使用嵌套视图时，Prism 7引发异常

如何解决使用嵌套视图时，Prism 7引发异常

几个月前，我Working with nested views using Prism with IsNavigationTarget which can return false发表了类似的问题，我仍然不确定什么是正确的方法。

假设您有一个视图A，在该视图A中您声明了一个区域A，然后将视图B注入了该区域A。类似地，在视图B中您注册了区域B，然后又注入了视图C。进入下图所示的区域B。

在ViewA的ViewModelA中，我有一个方法SetUpSubViews（）可以在其中调用：

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

用于View B的ViewModelB实现INavigationAware。因此，在OnNavigatedTo（）方法中，我调用：

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

用于View C的ViewModelC也实现了INavigationAware。

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

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

这意味着我想在每次浏览该视图时创建一个新视图。

ViewB和ViewC都实现了IRegionMemberLifetime接口，我在其中进行了设置：

#region IRegionMemberLifetime

public bool KeepAlive => false;

#endregion

这意味着我不想重用视图并且希望将其处理掉。

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ViewModelA上调用SetUpSubViews（）方法时，一切都很好。第二次当我调用它时，我看到了异常：

具有给定名称的区域已被注册 ...

我需要的是每次需要时都有一种从头开始重新创建view viewmodel对的方法。似乎在放置视图时，棱镜不会删除在删除的视图中声明的区域。向社区和棱镜开发者提出疑问，如何以正确的方式做到这一点？

当前的解决方案并不令人满意，这是我要做的：第1步-我在INavigationAware部分的ViewModelB和ViewModelC中设置

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

通知棱镜不创建新视图，这也可能意味着如果在视图中发现任何区域，也不要在区域管理器中注册该区域。

第2步-当我需要将视图注入区域时，我手动删除旧视图并创建新视图。所以我的SetUpSubViews（）方法看起来像这样：

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",NavigationCallback);}

类似地，我必须从ViewB的区域regionB中删除ViewC。（这是region.RemoveAll（）是关键行。）

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

可以，但是看起来不正确。

P.S。我也尝试了作用域管理器，但是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，并且如果我通过构造函数解析它，则会得到主要的全局管理器而不是作用域。

谢谢。

解决方法

这是一个相当麻烦的问题。我推荐Brian Lagunas本人提供的视频，他在那里提供解决方案和解释。例如这个。 https://app.pluralsight.com/library/courses/prism-problems-solutions/table-of-contents

如果可以观看。如果没有，我会尝试解释。

我相信的问题是，容器中的IRegionManager是一个单例，并且每当使用它时，它都是同一实例，因此，当您尝试在已注入的区域中注入区域时，它将无法工作，并且您需要单独的RegionManager用于嵌套视图。

这应该解决它。创建两个界面

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);
            }
        }
    }
}

创建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,StringComparison.Ordinal));
    }
}

在您的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));
    }

即将结束。现在，在您的ViewModelB中实现IRegionManagerAware并将其作为常规属性

public IRegionManager RegionManager { get; set; }

然后在您的ViewB上实现ICreateRegionManagerScope并将其作为get属性

public bool CreateRegionManagerScope => true;

现在应该可以了。

再次，我真的推荐Brian在Prism上在Pluralsight上播放的视频。当您开始使用Prism时，他有一些视频对您有很大帮助。

使用嵌套视图时，Prism 7引发异常

如何解决使用嵌套视图时，Prism 7引发异常

解决方法

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