How to get the "last active" Document Window using MVVM

Ok I was looking through the documentation again and the DockSite has a property called PrimaryDocument. If I understand this correctly this will always be the last active DocumentWindow, right? I guess technically this will be null when there are no Document Windows? What about adding a bind-able Boolean property to DocumentWindow called IsPrimary or IsPrimaryDocument (similar to IsOpen and IsActive). Then each Document View Model can bind to this property and they will know if they are the primary document without knowing anything about each other. What do you think?

Hi Eric,

Correct, the DockSite.PrimaryDocument property will tell you which document is effectively the last active document, even if another tool window is active at the time.  Check out the "Working with Documents" topic in the documentation for more info.

As for the additional view-model property, we can write down the idea for that.  Those properties get a bit tricky to sync around, so there would need to be a large benefit to adding it.  You can't just compare the DocumentWindow with DockSite.PrimaryDocument?

I understand that the DockSite knows which document is primary but the view models know nothing about the DockSite. As far as I understand it this is basic MVVM. I realize that I could add some code-behind to track the primary document (using its changed event) and then find the matching Document View Model and set a property on that view model. I could also add a behavior, like your older DockSiteBehavior, that would do the same job. Neither of these approaches violate MVVM and we are willing to use this approach. I just thought adding a bindable IsPrimary property, like the other IsOpen, IsActive, and IsSelected properties, would be a nice addition and make the DockSite even more MVVM friendly. Adding a property like IsPrimary seems very logical to me. It's more "state" information like IsOpen, IsActive, and IsSelected that the bindable view models need to know. In short we can make it work without an IsPrimary property but adding this property would make the dock site even more MVVM friendly. If you guys decide to add an IsPrimry property how long would it take to be in the API?

So I ended up adding the following code-behind.

private void dockSite_PrimaryDocumentChanged(object sender, DockingWindowEventArgs e) {
  foreach (DockingItemViewModelBase dockingItemViewModel in ((DockSite)sender).DocumentItemsSource) {
    dockingItemViewModel.IsPrimary = (e.Window != null && dockingItemViewModel == e.Window.DataContext);
  }
}

I also added an IsPrimary boolean property to my View Model.

If there was a bindable IsPrimary property we could do the following which would require no code-behind.

<Style TargetType="docking:DockingWindow">
  <Setter Property="IsActive" Value="{Binding Path=IsActive, Mode=TwoWay}" />
  <Setter Property="IsOpen" Value="{Binding Path=IsOpen, Mode=TwoWay}" />
  <Setter Property="IsSelected" Value="{Binding Path=IsSelected, Mode=TwoWay}" />
  <Setter Property="IsPrimary" Value="{Binding Path=IsPrimary, Mode=TwoWay}" />
</Style>

 Just something to consider.

Hi Eric,

Thanks, I've written down this info on the TODO list.  I can't give an estimate on timeframe since we have to prioritize it with our other work.  Your workaround solution looks solid and I would recommend doing that for now.