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 Wednesday, 27 February 2008
« High Speed Cell Modem Saves the Day | Main | CSLA .NET 3.5 property code reduction »

I'm psyched! I finally figured out how to get the Csla.Wpf.CslaDataProvider control to support a Delete command that removes child items from a databound list object.

As you might expect, the answer to the problem was staring me in the face the whole time - I just didn't see it. Isn't that the way things often work? ;)

Since day one, CslaDataProvider has support the Save, Undo and AddNew commands (using XAML commanding), so it has been possible to create Save, Cancel and Add buttons on a form purely through XAML - no code-behind required at all.

Now, with this new change it is also possible to implement a Remove button in a DataTemplate with no code-behind. The button can use the standard Delete command to tell the CslaDataProvider managing the BusinessListBase object to remove the child object bound to that row in the DataTemplate. The XAML looks like this:

<DataTemplate x:Key="lbTemplate">
  <Grid>
    <StackPanel Orientation="Horizontal">
      <TextBlock>Id:</TextBlock>
      <TextBox Text="{Binding Path=Id, Converter={StaticResource IdentityConverter}, ValidatesOnDataErrors=True}" Width="100" />
      <TextBlock>Name:</TextBlock>
      <TextBox Text="{Binding Path=Name, Converter={StaticResource IdentityConverter}, ValidatesOnDataErrors=True}" Width="250" />
      <Button
        Command="ApplicationCommands.Delete"
        CommandParameter="{Binding}"
        CommandTarget="{Binding Source={StaticResource RoleList}, Path=CommandManager, BindsDirectlyToSource=True}"
        HorizontalAlignment="Left">Remove</Button>
    </StackPanel>
  </Grid>
</DataTemplate>

The key was realizing that the CommandParameter could be bound to the entire object that is represented by this DataTemplate - the individual row object. This revelation meant that the CslaDataProvider control can in fact remove that item because an equality comparison is possible. So ApplicationCommands.Delete passes the child object reference to the data provider, which removes the item from the databound list object and that's it!

So this means you can now create a typical maintenance screen with no code behind the XAML. None. You have to write zero lines of VB/C#. Too cool!!

And it means that even on a more complex screen you still don't need to write save/cancel/add new/remove code. That's a lot of code savings! Often all you need to write is code dealing with navigation from this screen to some other screen, perhaps some exception handling code (which I put in a base class) and some authorization code to provide visual cues to the user about what they can and can't do.

Is this really the final step? I doubt it. I already have some ideas on how to expose the authorization knowledge encapsulated by the business object such that it can be directly used within the XAML. I don't know if I'll get that working, but if I can reduce/elminate the UI authorization code (that provides visual cues to the user) and get that into XAML then even more forms will become codeless.

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