Grouping child objects in WPF TreeView

Question

I am trying to get my tree view to group a collection of similar items by what they are. To keep things generic, my object hierarchy could look like this:

• Objects
  • Object Group #1
    • Item #1 (Type 'A')
    • Item #2 (Type 'A')
    • Item #3 (Type 'B')
    • Item #4 (Type 'B')

Right now my TreeView shows these objects exactly like the object model, but what I would like to do is insert a TreeView node for each object type so that it would look like this:

• Objects
  • Object Group #1
    • Type A
      • Item #1
      • Item #2
    • Type B
      • Item #3
      • Item #4

I saw in a similar question here that someone recommended to have two separate HierarchicalDataTemplates so I created one for 'Object Group #1' level which contains a TreeView with a list of the types, but this is really clumsy since it is a whole separate TreeView inside of some nodes. I have also been trying to use a CollectionViewSource to filter out the items in each category, but this doesn't do me very much good since I can't figure out how to display them.

I guess my question boils down to this: How do I make a HierarchicalDataTemplate group it's children? If someone could point me in the right direction I would appreciate it a lot.

I can post some code if anyone wants to see, but I am really just trying to figure out how to do what I want so my code is just a pretty straight forward databound treeview right now.

Answer 1

You can achieve this effect by binding the ItemsSource on your HierarchicalDataTempalate using an IValueConverter. This converter is simply does the following:

public class MyConverter : IValueConverter
{
  public object Convert(object value, ...)
  {
    return
      from item in (IEnumerable<MyItem>)value
      group item by item.Type into g
      select new { Type = g.Key, Items = g }
  }
  ...
}

Now your HierarchcialDataTemplate can be as follows:

<HierarchicalDataTemplate ItemsSource="{Binding SomePath, Converter={x:Static local:MyConverter}">

  <HierarchicalDataTemplate.ItemTemplate>
    <HierarchicalDataTemplate
      ItemsSource="{Binding Items}"
      TargetType="{x:Type local:MyItem}"

      ItemTemplate="{StaticResource MyItemTemplate}">
         <!-- may omit ItemTemplate in prior line to use implicit template -->

      <TextBlock Text="{Binding Type}" /> <!-- Header for type -->

    </HierarchicalDataTemplate>
  </HierarchicalDataTemplate.ItemTemplate>

  <!-- header for "Object Group #1" -->

</HierarchicalDataTemplate>