Run animation based on view model property?

Question

I'm trying to really adhere to the MVVM pattern, and as a result I'm having difficulty figuring out how I can get an animation on a view to fire every time some property on a view model is set (via binding).

In my MainWindow.xaml I have a ContentPresenter and I am binding the content of that presenter to the view model of the current view that's displayed to the user. For example, when the user first runs the application, ContentPresenter will be bound to StartViewModel. If the user then clicks on a button that takes him/her to a different screen to run a study, for example, the ContentPresenter needs to be bound to StudyViewModel so that the StudyView screen can be displayed. Then, if the user wishes to go back to the starting screen, the ContentPresenter at that point will once again be bound to the StartViewModel that was created when the application launched. It looks something like this:

<Window
...
  <Grid
  ...
        <ContentControl
            x:Name="CurrentContentView"
            Width="Auto"
            Grid.Row="2" 
            Content="{Binding CurrentContentViewModel}" />
  </Grid>
</Window>

CurrentContentViewModel is of type "ViewModelBase," which is a base class I've created for all view models that implements INotifyPropertyChanged. I keep an ObservableCollection of ViewModelBase behind the scenes and every time some event happens that should cause the user to go to a new screen, I look for the appropriate view model in that collection and set CurrentContentViewModel to that view model. If it doesn't exist yet in my collection, I add it to the collection first and then set CurrentContentViewModel to that view model.

My question is: how can I cause my views to run an "intro" animation every time the view is displayed? I can't set the storyboard to be run every time a view is simply loaded within my ContentPresenter, because I also want to run the animation again when the user comes back to a view that was already loaded previously in the application's lifetime. On top of that, my view is "dumb" and has no idea that it has once again been displayed in the MainWindow because the ContentPresenter on my MainWindow is bound to a view model rather than a view (so it's almost like the VIEW MODEL would have to fire some event every time it gets bound to the content presenter and the view would have to respond to that event). Is there a way I can declare some sort of primitive type like a Boolean in my xaml markup that can bind to a boolean in my view model?

Thanks in advance for any ideas anyone might have, and sorry if this question has already been asked. I found some similar posts, but nothing quite like what I'm wanting to do.

UPDATE - SOLUTION FOUND

As it turns out (at least with the implementation I currently have), this was a much easier problem to solve than I realized. If you are building your views using the data template pattern (i.e. define a data template for each of your view models in a resource dictionary and then add that resource dictionary to the app.xaml file's list of resources), the view gets recreated every time it is removed from a content control. In my case, each time I bind my Content Control's Content to a different view model than the previous one, the view that was "built" for that view model actually gets disposed. Then, when the view model is once again reassigned to the Content Control, the view gets built all over again. Therefore, I am able to put my screen intro animations in the Loaded event of the view's XAML and my screen outro animations in the UnLoaded event of the view's XAML.

I wanted to share this in case anyone else is facing a similar problem - I was unaware of this type of behavior using data templates to display the view for a view model.

Answer 1

It rather depends on what animation you're after. For example, if you want to slide out the current view and then slide in the next, you could simply have your controlling view model raise two events: CurrentContentViewModelChanging and CurrentContentViewModelChanged. Your view can trigger appropriate animations based on those events.

However, if your animation needs to show the new view at the same time as the old view, things get a little trickier. You either need to expose the old view at the same time as the new, or you need your view to take a snapshot of the old view just before the new view is substituted in. Again, you should be able to use the same events to achieve this.