How to create new instance of a view/viewmodel when navigation occurs with Prism

Question

I am trying to control when a new view is created and when an existing view is shown.

This is a very similar scenario as outlined in the "Navigating to Existing Views" section in the Prism documentation, but I can't get it to work fully: http://msdn.microsoft.com/en-us/library/gg430861(v=pandp.40).aspx

I am finding I can create the view/view model to begin with ok, but I am then unable to create a new instance of it. I.e. I want more than one instance to exist at once.

Here's an example of the view model:

[Export]
[PartCreationPolicy(CreationPolicy.NonShared)]
public class DataEntryPageViewModel : INavigationAware, IRegionMemberLifetime
{
    private Guid id;

    [ImportingConstructor]
    public DataEntryPageViewModel()
    {
        id = Guid.NewGuid();
    }

    public bool IsNavigationTarget(NavigationContext navigationContext)
    {
        // In actual fact there would be more logic here to determine
        // whether this should be shown to the user
        return false;
    }

    public void OnNavigatedFrom(NavigationContext navigationContext)
    {
    }

    public void OnNavigatedTo(NavigationContext navigationContext)
    {
    }

    public bool KeepAlive
    {
        // For the purposes of this example we don't want the view or the viewModel
        // to be disposed of.
        get { return true; }
    }
}

I am navigating to this as follows:

m_RegionManager.RequestNavigate(
               "MainRegion",
               new Uri("/DataEntryPageView", UriKind.Relative));

So the first time I call the above the view is shown.

The next time I call RequestNavigate the IsNavigationTarget is hit and it returns false. What I then want it to do is to create a new instance but that doesn't happen. I know it's not happening because the constructor does not get hit and the UI does not update to show the new instance of the view.

Any ideas how I can make it create a new instance?

Many thanks,

Paul

Edit

I have noticed that the second time I call RequestNavigate (to request another instance of the same view) the callback reports an error "View already exists in region." It therefore seems that I can have multiple instances of different views in a region, but not multiple instances of the same view. My understand of this isn't great though so I could be wrong.

Answer 1

I have now been able to get this to work, it was because I didn't have

[PartCreationPolicy(CreationPolicy.NonShared)]

on the class declaration of the view. I had it on the ViewModel.

So this is now resulting in the behaviour I expected.

Thanks though to Zabavsky and Alan for your suggestions.

Answer 2

Why are you not creating the view when you want a new one to be created? It looks to me like you are using MEF.

1. Use the container to resolve a new instance of your view
2. Add the new instance of the view to the MainRegion
3. Then call Navigate and handle the appropriate logic in IsNavigationTarget

Answer 3

You should use the [Export] attribute in your view with a contract name: [Export("DataEntryPageView")].