RadComboBox NullValueTemplate

[Jose Jones]

Hi...

I would like to inherit from the RadComboBox to have a NullableComboBox which allows us to set a distinct template for anull value within the combobox declaration.

Something like this

<CustomControls:RadCustomComboBox
                 ItemsSource="{Binding SomeEntities, Converter={StaticResource nullableItemSourceConverter}}">
          <CustomControls:RadCustomComboBox.NullValueContentTemplate>
            <DataTemplate>
              <TextBlock Text="Something representing a null value" />
            </DataTemplate>
          </CustomControls:RadCustomComboBox.NullValueContentTemplate>
          <CustomControls:RadCustomComboBox.ItemTemplate>
            <DataTemplate>
              <StackPanel Orientation="Horizontal">
                <TextBlock Text="{Binding Path=Attribute1}" />
                <TextBlock Text="{Binding Path=Attribute2}" Foreground="DarkRed"  />
              </StackPanel>
            </DataTemplate>
          </CustomControls:RadCustomComboBox.ItemTemplate>
        </CustomControls:RadCustomComboBox>

The Null value is inserted in the list using a quite simple converter as I don't want my ViewModel to expose a list with a null value.

I'm able to display the templated item.  This works fine.  Unfortunatly, I'm just not able to get the SelectionChanged event to be raised when I select it.  I was able to do this stuff using a regular ComboBox and I noticed that the PrepareContainerForItemOverride method is raised earlier in the comboxBox than in the RadComboBox (which is raised only when the user clicks on the combobox).

I'm requesting some help, I would greatly appreciate to benefit from the rich feature set of the radcombobox.

Here's the code for the custom control

 public class RadCustomComboBox : RadComboBox
    {
        public static readonly DependencyProperty NullValueContentProperty = DependencyProperty.Register("NullValueContent", typeof(object), typeof(RadCustomComboBox), new PropertyMetadata(new PropertyChangedCallback(RadCustomComboBox.OnNullValueContentPropertyChanged)));
        public static readonly DependencyProperty NullValueContentTemplateProperty = DependencyProperty.Register("NullValueContentTemplate", typeof(DataTemplate), typeof(RadCustomComboBox), new PropertyMetadata(new PropertyChangedCallback(RadCustomComboBox.OnNullValueContentTemplatePropertyChanged)));
        protected RadComboBoxItem NullValueListBoxItem { get; set; }


        /// <summary>
        /// Gets or sets the NullValueContent value.
        /// </summary>
        public object NullValueContent
        {
            get { return (object)base.GetValue(RadCustomComboBox.NullValueContentProperty); }
            set { base.SetValue(RadCustomComboBox.NullValueContentProperty, value); }
        }

        /// <summary>
        /// Gets or sets the NullValueContentTemplate value.
        /// </summary>
        public DataTemplate NullValueContentTemplate
        {
            get { return (DataTemplate)base.GetValue(RadCustomComboBox.NullValueContentTemplateProperty); }
            set { base.SetValue(RadCustomComboBox.NullValueContentTemplateProperty, value); }
        }


        /// <summary>
        /// Initializes a new instance of the <see cref="RadCustomComboBox"/> class.
        /// </summary>
        public RadCustomComboBox()
            : base()
        {
            base.SelectionChanged += new SelectionChangedEventHandler(RadCustomComboBox_SelectionChanged);
        }

        /// <summary>
        /// Handles the SelectionChanged event of the RadCustomComboBox control.
        /// </summary>
        /// <param name="sender">The source of the event.</param>
        /// <param name="e">The <see cref="Telerik.Windows.Controls.SelectionChangedEventArgs"/> instance containing the event data.</param>
        private void RadCustomComboBox_SelectionChanged(Object sender, SelectionChangedEventArgs e)
        {
            if (this.SelectedItem == this.NullValueListBoxItem)
            {
                this.SelectedItem = null;
                this.SelectedValue = null;
            }

            if (!string.IsNullOrEmpty(SelectedValuePath))
            {
                object value = null;
                if (SelectedItem != null)
                    value = SelectedItem.GetType().GetProperty(SelectedValuePath).GetValue(SelectedItem, null);

