTemplate programmatically. However, I can't seem to find a way to do this. ItemsPanelTemplate does not seem to have a property/method that lets me set the visual tree. WPF ItemsPanelTemplate has the VisualTree but Silverlight does not.

Basically, what is the programmatic equivalent of this XAML?

    <ComboBox>
        <ComboBox.ItemsPanel>
            <ItemsPanelTemplate>
                <StackPanel/>
            </ItemsPanelTemplate>
        </ComboBox.ItemsPanel>
    </ComboBox>

Edit:
Okay apparently that is not an easy question, so I started a bounty and I'm going to give some more background in case there is another way to go about this. I want to provide keyboard support for the Silverlight ComoboBox. Out of the box it only supports the up and down arrows but I also want it to work so that when the user hits a letter, the ComboBox jumps to the first item of that letter, similar to how ComboBoxes work in a browser or Windows app.

I found this blog post, which got me half way. Adapting that behavior code, the ComboBox will change selection based on letter input. However, it does not work when the ComboBox is opened. The reason for this, according to this blog post is that when the ComboBox is opened, you are now interacting with its ItemsPanel and not the ComboBox itself. So according to that post I actually need to add a StackPanel to the ItemsPanelTemplate and subscribe to the StackPanel's KeyDown event, in order to take action when the ComboBox is opened.

So that is what prompted my question of how to get a StackPanel into the ItemsPanelTemplate of a ComboBox, from a behavior. If that is not possible, are there alternative ways of getting this to work? Yes, I know I could go to each ComboBox in the application and add a StackPanel and the event. But I want to do this through a behavior so that I don't have to modify every ComboBox in the app, and so I can reuse this logic across applications.

AnthonyWJones' answer below using XamlReader gets me part way, in that I can create the StackPanel and get it into the template. However, I need to be able to get at that SP programmatically in order to subscribe to the event.

Answer 1

This should work. I've shown how you can change the orientation below. You can add additional SetValue calls to modify other properties.

cb.ItemsPanel = new ItemsPanelTemplate();
var stackPanelFactory = new FrameworkElementFactory(typeof (StackPanel));
// Modify it like this:
stackPanelFactory.SetValue(StackPanel.OrientationProperty, Orientation.Horizontal);
// Set the root of the template to the stack panel factory:
cb.ItemsPanel.VisualTree = stackPanelFactory;

You can find more detailed information in this article: http://www.codeproject.com/KB/WPF/codeVsXAML.aspx

Answer 2

What you actually want to build programmatically is this:-

<ItemsPanelTemplate>
    <StackPanel />
</ItemsPanelTemplate>

Your behaviour will then assign this to the ItemsPanel property of the ComboBox it is attached to. Currently your behaviour is pure code but there is no way to create the above purely in code.

Since this is such a small piece for of Xaml the easiest approach is to use the XamlReader:-

ItemsPanelTemplate itemsPanelTemplate = XamlReader.Load("<ItemsPanelTemplate><StackPanel /></ItemsPanelTemplate>");

Answer 3

I think, best way for you - extend combobox functionality not via behavior but using inheritance. So, you can create own control MyComboBox:ComboBox. Create style for it - get default ComboBox Style here

And write instead (look for ScrollViewer by name):

< ScrollViewer x:Name="ScrollViewer" BorderThickness="0" Padding="1" >

   < ItemsPresenter />

< /ScrollViewer >

this

< ScrollViewer x:Name="ScrollViewer" BorderThickness="0" Padding="1" >

< StackPanel x:Name="StackPanel" >

   < ItemsPresenter />

< /StackPanel >

< /ScrollViewer >

This StackPanel you can get in code:

public class MyComboBox: ComboBox{

    public CM()
    {
        DefaultStyleKey = typeof (MyComboBox);
    }
    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();
        StackPanel stackPanel = (StackPanel)GetTemplateChild("StackPanel");
        stackPanel.KeyUp += (s, e) => { /*do something*/ };
    }

}

Inheritance is more powerful. It's allow work with template elements. If you decided to inject ItemsPanel, you must understand that:

1)it's impossible from code with keeping reference on injected panel.
2)to get reference to injected panel, this panel must registered itself in some storage, e.g.

< ComboBox>

   < ComboBox.ItemsPanel>

       < ItemsPanelTemplate>

           < StackPanel>

             < i:Interaction.EventTriggers>

               < i:EventTrigger EventName="Loaded">

                  < RegisterMyInstanceInAccessibleFromCodePlaceAction/>

               < /i:EventTrigger>

             < /i:Interaction.EventTriggers>

          < /StackPanel>

       < /ItemsPanelTemplate>

   < /ComboBox.ItemsPanel>

< /ComboBox>

Good luck!

Answer 4

I found your post while trying to set the ItemsPanel from code so that I can implement a VirtualizingStackPanel. When there are hundreds of items in my list, performance sucks. Anyway .. here's how I did it.

1) Custom control
2) Custom Behavior -- you could also just apply this behavior to the normal ComboBox - either at each instance, or through a style.

-- you might also expose the timeout value so that can be overridden in xaml ..
-- also, it seems this doesn't work when the dropdown itself is open. not sure why exactly .. never looked into it

1..

public class KeyPressSelectionComboBox : ComboBox
{ 

   private BindingExpression _bindingExpression;  


    public KeyPressSelectionComboBox()
        : base()
    {
        Interaction.GetBehaviors(this).Add(new KeyPressSelectionBehavior());
        Height = 22;

        this.SelectionChanged += new SelectionChangedEventHandler(KeyPressSelectionComboBox_SelectionChanged);
    }

    void KeyPressSelectionComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        if (_bindingExpression == null)
        {
            _bindingExpression = this.GetBindingExpression(ComboBox.SelectedValueProperty);
        }
        else
        {
            if (this.GetBindingExpression(ComboBox.SelectedValueProperty) == null)
            {
                this.SetBinding(ComboBox.SelectedValueProperty, _bindingExpression.ParentBinding);
            }
        }
    }  

}

2...

