DataGrid Hierarchical data, Expand nodes programmatically.

How can I expand/collapse nodes in a Hierarchical Data Grid? I need this in two contexts:

1. My window has a button "Expand All" and a button "Collapse All", which will expand/collapse all the parent rows in my grid
2. My grid needs to always have the first row expanded (in the image, the "ALL ACCOUNTS" needs to be always expanded (the user should not be able to collapse it).

Thanks

Hello

We have added Expand and Collapse methods to the data grid. You can call these methods and pass in the data object of the hierarchical row that you want to toggle. For example, if you want to collapse all items, iterate through each object in your items source (except for the first item), and pass each item into the DataGrid.Collapse method. This will collapse all the top level items.

We have also added the DataGrid.HierarchicalItemStateChanging event. You can attach an event handler to this to provide your own logic for weather or not a user can expand or collapse an item. The event arguments give you the data item that is requesting to be expanded or collapsed. In the event handler in your scenario, check to see if the given item is the first item in your items source. If it is, then set the CanChangeState property on the event arguments to false. Otherwise do nothing to indicate the user can toggle the state. Here is an example where "Data" is a reference to the items source. Note that this only effects user interaction. You can still programatically effect the expanded state of any item.

private void DataGrid_HierarchicalItemStateChanging(object sender, DataGridHierarchicalItemStateChangingEventArgs e)
{
    if (e.Item == Data[0])
    {
        e.CanChangeState = false;
    }
}

These additions will be available in the next nightly build. The next nightly build will be available at around 1200 GMT (about 11 hours from now). You can download nightly builds from your account page: http://www.mindscapehq.com/store/myaccount

Jason Fauchelle

Great. Thanks!

Two quick points.

1. The Expand method sometimes throws the exception below. I think that happens if you call it before the grid got the chance to load a new ItemsSource, because it is thrown only once at load time. I wrapped the call in try {...Expand } catch {} and it got solved for me.

   System.NullReferenceException: Object reference not set to an instance of an object. at Mindscape.WpfElements.WpfDataGrid.DataGridItemsSource.#oGb(DataGridItemWrapper wrapper) at Mindscape.WpfElements.WpfDataGrid.DataGridItemsSource.#7Db() at Mindscape.WpfElements.WpfDataGrid.DataGridItemsSource.#gGb(Object sender, EventArgs e) at Mindscape.WpfElements.WpfDataGrid.DataGridItemWrapper.#6qb() at Mindscape.WpfElements.WpfDataGrid.DataGridItemWrapper.setIsExpanded(Boolean value) at Mindscape.WpfElements.WpfDataGrid.DataGridItemsSource.Expand(Object item) at Mindscape.WpfElements.DataGrid.Expand(Object item) at Djali.Views.Behaviors.TopRiskBehavior.b_0(Int64 _) in c:\src-tfs\Development\Djali\Djali\Views\Behaviors\TopRiskBehavior.cs:line 23

2. Given that I would not allow my users to collapse the top node, ever; how could I hide the "Expand Arrow"... which would always be shown pointed down (i.e. expanded), but would not server any purpose?

Hello Nestor

1. The Expand and Collapse methods will only work after the items source has been set. I have included null checks at that point to avoid this exception.

2. In the next nightly build you will find the DataGrid.ExpanderToggleButtonStyleKey. You can use this to set a custom toggle button style used for hierarchical items and group headers. Assuming that you want to apply the custom expander button style to all your data grids, you can add a style that uses this key into the resource dictionary of App.xaml. For your scenario, you can include a trigger that checks the IsEnabled property and then hide the button when appropriate. IsEnabled is set to false if your custom expansion logic returned false. Here is an example that extends the Alloy expander button style:

   <Style x:Key="{x:Static ms:DataGrid.ExpanderToggleButtonStyleKey}" TargetType="ToggleButton">
     <Setter Property="SnapsToDevicePixels" Value="True" />
     <Setter Property="FocusVisualStyle"   Value="{x:Null}" />
     <Setter Property="Focusable" Value="False" />
     <Setter Property="Template">
       <Setter.Value>
         <ControlTemplate TargetType="ToggleButton">
           <Border Background="Transparent">
             <Path Name="Path" Data="M 0 0 L 6 4 L 0 8" Fill="{StaticResource TreeViewArrowBrush}" Margin="4" />
           </Border>
           <ControlTemplate.Triggers>
             <Trigger Property="IsChecked" Value="True">
               <Setter TargetName="Path" Property="Data" Value="M 0 0 L 8 0 L 4 6" />
             </Trigger>
             <DataTrigger Binding="{Binding IsSelected, RelativeSource={RelativeSource AncestorType={x:Type ms:DataGridRow}}}" Value="True">
               <Setter TargetName="Path" Property="Fill" Value="{StaticResource SelectedTreeViewArrowBrush}" />
             </DataTrigger>
             <Trigger Property="IsMouseOver" Value="True">
               <Setter TargetName="Path" Property="Fill" Value="{StaticResource MouseOverTreeViewArrowBrush}" />
             </Trigger>
             <Trigger Property="IsEnabled" Value="False">
               <Setter TargetName="Path" Property="Visibility" Value="Hidden" />
             </Trigger>
           </ControlTemplate.Triggers>
         </ControlTemplate>
       </Setter.Value>
     </Setter>
   </Style>
   

Let me know if you have any questions about this.

Jason Fauchelle

I'm not following you. By "custom expansion logic returned false" you mean when I set e.CanChangeState = false inside the event HierarchicalItemStateChanging ?

Never mind... I tried it and it worked. Thanks.

Would it be possible to allow the IsExpanded property to be binded to the View Model? In that way I can control precisely whether I was a node to appear expanded or collapsed after insertion.

Hello Nestor

We won't be adding this feature in right now, but I have made a note for a possible future feature. Expansion state isn't typically something you'd have in the model objects, though I can see this would be convenient. I hope you are able to live with the Expand and Collapse methods for now.

Jason Fauchelle

OK. I can live with that. Thanks. Nestor