WPF Simple "Data Storage" Control

Question

Is there a "Control" in WPF, which acts as a do-nothing, show-nothing storage repository for a single piece of bound data? Basically, an invisible control which exposes only a "Tag" DependencyProperty.

I'd like to use such a control as a way to consolidate some of my 'frequently used' data bindings into a single location. If I later change the name of the ViewModel property, I only have to update the binding in one place; my 'tag' control.

Ex:

<!-- Does this type of control exist? -->
<StorageControl x:Name="someData" Tag="{Binding MyProperty}" />
<StorageControl x:Name="moreData" Tag="{Binding MyOtherProperty}" />

<!-- further down the XAML file... -->
<TextBlock Text="{Binding Tag, ElementName=someData}">
    <Image.Style> 
        <!-- Style makes reference to "someData" and "moreData," -->
        <!-- with data triggers effecting the appearance. -->
    </Image.Style>
</TextBlock>
<Image>
    <Image.Style> 
        <!-- Style makes reference to "someData" and "moreData," -->
        <!-- with data triggers effecting the appearance. -->
    </Image.Style>
</Image>

Such a control would also allow me to 'flatten' an other-wise complicated and deeply-nested ViewModel structure, within a single View. I could also use it as a placeholder when stubbing out a user interface mockup, and I have not yet generated the ViewModel.

Another ideal usage scenario would be as an alternative to creating a single-use "IValueConverter" class. As an example: rather than trying to fliter a data-bound value through an IValueConverter, in order to get a resulting "Opacity," I could just 'style' the StorageControl to present the correct value using DataTriggers:

<StorageControl x:Name="opacityHost">
    <StorageControl.Style>
        <Style TargetType="StorageControl">
            <Setter Property="Tag" Value="1.0" />
            <Style.Triggers>
                <DataTrigger Binding="{Binding MyProperty}">
                    <!-- This opacity trigger can now be used -->
                    <!-- by many other controls, without overriding their style -->
                    <Setter Property="Tag" Value="0.5" />
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </StorageControl.Style>
</StorageControl>

I realize I could craft my own, but this seems like a generic-enough control that it might already exist.

Would I just use FrameworkElement in this scenario?

Answer 1

As far as I remember, such control does not exist, but you can easily write it. It literally would consist of a:

class MyDataHolder : Control
{
    // now add here one DependencyProperty that DONT have "AffectsXYZ" flags
    // or many DepProps if you want to 'gather' more things in one place
}

Actually, considering that you always can set Visible=Collapsed and Bindings still work, you might also use simple ContentPresenter/ContentControl, collapsed, and use their Content property has the "Tag". I used that trick a few times when I was in a real hurry.

However, mind that this is not a way to go. It will make some things easier, but evetually you will be going the naiive WinForms way taught on some courses that everyone strived to avoid due to many problems.

If you inspect carefully the 'Control+DP' way, you will learn few tricks how to make better "lookless controls". Also, if you think about it more, you will eventually strip the "DP" and "Control" part and you will "discover" the ViewModel, which will enable you to test your UI behaviours.

Lookless controls+Styles allow you to write Biindings once and reuse the style.

"Composite Styles" (search on the web! keywords: composite, merge, multi, etc) allow you to apply multiple styles to the same control, with some limitations of course.

Finally, if would like to add to all that the AttachedDependencyProperty, search about idea of "Behaviors", that will allow you to inject arbitrary code by XAML. This will allow you to write i.e. relatively simple scanners/observers that will walk though visual tree and apply any custom styles, bindings, etc automatically.

No free luch though - some of these end up in more complex code. Debugging clashes between 2 or 5 behaviours applied to the same control can be, well, haunting.

EDIT:

notes/links to "Composite Styles" I've managed to find:

• Style.BasedOn -- mostbasic approach (i.e. http://leecampbell.blogspot.com/2008/08/wpf-styles-basedon-keyless-resource.html)
• XAML Combine Styles
• http://www.zagstudio.com/blog/384#.U4yvXXLV8Uo
• http://swdeveloper.wordpress.com/2009/01/03/wpf-xaml-multiple-style-inheritance-and-markup-extensions/