Lifetime for the view / viewmodel when using data templates - wpf

Question

I am defining a strategy where a main view will use data templates to switch between the views. Currently it can switch between 3 Views:
ApplicationView: it is actually the view that consists of lots of
different views, mostly layered out using tabs / docking. this is a
view that deals with application data.
LogInView: it is used for logging the user in.
DialogView: it is used to display dialog views. This view will also use data templates to select a proper view that is required.
The idea is that when a dialog view needs to be displayed, it is set as current view on the main view. After the selection is done, this information is passed to ApplicationView, or a view that is part of ApplicationView. While DialogView is shown, ApplicationView, must not be released from memory, since it ApplicationViewModel will still be manipulating with data (it needs to constantly work in the background).
I am thinking of achieving this using DataTemplates, and binding ContentControl's Content to CurrentView:
// in MainView
DataTemplate DataType="{x:Type vm:ApplicationViewModel}">
<vw:ApplicationView />
</DataTemplate>
.....
// in MainViewModel
public ViewModelBase CurrentView { get; set; }
Basically I am trying to avoid using modal windows for dialogs.
1) Is this strategy OK, or there are some problems that I am not aware with it?
2) When I switch to DialogView (I am actually switching viewmodels), what happens with the ApplicationView/ApplicationViewModel? Do I need to store ApplicationViewModel's reference somewhere, so it doesn't get garbage collected? I haven't tested this, but probably when I set CurrentView a new instance of ViewModel/View will be created.
3) Connected to second question, when using DataTemplates, what happens to View/ ViewModel that was previously used, and is now replaced with different view/viewmodel?

Answer 1

I don't see anything wrong with the way you're switching views, although typically you don't want to get rid of the application when you're displaying a dialog.
What I've done in the past is to put both the CurrentView, and the DialogView in a Grid so they are positioned on top of each other, then have the ApplicationViewModel contain an IDialogViewModel and IsDialogVisible properties, and when you want to display the dialog simply populate those two fields. (see below for an example)
You will have to store the ApplicationViewModel somewhere if you want to go back to it and avoid creating a new ApplicationViewModel
WPF disposes of UI objects that are no longer visible, so switching the CurrentView from Login to Application will get rid of the LoginView and create an ApplicationView
The ContentControl's Content is getting set to your ViewModel, so the ViewModel is actually being put in the applications VisualTree. Whenever WPF encounters an object in its VisualTree that it doesn't know how to draw, it will draw it using a TextBlock containing the .ToString() of the the object. By defining a DataTemplate, you are telling WPF how to draw the object instead of using its default .ToString() method. Once the object leaves the VisualTree, any visual objects that were created to render the object will get destroyed.
Although I would keep using what you currently have for switching Views, I would not use that method for the Login, Application, and Dialog views.
Typically the LoginView should only be displayed once when logging in, although it might get displayed again in a Dialog if you allow users to switch logins once logging in. Because of this, I typically show the LoginView in the startup code, then display the ApplicationView once login is successful.
protected override void OnStartup(StartupEventArgs e)
{
base.OnStartup(e);
var login = new LoginDialog();
var loginVm = new LoginViewModel();
login.DataContext = loginVm;
login.ShowDialog();
if (!login.DialogResult.GetValueOrDefault())
{
Environment.Exit(0);
}
// Providing we have a successful login, startup application
var app = new ApplicationView();
var context = new ApplicationViewModel(loginVm.CurrentUser);
app.DataContext = context;
app.Show();
}
Like I said earlier, I wouldn't want to hide the Application when I display a Dialog, so I would make the Dialog part of the Application
Here's an example of the DataTemplate I would use for my ApplicationViewModel, using my own custom Popup from my blog for the Dialog
<Grid x:Name="ApplicationView">
<ContentControl Content="{Binding CurrentView}" />
<local:PopupPanel x:Name="DialogPopup"
Content="{Binding DialogContent}"
local:PopupPanel.IsPopupVisible="{Binding IsDialogVisible}"
local:PopupPanel.PopupParent="{Binding ElementName=ApplicationView}" />
</Grid>

Answer 2

Personally, I would find it easier to use ZOrdering within a standard grid, and put everything in the same view - using the ViewModel to manage visibility.
E.g
<Grid>
<Grid Visibility="{Binding IsView1Visible,
Converter={StaticResource BoolToVisibilityConverter}}">
<!-- view 1 contents -->
</Grid>
<Grid Visibility="{Binding IsView2Visible,
Converter={StaticResource BoolToVisibilityConverter}}">
<!-- view 2 contents -->
</Grid>
<Grid Visibility="{Binding IsView3Visible,
Converter={StaticResource BoolToVisibilityConverter}}">
<!-- view 3 contents -->
</Grid>
<Grid Visibility="{Binding IsDialogVisible,
Converter={StaticResource BoolToVisibilityConverter}}">
<!-- dialog contents contents -->
</Grid>
</Grid>