WPF: How to show a "Please Wait" tab - wpf

Question

In my TabControl wrapper control, I want to provide the following function:
void AddTab(Func<object> tabContentGenerator)
The function should add a new TabItem with a “Please wait” content, and then call the tabContentGenerator function to get the object to show and replace that TabItem’s Content with the returned object.
I tried to implement the call to tabContentGenerator in a BackgrounWorker. However, the tabContentGenerator function usually creates a UserControl to be used as the content, and this causes an exception when called in a BackgroundWorker. Do you have another idea on how to achieve the required behavior of having “Please wait” tab item that is later replaced with the real content (that needs to be generated in an STA thread)?

Answer 1

As you've probably figured out you need to construct framework objects on the UI thread, not in the background worker. I assume this is the nub of the question.
This looks like some sort of plug-in framework, where tabContentGenerator is injected. If so, I'd use two actions, one that does the long running work and the other that creates the controls. Your extended TabControl would run the first in DoWork and the second in WorkerCompleted.
For example (pseudo-code):
public void AddTab(Action backgroundAction, Func<FrameworkElement> constructUiAction)
{
var tab = ...
var worker = new BackgroundWorker();
worker.DoWork += (sender, e) => { backgroundAction(); };
worker.RunWorkerCompleted += (sender, e) =>
{
var ui = constructUiAction();
if (ui != null) tab.Content = ui;
};
worker.RunWorkerAsync();
}
The other option is to have the action return a FrameworkElementFactory which it then uses to instantiate the GUI on the UI thread via a ControlTemplate. FrameworkElementFactory is not a DispatcherObject and can be created on a non-GUI thread. It's harder to create the UI from factories, but if the client specifies a control template in a resource in XAML they can get the FrameworkFactoryElement from its visual tree (e.g. ((ControlTemplate)FindResource("MyTemplate")).VisualTree).
public void AddTab(Func<FrameworkElementFactory> tabContentGenerator)
{
var tab = ...
var worker = new BackgroundWorker();
FrameworkElementFactory uiFactory = null;
worker.DoWork += (sender, e) => { uiFactory = tabContentGenerator(); }
worker.RunWorkerCompleted += (sender, e) =>
{
if (uiFactory != null)
{
var template = new ControlTemplate();
template.VisualTree = uiFactory;
template.Seal();
tab.Content = template.LoadContent();
};
}
worker.RunWorkerAsync();
}

Answer 2

You might have a look at http://wpftoolkit.codeplex.com/wikipage?title=Extended%20WPF%20Toolkit%20Controls This is showing an indefinite progress bar.

Answer 3

A simple way to do this is to add a Waiting property to your view model, create a UI element for use when the tab is waiting, and put a style on the content, e.g.:
<Grid>
<DockPanel>
<DockPanel.Style>
<Style TargetType="DockPanel">
<Setter Property="Visibility" Value="Visible"/>
<DataTrigger Binding="{Binding Waiting}" Value="True">
<Setter Property="Visibility" Value="Collapsed"/>
</DataTrigger>
</Style>
<! -- normal content goes here -->
</DockPanel>
<DockPanel>
<DockPanel.Style>
<Style TargetType="DockPanel">
<Setter Property="Visibility" Value="Collapsed"/>
<DataTrigger Binding="{Binding Waiting}" Value="True">
<Setter Property="Visibility" Value="Visible"/>
</DataTrigger>
</Style>
<! -- waiting content goes here -->
</DockPanel>
</Grid>
The command that launches the BackgroundWorker should update Waiting to true (and have the object raise PropertyChanged) before calling DoWork. The BackgroundWorker's RunWorkerCompleted event handler sets Warning back to false.
You'll note that I'm not using any AddTab method to create my tab. Under most circumstances, you should never be writing code that directly creates WPF objects. It's much, much more productive to do it declaratively. When I was learning WPF, I found myself saying, a lot, "I need to create this in code because I can't do X in XAML." The right answer to that is to learn how to do X in XAML, because you almost certainly can, whatever X is.