                SetValue(RadCustomComboBox.SelectedValueProperty, value);
            }
        }

        /// <summary>
        /// Invoked when the <see cref="P:System.Windows.Controls.ItemsControl.Items"/> property changes.
        /// </summary>
        /// <param name="e">Information about the change.</param>
        protected override void OnItemsChanged(System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
        {
            switch (e.Action)
            {
                case NotifyCollectionChangedAction.Reset:
                    this.NullValueListBoxItem = null;
                    break;
                case NotifyCollectionChangedAction.Remove:
                case NotifyCollectionChangedAction.Replace:
                    if (e.OldItems.Contains(null)) this.NullValueListBoxItem = null;
                    break;
            }
            object obj = this.SelectedItem;
            base.OnItemsChanged(e);

            this.SelectedItem = obj;

        }

        /// <summary>
        /// Called when [null value content changed].
        /// </summary>
        /// <param name="oldValue">The old value.</param>
        /// <param name="newValue">The new value.</param>
        private void OnNullValueContentChanged(object oldValue, object newValue)
        {
            if (this.NullValueListBoxItem != null)
            {
                this.NullValueListBoxItem.Content = newValue;
                this.OnApplyTemplate();
            }
        }

        /// <summary>
        /// Called when [null value content template property changed].
        /// </summary>
        /// <param name="d">The d.</param>
        /// <param name="e">The <see cref="System.Windows.DependencyPropertyChangedEventArgs"/> instance containing the event data.</param>
        private static void OnNullValueContentTemplatePropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            ((RadCustomComboBox)d).OnNullValueContentTemplateChanged((DataTemplate)e.OldValue, (DataTemplate)e.NewValue);
        }


        /// <summary>
        /// Called when [null value content template changed].
        /// </summary>
        /// <param name="oldValue">The old value.</param>
        /// <param name="newValue">The new value.</param>
        private void OnNullValueContentTemplateChanged(DataTemplate oldValue, DataTemplate newValue)
        {
            if (this.NullValueListBoxItem != null)
            {
                this.NullValueListBoxItem.ContentTemplate = newValue;
                this.OnApplyTemplate();
            }
        }

        protected virtual void OnSelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            if (this.SelectedItem == this.NullValueListBoxItem)
            {
                this.SelectedItem = null;
                this.SelectedValue = null;
            }

        }

        /// <summary>
        /// Prepares the specified element to display the specified item.
        /// </summary>
        /// <param name="element">Element used to display the specified item.</param>
        /// <param name="item">Specified item.</param>
        protected override void PrepareContainerForItemOverride(DependencyObject element, object item)
        {
            base.PrepareContainerForItemOverride(element, item);

            var item2 = element as RadComboBoxItem;

            if (item == null && item2 != null)
            {
                item2.Content = NullValueContent;
                item2.ContentTemplate = NullValueContentTemplate;
                NullValueListBoxItem = item2;
            }
        }

        /// <summary>
        /// Called when [null value content property changed].
        /// </summary>
        /// <param name="d">The d.</param>
        /// <param name="e">The <see cref="System.Windows.DependencyPropertyChangedEventArgs"/> instance containing the event data.</param>
        private static void OnNullValueContentPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            ((RadCustomComboBox)d).OnNullValueContentChanged(e.OldValue, e.NewValue);
        }
    }

Thanks!

[Valeri Hristov]

Hello,

I will need some time to see what's wrong with your code, I will write back here when I have more information. Meanwhile, I would like to suggest using the ClearValue button in RadComboBox, that seems to do exactly what you need, except it is displayed like a button, not like a regular item. There is more information about this button at the end of the following article:
http://www.telerik.com/help/silverlight/radcombobox-features-selection.html

Sincerely yours,
Valeri Hristov
the Telerik team

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[Valeri Hristov]

Hi,

I just wanted to share that RadComboBox will contain EmptyText and EmptySelectionBoxTemplate properties that will allow you to customize the control when it has no selection :)

All the best,
Valeri Hristov
the Telerik team

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