使用嵌套视图时 Prism 7 抛出异常

几个月前我发布了类似的问题使用 Prism 和 IsNavigationTarget 处理嵌套视图，可能返回 false，我仍然不确定正确的方法是什么。

假设你有一个视图A，在这个视图A中你声明了一个区域A，然后你在这个区域A中注入了一个视图B。同样，在视图B中你注册了区域B，然后你在这个区域中注入了一个视图C B、如下图所示：

在 ViewS 的 ViewModel 中，我有一个方法 SetUp SubViews() ，我在其中调用：

_regionManager.RequestNavigate("regionA", "ViewB", NavigationCallback);

视图 B 的 ViewModelB 实现 INavigationAware。因此，在 OnNavigateTo() 方法中我调用：

_regionManager.RequestNavigate("regionB", "ViewC", NavigationCallback);

View C 的 ViewModelC 也实现了 INavigationAware。

现在，我在 IsNavigationTarget() 方法中的 ViewModelB 和 ViewModelC 中都有：

public bool IsNavigationTarget(NavigationContext navigationContext)
    {            
        return false;
    }

这意味着我想在每次导航该视图时创建新视图。

ViewB和ViewC都实现了IRegionMemberLifetime接口，我在其中设置：

#region IRegionMemberLifetime

public bool KeepAlive => false;

#endregion

这意味着我不想重用视图并且希望将其丢弃。

视图中的区域声明如下：

<ContentControl prism:RegionManager.RegionName="{x:Static localRegions:LocalRegions.RegionB}" />

现在，当我第一次在 ViewModel 上调用 SetUp SubViews() 方法时，一切都很好。当我第二次调用它时，我看到了异常：

具有给定名称的区域已注册 ...

我需要的是有一种方法可以在每次需要时从头开始重新创建视图视图模型对。似乎当视图被处置时，棱镜不会删除在已删除视图中声明的区域。请问社区和棱镜开发者，如何正确地做到这一点？

当前的解决方案并不令人满意，这就是我所做的： 步骤 1 - 我在 INavigationAware 部分中设置 ViewModelB 和 ViewModelC

    public bool IsNavigationTarget(NavigationContext navigationContext)
    {            
        return true;
    }

这向 prism 发出信号，要求不要创建新视图，并且可能还意味着，如果在视图中找到任何区域，则不要将其注册到区域管理器中。

第 2 步 - 当我需要将视图注入到区域时，我手动删除旧视图并创建新视图。所以我的 SetUpSubViews() 方法如下所示：

protected void SetUpSubViews(){
            //get region by name
            var region = _regionManager.Regions["regionA"];
            // push to remove all views from the region
            region.RemoveAll();
            // navigate to view
           _regionManager.RequestNavigate("regionA", "ViewB", NavigationCallback);}

同样，我必须从 View 上的区域区域中删除 ViewS 有 Region.RemoveAll() 是关键行。）

Step3 - 我没有在 viewB 和 viewC 上实现 IRegionMemberLifetime 接口。

它有效，但看起来不正确。

附：我也尝试了作用域管理器，但我不知道如何将新创建的作用域管理器传播到视图模型，因为它们是自动创建的，如果我通过构造函数解决它，我会得到主全局管理器而不是作用域。

Thanks.

这是一个相当麻烦的问题。我推荐 Brian Lagunas 本人的视频，他在其中提供了解决方案和解释。比如这个。https://app.pluralsight.com/library/courses/prism-problems-solutions/table-of-contents

如果你能看的话。如果没有，我会尽力解释。

我相信的问题是IRegionManager从容器中是一个单例，无论何时使用它，它都是同一个实例，因此当您尝试在已注入的区域中注入一个区域时，它将不起作用，您需要有一个单独的实例RegionManager对于嵌套视图。

这应该可以解决它。 创建两个接口

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

创建一个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));
    }

即将到达终点。 现在在你的ViewModelB实施IRegionManagerAware并将其作为正常财产

public IRegionManager RegionManager { get; set; }

然后在你的ViewB实施ICreateRegionManagerScope并将其作为获取属性

public bool CreateRegionManagerScope => true;

现在应该可以了。

我再次真心推荐 Brian 在 Prism 上发布的 Pluralsight 视频。他有几个视频对您开始使用 Prism 时有很大帮助。