EventToCommand in DataTemplate - wpf

Question

I'm using the EventToCommand Class from the MVVM-light-Toolkit to handle the AutoGeneratingColumn-Event in the WPF-DataGrid. It works fine in my Main-DataGrid, but I use another DataGrid in the RowDetailsTemplate and here I got a problem:
The AutoGeneratingColumn fires before the EventToCommand-Object was generated. Is there a solution for this problem?
Here is a piece of my Xaml-Code:
<DataGrid DockPanel.Dock="Top" AutoGenerateColumns="True" Name="table" VerticalAlignment="Top" ItemsSource="{Binding PartBatchList}" IsReadOnly="True">
<i:Interaction.Triggers>
<i:EventTrigger EventName="AutoGeneratingColumn">
<hgc:EventToCommand Command="{Binding AutoGeneratingColumnCommand}" PassEventArgsToCommand="True"/>
</i:EventTrigger>
</i:Interaction.Triggers>
<DataGrid.RowDetailsTemplate>
<DataTemplate>
<StackPanel Margin="30,0,30,30" Orientation="Vertical">
<Border CornerRadius="4" Padding="5" Background="White">
<DataGrid ItemsSource="{Binding Workpieces}"
CanUserReorderColumns="False" CanUserResizeColumns="False" CanUserSortColumns="False"
AutoGenerateColumns="True" AutoGeneratingColumn="WorkpieceListAutoGeneratingColumn">
<i:Interaction.Triggers>
<i:EventTrigger EventName="AutoGeneratingColumn">
<hgc:EventToCommand Command="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type DataGrid},AncestorLevel=2}, Path=DataContext.AutoGeneratingColumnCommand}" PassEventArgsToCommand="True"/>
</i:EventTrigger>
</i:Interaction.Triggers>
</DataGrid>
</Border>
</StackPanel>
</DataTemplate>
</DataGrid.RowDetailsTemplate>
</DataGrid>
The Event-Handler WorkpieceListAutoGeneratingColumn in the Code-Behind File is called, the Command in my ViewModel is never called.
Andreas

Answer 1

The reason should be that you can't have an event andler and an event to command on the same object/event combination. Remove the AutoGeneratingColumn="WorkpieceListAutoGeneratingColumn" from your DataGrid an the command should be called.
Had the problem once myself :-)
Edit
If you need the eventhandler in the code behind, remove the EventToCommand and call the command in your code behind, e.g.
public void WorkpieceListAutoGeneratingColumn(object sender, DataGridAutoGeneratingColumnEventArgs args) {
var vm = ((YourViewModel) this.DataContext);
if (vm.AutoGeneratingColumnCommand.CanExecute(eventArgs))
vm.AutoGeneratingColumnCommand.Execute(eventArgs);
}
But, I think that the first option is the better one.
Edit 2
OK, had some look around and it seems that <i:Interaction.Triggers/> only works after the object is already rendered and user interaction takes place (hence the name?). Well, this means that there are simply just some events - the ones that are called during the construction of the object - that cannot be handled by the EventToCommand mechanism. In these cases it is OK to use code behind to call your command from there, see my first edit.

Answer 2

You don't have to use evil code behind ;-) You can do this using an attached behaviour...
public class AutoGeneratingColumnEventToCommandBehaviour
{
public static readonly DependencyProperty CommandProperty =
DependencyProperty.RegisterAttached(
"Command",
typeof(ICommand),
typeof(AutoGeneratingColumnEventToCommandBehaviour),
new PropertyMetadata(
null,
CommandPropertyChanged));
public static void SetCommand(DependencyObject o, ICommand value)
{
o.SetValue(CommandProperty, value);
}
public static ICommand GetCommand(DependencyObject o)
{
return o.GetValue(CommandProperty) as ICommand;
}
private static void CommandPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
var dataGrid = d as DataGrid;
if (dataGrid != null)
{
if (e.OldValue != null)
{
dataGrid.AutoGeneratingColumn -= OnAutoGeneratingColumn;
}
if (e.NewValue != null)
{
dataGrid.AutoGeneratingColumn += OnAutoGeneratingColumn;
}
}
}
private static void OnAutoGeneratingColumn(object sender, DataGridAutoGeneratingColumnEventArgs e)
{
var dependencyObject = sender as DependencyObject;
if (dependencyObject != null)
{
var command = dependencyObject.GetValue(CommandProperty) as ICommand;
if (command != null && command.CanExecute(e))
{
command.Execute(e);
}
}
}
}
Then use it in XAML like this...
<DataGrid ItemsSource="{Binding MyGridSource}"
AttachedCommand:AutoGeneratingColumnEventToCommandBehaviour.Command="{Binding CreateColumnsCommand}">
</DataGrid>