/// <summary>
/// This behavior can be attached to a ListBox or ComboBox to 
/// add keyboard selection
/// </summary>
public class KeyPressSelectionBehavior : Behavior<Selector>
{

    private string keyDownHistory = string.Empty;
    private double _keyDownTimeout = 2500;

    private DateTime _lastKeyDownTime;
    private DateTime LastKeyDownTime
    {
        get
        {
            if (this._lastKeyDownTime == null)
                this._lastKeyDownTime = DateTime.Now;

            return this._lastKeyDownTime;
        }
        set { _lastKeyDownTime = value; }
    }


    /// <summary>
    /// Gets or sets the Path used to select the text
    /// </summary>
    public string SelectionMemberPath { get; set; }

    /// <summary>
    /// Gets or sets the Timeout (ms) used to reset the KeyDownHistory item search string
    /// </summary>
    public double KeyDownTimeout
    {
        get { return _keyDownTimeout; }
        set { _keyDownTimeout = value; }
    }


    public KeyPressSelectionBehavior() { }

    /// <summary>
    /// Attaches to the specified object: subscribe on KeyDown event
    /// </summary>
    protected override void OnAttached()
    {
        base.OnAttached();
        this.AssociatedObject.KeyDown += DoKeyDown;
    }

    void DoKeyDown(object sender, KeyEventArgs e)
    {
        // Create a list of strings and indexes
        int index = 0;
        IEnumerable<Item> list = null;

        var path = SelectionMemberPath ??
            this.AssociatedObject.DisplayMemberPath;
        var evaluator = new BindingEvaluator();

        if (path != null)
        {
            list = this.AssociatedObject.Items.OfType<object>()
                .Select(item =>
                {
                    // retrieve the value using the supplied Path
                    var binding = new Binding();
                    binding.Path = new PropertyPath(path);
                    binding.Source = item;

                    BindingOperations.SetBinding(evaluator,
                        BindingEvaluator.TargetProperty, binding);
                    var value = evaluator.Target;

                    return new Item
                    {
                        Index = index++,
                        Text = Convert.ToString(value)
                    };
                });
        }
        else
        {
            list = this.AssociatedObject.Items.OfType<ListBoxItem>()
                .Select(item => new Item
                {
                    Index = index++,
                    Text = Convert.ToString(item.Content)
                });
        }
        // Sort the list starting at next selectedIndex to the end and 
        // then from the beginning
        list = list.OrderBy(item => item.Index <=
            this.AssociatedObject.SelectedIndex ?
            item.Index + this.AssociatedObject.Items.Count : item.Index);

        // calculate how long has passed since the user typed a letter
        var elapsedTime = DateTime.Now - this.LastKeyDownTime;
        if (elapsedTime.TotalMilliseconds <= this.KeyDownTimeout)
        { /* if it's less than the timeout, add to the search string */
            this.keyDownHistory += GetKeyValue(e);
        }
        else
        { /* otherwise replace it */
            this.keyDownHistory = GetKeyValue(e);
        }

        // Find first starting with the search string            
        var searchText = this.keyDownHistory;
        var first = list.FirstOrDefault(item => 
            item.Text.StartsWith(searchText, StringComparison.InvariantCultureIgnoreCase));

        if (first != null)
        { /* found */
            this.AssociatedObject.SelectedIndex = first.Index;
        }
        else
        { /* not found - so reset the KeyDownHistory */
            this.keyDownHistory = string.Empty;
        }


        // reset the last time a key was pressed
        this.LastKeyDownTime = DateTime.Now;
    }

    /// <summary>
    /// Gets the value of the pressed key, 
    /// specifically converting number keys from their "Dx" value to their expected "x" value
    /// </summary>
    /// <param name="e"></param>
    /// <returns></returns>
    private static string GetKeyValue(KeyEventArgs e)
    {
        string rValue = string.Empty;

        switch (e.Key)
        {
            default:
                rValue = e.Key.ToString();
                break;
            case Key.D0:
            case Key.NumPad0:
                rValue = (0).ToString();
                break;
            case Key.D1:
            case Key.NumPad1:
                rValue = (1).ToString();
                break;
            case Key.D2:
            case Key.NumPad2:
                rValue = (2).ToString();
                break;
            case Key.D3:
            case Key.NumPad3:
                rValue = (3).ToString();
                break;
            case Key.D4:
            case Key.NumPad4:
                rValue = (4).ToString();
                break;
            case Key.D5:
            case Key.NumPad5:
                rValue = (5).ToString();
                break;
            case Key.D6:
            case Key.NumPad6:
                rValue = (6).ToString();
                break;
            case Key.D7:
            case Key.NumPad7:
                rValue = (7).ToString();
                break;
            case Key.D8:
            case Key.NumPad8:
                rValue = (8).ToString();
                break;
            case Key.D9:
            case Key.NumPad9:
                rValue = (9).ToString();
                break;

        }

        return rValue;
    }

    /// <summary>
    /// Helper class
    /// </summary>
    private class Item
    {
        public int Index;
        public string Text;
    }

    /// <summary>
    /// Helper class used for property path value retrieval
    /// </summary>
    private class BindingEvaluator : FrameworkElement
    {

        public static readonly DependencyProperty TargetProperty =
            DependencyProperty.Register(
                "Target",
                typeof(object),
                typeof(BindingEvaluator), null);

        public object Target
        {
            get { return GetValue(TargetProperty); }
            set { SetValue(TargetProperty, value); }
        }
    }

}