How to set GroupStyle for custom control - wpf

Question

I have created a custom control library project and did the followings:
The custom control is derived from ComboBox
Add a resource dictionary file rd.xaml under Themes folder
Define some styles in the rd.xaml file
<Style x:Key="GroupComboBoxStyle" TargetType="{x:Type local:GroupComboBox}">
<Setter Property="ItemContainerStyle" >
<Setter.Value>
<Style TargetType="{x:Type ComboBoxItem}">
<Setter Property="IsEnabled" Value="{Binding Available}"/>
</Style>
</Setter.Value>
</Setter>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type local:GroupComboBox}">
<Border Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}">
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
<Setter Property="ItemsPanel">
<Setter.Value>
<ItemsPanelTemplate>
<WrapPanel IsItemsHost="True" Orientation="Horizontal" Width="150" Height="Auto" >
<!-- add scroll bar -->
</WrapPanel>
</ItemsPanelTemplate>
</Setter.Value>
</Setter>
<Setter Property="ItemTemplate">
<Setter.Value>
<DataTemplate>
<TextBlock Text="{Binding Item}" Width="40"/>
</DataTemplate>
</Setter.Value>
</Setter>
</Style>
<CollectionViewSource x:Key="groupedData" Source="{Binding Items}">
<CollectionViewSource.GroupDescriptions>
<PropertyGroupDescription PropertyName="Category"/>
</CollectionViewSource.GroupDescriptions>
</CollectionViewSource>
<Style x:Key="groupComboBoxItemStyle" TargetType="{x:Type ComboBoxItem}">
<Setter Property="Width" Value="50" />
</Style>
<GroupStyle x:Key="groupStyle">
<GroupStyle.HeaderTemplate>
<DataTemplate>
<Border BorderBrush="Black" BorderThickness="2">
<TextBlock Text="{Binding Name}" HorizontalAlignment="Stretch" Background="YellowGreen"/>
</Border>
</DataTemplate>
</GroupStyle.HeaderTemplate>
</GroupStyle>
Then I want to set the group style to my custom control at runtime
But the groupstyle cannot be found, how to get it from the resource dictionary file?
public GroupComboBox()
{
GroupStyle style = new GroupStyle();
// get the groupstyle
style.HeaderTemplate = (DataTemplate)this.FindResource("groupStyle");
this.GroupStyle.Add(style);
}

Answer 1

A WPF CustomControl is supposed to be lookless. This means, that the code should only contain the control's logic but nothing related to how it looks, styling, etc. This should all be done using the style which is created for you in the Generic.xaml.
Anyway, it is totally valid to wish for a green background in your header... I would suggest to create a bindable dependency property for a DefaultGroupStyle in your control. I've implemented and tested it and it does the trick:
The control GroupComboBox:
public class GroupComboBox : ComboBox
{
public static readonly DependencyProperty DefaultGroupStyleProperty =
DependencyProperty.Register("DefaultGroupStyle", typeof (GroupStyle), typeof (GroupComboBox), new PropertyMetadata(default(GroupStyle), OnDefaultGroupStyleChanged));
private static void OnDefaultGroupStyleChanged(DependencyObject s, DependencyPropertyChangedEventArgs a)
{
var c = (GroupComboBox) s;
if (a.NewValue == null) return;
if (c.GroupStyle.Count == 0)
c.GroupStyle.Add((GroupStyle) a.NewValue);
}
public GroupStyle DefaultGroupStyle
{
get { return (GroupStyle) GetValue(DefaultGroupStyleProperty); }
set { SetValue(DefaultGroupStyleProperty, value); }
}
static GroupComboBox()
{
DefaultStyleKeyProperty.OverrideMetadata(typeof(GroupComboBox), new FrameworkPropertyMetadata(typeof(GroupComboBox)));
}
}
and the styles in Generic.xaml (feel free to move the styles to another file but don't forget to merge it into Generic.xaml. Note that I removed the key on the default style for the ComboBox. It won't get applied automatically otherwise...
<ResourceDictionary
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfCustomControlLibrary1">
<GroupStyle x:Key="GroupStyle">
<GroupStyle.HeaderTemplate>
<DataTemplate>
<Border BorderBrush="Black" BorderThickness="2">
<TextBlock Text="{Binding Name}" HorizontalAlignment="Stretch" Background="YellowGreen"/>
</Border>
</DataTemplate>
</GroupStyle.HeaderTemplate>
</GroupStyle>
<Style TargetType="{x:Type local:GroupComboBox}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type local:GroupComboBox}">
<Border Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}">
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
<Setter Property="DefaultGroupStyle" Value="{StaticResource GroupStyle}" />
</Style>
</ResourceDictionary>
Please let me know if this works for you and feel free to ask, if there's anything unclear.