Prism 7 throws and exception when working with nested views

Question

I posted similar question few months ago Working with nested views using Prism with IsNavigationTarget which can return false, I\'m still not sure what is the proper way to do i

Answer 1

This is quite a troublesome problem. I recommend videos from Brian Lagunas himself where he provides a solution and explanation. For example this one. https://app.pluralsight.com/library/courses/prism-problems-solutions/table-of-contents

If you can watch it. If not I will try to explain.

The problem I believe is that IRegionManager from the container is a singleton and whenever you use it it is the same instance, so when you are trying to inject a region in an already injected region it will not work and you need to have a separate RegionManager for nested views.

This should fix it. Create two interfaces

public interface ICreateRegionManagerScope
{
    bool CreateRegionManagerScope { get; }
}
public interface IRegionManagerAware
{
    IRegionManager RegionManager { get; set; }
}

Create a RegionManagerAwareBehaviour

public class RegionManagerAwareBehaviour : RegionBehavior
{
    public const string BehaviorKey = "RegionManagerAwareBehavior";

    protected override void OnAttach()
    {
        Region.Views.CollectionChanged += Views_CollectionChanged;
    }

    void Views_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
    {
        if (e.Action == NotifyCollectionChangedAction.Add)
        {
            foreach (var item in e.NewItems)
            {
                IRegionManager regionManager = Region.RegionManager;

                // If the view was created with a scoped region manager, the behavior uses that region manager instead.
                if (item is FrameworkElement element)
                {
                    if (element.GetValue(RegionManager.RegionManagerProperty) is IRegionManager scopedRegionManager)
                    {
                        regionManager = scopedRegionManager;
                    }
                }

                InvokeOnRegionManagerAwareElement(item, x => x.RegionManager = regionManager);
            }
        }
        else if (e.Action == NotifyCollectionChangedAction.Remove)
        {
            foreach (var item in e.OldItems)
            {
                InvokeOnRegionManagerAwareElement(item, x => x.RegionManager = null);
            }
        }
    }

    private static void InvokeOnRegionManagerAwareElement(object item, Action<IRegionManagerAware> invocation)
    {
        if (item is IRegionManagerAware regionManagerAwareItem)
        {
            invocation(regionManagerAwareItem);
        }

        if (item is FrameworkElement frameworkElement)
        {
            if (frameworkElement.DataContext is IRegionManagerAware regionManagerAwareDataContext)
            {
                // If a view doesn't have a data context (view model) it will inherit the data context from the parent view.
                // The following check is done to avoid setting the RegionManager property in the view model of the parent view by mistake.
                if (frameworkElement.Parent is FrameworkElement frameworkElementParent)
                {
                    if (frameworkElementParent.DataContext is IRegionManagerAware regionManagerAwareDataContextParent)
                    {
                        if (regionManagerAwareDataContext == regionManagerAwareDataContextParent)
                        {
                            // If all of the previous conditions are true, it means that this view doesn't have a view model
                            // and is using the view model of its visual parent.
                            return;
                        }
                    }
                }

                invocation(regionManagerAwareDataContext);
            }
        }
    }
}

Create ScopedRegionNavigationContentLoader

public class ScopedRegionNavigationContentLoader : IRegionNavigationContentLoader
{
    private readonly IServiceLocator serviceLocator;

    /// <summary>
    /// Initializes a new instance of the <see cref="RegionNavigationContentLoader"/> class with a service locator.
    /// </summary>
    /// <param name="serviceLocator">The service locator.</param>
    public ScopedRegionNavigationContentLoader(IServiceLocator serviceLocator)
    {
        this.serviceLocator = serviceLocator;
    }

    /// <summary>
    /// Gets the view to which the navigation request represented by <paramref name="navigationContext"/> applies.
    /// </summary>
    /// <param name="region">The region.</param>
    /// <param name="navigationContext">The context representing the navigation request.</param>
    /// <returns>
    /// The view to be the target of the navigation request.
    /// </returns>
    /// <remarks>
    /// If none of the views in the region can be the target of the navigation request, a new view
    /// is created and added to the region.
    /// </remarks>
    /// <exception cref="ArgumentException">when a new view cannot be created for the navigation request.</exception>
    public object LoadContent(IRegion region, NavigationContext navigationContext)
    {
        if (region == null) throw new ArgumentNullException("region");
        if (navigationContext == null) throw new ArgumentNullException("navigationContext");

        string candidateTargetContract = this.GetContractFromNavigationContext(navigationContext);

