[BUG] WPF - ViewModelViewHost creates the old view again when switching to a new view model

[TolikPylypchuk]

Describe the bug
When a view model is changed for a ViewModelViewHost, it creates the old view again and only after that creates a new view, which can cause brief flickering of views. This happens in a WPF app on .NET Core 3.0 (not sure about other frameworks). ReactiveUI version is 10.5.31.

Steps To Reproduce
I've created a simple repo to reproduce the issue.

The view flickering is not actually visible there, but if you debug the app in Visual Studio, you'll see the log output. When switching the view from View1 to View2, the following log can be seen:

MainViewModel: Switching the view.
View1: Creating the view 1
View2: Creating the view 2

Expected behavior
The old view is not created again when switching to a new view.

Environment

• OS: Windows 10 Home 1909

Additional context

The source of error is probably in the constructor of the ViewModelViewHost class:

var contractChanged = _updateViewModel.Select(_ => ViewContractObservable).Switch();
var viewModelChanged = _updateViewModel.Select(_ => ViewModel);

var vmAndContract = contractChanged.CombineLatest(viewModelChanged, (contract, vm) => new { ViewModel = vm, Contract = contract });

vmAndContract.Subscribe(x => ResolveViewForViewModel(x.ViewModel, x.Contract));

I've debugged this code, and x => ResolveViewForViewModel(x.ViewModel, x.Contract) is hit twice when switching the view model - first time with the old view model (hence creating the old view) and the second time with the new view model. I think this is becuase when a view model is changed, the contractChanged is fired, and then the viewModelChanged is fired, which is why vmAndContract is fired twice.

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[Brimce]

Hi,
This bug can also be observed on the RoutedViewHost.
I have found it comes from the TransitioningContentControl which inherits ContentControl.

Navigation with RoutedViewHost or change of ViewModelViewHost.ViewModel has two consequences :

1. Change of Content property inherited from the ContentControl. (Dispose the old view)
2. Start of transition animation. (Creates the old view again)

To fix this i've first tried to inherits from Control and create my own Content property.
But i faced multiple viewModel activation issue with the slide left transition.

I've then created a TransitioningContentControl that add a new ContentPresenter to the container children on each Content change. And remove the old ContentPresenter when transition is ended. The storyboard to animate the transition is created on the fly.

You can find my code here

WpfTestApp allows to compare the different solutions.

1. Original TransitioningContentControl (navigation between OrginalView and OrginalViewBis)
2. First attempt TransitioningContentControl1 (navigation between FirstView and FirstViewBis)
3. Final TransitioningContentControl2 (navigation between SecondView and SecondViewBis)

You can manage console log in App.cs

I will be happy to have feedback on my code

[github-actions]

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