Question

I am new to both WPF and MVVM so I will beg forgiveness upfront if this is a silly question.

Problem:
I am trying to create a grouped list of items using the MVVM design pattern. I can do it with code behind, but would prefer a more elegant solution.

Details

• Let’s say I have a collection of animals: horse, wolf, monkey, tiger, polar bear, zebra, bat, etc.
• These animals are grouped by continents: North America, Africa, Antarctica, etc.

Goal: Within a wrap panel, I would like to create grouped toggle buttons. For example, there would be a “North America” GroupBox with ToggleButtons for each animal found in North America. Next, there would be a GroupBox with the header “Africa” and inside the group box would be all the animals in Africa.

Using the MVVM design pattern, I can bind to an ObservableCollection and, using a data template, create the toggle buttons I need. Where I’m struggling is I don’t know how to group the animals. All I need are guidelines to follow. Any help would be appreciated.

Answer 1

In the View, set the items source to a CollectionViewSource that itself binds to the Animals collection on the ViewModel. The CollectionViewSource can be grouped, something like this:

<CollectionViewSource.GroupDescriptions>
   <PropertyGroupDescription PropertyName="Continent" />
</CollectionViewSource.GroupDescriptions>

You’ll also need to set a group style on whatever items control you’ve got – something like this

<ItemsControl.GroupStyle>
   <GroupStyle>
      <GroupStyle.HeaderTemplate>
         <DataTemplate>
            <TextBlock FontWeight="Bold" FontSize="15"
               Text="{Binding Path=Name}"/>
         </DataTemplate>
      </GroupStyle.HeaderTemplate>
   </GroupStyle>
</ItemsControl.GroupStyle>

Taken from the example on this page – http://msdn.microsoft.com/en-us/library/system.windows.controls.itemscontrol.groupstyle.aspx

That’s setting the HeaderTemplate, but if you play around a bit you should be able to set a container style for each group.

Hope this helps!

Update:
I wasn’t too sure about this so I had a quick bash at the code. Assuming ‘toggle button’ is ‘radio button’, this is what I’ve come up with:

<Grid>
    <Grid.Resources>
        <CollectionViewSource x:Key="Animals" Source="{Binding}">
            <CollectionViewSource.GroupDescriptions>
                <PropertyGroupDescription PropertyName="Continent" />
            </CollectionViewSource.GroupDescriptions>
        </CollectionViewSource>
    </Grid.Resources>

    <ItemsControl ItemsSource="{Binding Source={StaticResource Animals}}">
        <ItemsControl.GroupStyle>
            <x:Static Member="GroupStyle.Default" />
        </ItemsControl.GroupStyle>

        <ItemsControl.ItemTemplate>
            <DataTemplate>
                <RadioButton Content="{Binding Name}" GroupName="{Binding Continent}" />
            </DataTemplate>
        </ItemsControl.ItemTemplate>
    </ItemsControl>
</Grid>

In addition, you can display each group as a GroupBox by replacing the line <x:Static Member="GroupStyle.Default" /> with:

<GroupStyle>
   <GroupStyle.ContainerStyle>
      <Style TargetType="{x:Type GroupItem}">
         <Setter Property="Template">
            <Setter.Value>
               <ControlTemplate>
                  <GroupBox Margin="10" Header="{Binding Name}">
                     <GroupBox.Content>
                        <ItemsPresenter />
                     </GroupBox.Content>
                  </GroupBox>
               </ControlTemplate>
            </Setter.Value>
         </Setter>
      </Style>
   </GroupStyle.ContainerStyle>
</GroupStyle>

However, the radio buttons will not be mutually exclusive on their own (I presume because they are wrapped in ListItem controls, or something else that makes them a single child of a grouping parent). That code was stolen/modified from the GroupStyle entry in MSDN if you want to go back for more information (their example had expanders to show/hide groups): http://msdn.microsoft.com/en-us/library/system.windows.controls.groupstyle.aspx

Let me know if I’ve missed the point at all.