        var candidates = this.GetCandidatesFromRegion(region, candidateTargetContract);

        var acceptingCandidates =
            candidates.Where(
                v =>
                {
                    var navigationAware = v as INavigationAware;
                    if (navigationAware != null && !navigationAware.IsNavigationTarget(navigationContext))
                    {
                        return false;
                    }

                    var frameworkElement = v as FrameworkElement;
                    if (frameworkElement == null)
                    {
                        return true;
                    }

                    navigationAware = frameworkElement.DataContext as INavigationAware;
                    return navigationAware == null || navigationAware.IsNavigationTarget(navigationContext);
                });


        var view = acceptingCandidates.FirstOrDefault();

        if (view != null)
        {
            return view;
        }

        view = this.CreateNewRegionItem(candidateTargetContract);

        region.Add(view, null, CreateRegionManagerScope(view));

        return view;
    }

    private bool CreateRegionManagerScope(object view)
    {
        bool createRegionManagerScope = false;

        if (view is ICreateRegionManagerScope viewHasScopedRegions)
            createRegionManagerScope = viewHasScopedRegions.CreateRegionManagerScope;

        return createRegionManagerScope;
    }

    /// <summary>
    /// Provides a new item for the region based on the supplied candidate target contract name.
    /// </summary>
    /// <param name="candidateTargetContract">The target contract to build.</param>
    /// <returns>An instance of an item to put into the <see cref="IRegion"/>.</returns>
    protected virtual object CreateNewRegionItem(string candidateTargetContract)
    {
        object newRegionItem;
        try
        {
            newRegionItem = this.serviceLocator.GetInstance<object>(candidateTargetContract);
        }
        catch (ActivationException e)
        {
            throw new InvalidOperationException(
                string.Format(CultureInfo.CurrentCulture, "Cannot create navigation target", candidateTargetContract),
                e);
        }
        return newRegionItem;
    }

    /// <summary>
    /// Returns the candidate TargetContract based on the <see cref="NavigationContext"/>.
    /// </summary>
    /// <param name="navigationContext">The navigation contract.</param>
    /// <returns>The candidate contract to seek within the <see cref="IRegion"/> and to use, if not found, when resolving from the container.</returns>
    protected virtual string GetContractFromNavigationContext(NavigationContext navigationContext)
    {
        if (navigationContext == null) throw new ArgumentNullException(nameof(navigationContext));

        var candidateTargetContract = UriParsingHelper.GetAbsolutePath(navigationContext.Uri);
        candidateTargetContract = candidateTargetContract.TrimStart('/');
        return candidateTargetContract;
    }

    /// <summary>
    /// Returns the set of candidates that may satisfiy this navigation request.
    /// </summary>
    /// <param name="region">The region containing items that may satisfy the navigation request.</param>
    /// <param name="candidateNavigationContract">The candidate navigation target as determined by <see cref="GetContractFromNavigationContext"/></param>
    /// <returns>An enumerable of candidate objects from the <see cref="IRegion"/></returns>
    protected virtual IEnumerable<object> GetCandidatesFromRegion(IRegion region, string candidateNavigationContract)
    {
        if (region == null) throw new ArgumentNullException(nameof(region));
        return region.Views.Where(v =>
            string.Equals(v.GetType().Name, candidateNavigationContract, StringComparison.Ordinal) ||
            string.Equals(v.GetType().FullName, candidateNavigationContract, StringComparison.Ordinal));
    }
}

In your App.xaml

protected override void RegisterTypes(IContainerRegistry containerRegistry)
{
 containerRegistry.RegisterSingleton<IRegionNavigationContentLoader,ScopedRegionNavigationContentLoader>();
    
}

protected override void ConfigureDefaultRegionBehaviors(IRegionBehaviorFactory regionBehaviors)
    {
       base.ConfigureDefaultRegionBehaviors(regionBehaviors);

       regionBehaviors.AddIfMissing(RegionManagerAwareBehaviour.BehaviorKey, typeof(RegionManagerAwareBehaviour));
    }

Coming to the finish. Now in your ViewModelB implement IRegionManagerAware and have it as a normal property

public IRegionManager RegionManager { get; set; }

Then at your ViewB implement ICreateRegionManagerScope and have it as a get property

public bool CreateRegionManagerScope => true;

Now it should work.

Again I truly recommend the videos at Pluralsight from Brian on Prism. He has a couple of videos that help a lot when you are starting with a Prism.