Silverlight 4 DataTemplate DataType

Question

Silverlight 4 is out and it seems we've missed the DataTemplate DataType functionality in this release again, which is pretty pivotal for MVVM support IMHO. For my WPF apps, at this point, I'm pretty used to globally adding DataTemplates for my Views to my Application.Resources with DataTypes for my corresponding ViewModels:

ie.

<DataTemplate DataType="{x:Type viewModels:myViewModel}">
<views:myView/>
</DataTemplate>

I like this approach, since all my bound ViewModels automatically display the correct content...especially useful when I have some ItemSource in my view bound to a collection of ViewModels... This, for example, will automatically make sure each tab in a TabControl bound to a Collection<SomeViewModel> displays the view associated with SomeViewModel.

Some things I tried for SL 3 include:

• Creating a "DataTemplatePresenterContentControl" which automatically applies a DataTemplate for the Content when the control has loaded

• Using a TypeConverter, applied dynamically on control load, walking down the visual tree looking for data bound objects

• Using a style, applied dynamically on control load, walking down the visual tree looking for data bound objects

However, none of these approaches really address the situation I mentioned above in an acceptable way, which is really key.

So, since this still isn't possible out of the box in Silverlight 4, I'd appreciate to know if anyone has yet come up with some reasonable alternatives.

Thanks.

Uta

Answer 1

The way I do it in a couple of commercial projects is as follows:

I have a standard IValueConverter

public class ViewTemplateChooser : IValueConverter
{
    /// <summary>
    /// Modifies the source data before passing it to the target for display in the UI.
    /// </summary>
    /// <returns>
    /// The value to be passed to the target dependency property.
    /// </returns>
    /// <param name="value">The source data being passed to the target.</param><param name="targetType">The <see cref="T:System.Type"/> of data expected by the target dependency property.</param><param name="parameter">An optional parameter to be used in the converter logic.</param><param name="culture">The culture of the conversion.</param>
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value is MyViewModel)
        {
            return new MyView { DataContext = value };
        }

        return value;
    }

    /// <summary>
    /// Modifies the target data before passing it to the source object.  This method is called only in <see cref="F:System.Windows.Data.BindingMode.TwoWay"/> bindings.
    /// </summary>
    /// <returns>
    /// The value to be passed to the source object.
    /// </returns>
    /// <param name="value">The target data being passed to the source.</param><param name="targetType">The <see cref="T:System.Type"/> of data expected by the source object.</param><param name="parameter">An optional parameter to be used in the converter logic.</param><param name="culture">The culture of the conversion.</param>
    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

The converter would require a namespace registration

xmlns:Converters="clr-namespace:YourProject.Converters" 

Then you reference the converter in your resources section:

<UserControl.Resources>
    <Converters:ViewTemplateChooser x:Key="TemplateChooser" />
</UserControl.Resources>

and finally I use the converter to convert the ViewModel into a View with the Datacontext of the view set to the ViewModel

<ContentControl Content="{Binding Workspace, Converter={StaticResource TemplateChooser}}" Margin="5,35,5,5" Grid.Column="1" />

The converter can be modified to implement Navigation strategies, I tried to make the example as simple as possible.

I hope this helps, you don't have to go to extremes - or third party libraries - to get what you are looking for.

Shcherbakov

Answer 2

In WPF and Silverlight, I use Prism to do this. I find it to be much more versatile to switch out views based on types. It requires a bit to get it strapped in, but once it is in, the possibilities are endless.

Edit

I do this by binding the RegionName to a property in my ViewModel (Could be GetType().Name if you want). Then, I register the types for the names, and it just works.

In the case of something like a ListBox, I set up the data template to be:

<ContentControl Regions:RegionManager.RegionName="{Binding SomeName}" />

If you don't want SomeName to be on the object you are binding to, consider a ValueConverter that returns the type name:

<ContentControl Regions:RegionManager.RegionName="{Binding SomeName, Converter={StaticResource ObjectToTypeConverter}}" />

Does that help?

Norwood

I'm using Prism's RegionManager too, but could you elaborate a bit more please on the specifics of how you're doing this? – Uta

Yes, thanks. I like the approach. But it still does not address the one issue I mentioned above; binding to an IEnumerable<T> - for example binding a TabControl to a Collection<MyViewModelClass> and expecting each tab to display the MyViewForMyViewModelClass UserControl. Or is there a way to adapt your approach to support that? Thanks. – Uta

Right.. in the template for the TabControl, include the ContentControl with a region name that is type-specific. Then, just register different views to show for the different type-specific region names. – Norwood

Ok, cool. I'm liking this. Then how/where to I call to register the view with the region (ie. RegionManager.RegisterViewWithRegion)? – Uta