mvvm - 我怎样才能避免这个无限循环？

Question

感觉必须有一些半简单的解决方案，但我就是想不通。

编辑:前面的示例更清楚地显示了无限循环，但这提供了更多上下文。查看预编辑以快速了解问题。

以下 2 个类表示模型 View View 模型 (MVVM) 模式的 View 模型。

/// <summary>
/// A UI-friendly wrapper for a Recipe
/// </summary>
public class RecipeViewModel : ViewModelBase
{
    /// <summary>
    /// Gets the wrapped Recipe
    /// </summary>
    public Recipe RecipeModel { get; private set; }

    private ObservableCollection<CategoryViewModel> categories = new ObservableCollection<CategoryViewModel>();

    /// <summary>
    /// Creates a new UI-friendly wrapper for a Recipe
    /// </summary>
    /// <param name="recipe">The Recipe to be wrapped</param>
    public RecipeViewModel(Recipe recipe)
    {
        this.RecipeModel = recipe;
        ((INotifyCollectionChanged)RecipeModel.Categories).CollectionChanged += BaseRecipeCategoriesCollectionChanged;

        foreach (var cat in RecipeModel.Categories)
        {
            var catVM = new CategoryViewModel(cat); //Causes infinite loop
            categories.AddIfNewAndNotNull(catVM);
        }
    }

    void BaseRecipeCategoriesCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        switch (e.Action)
        {
            case NotifyCollectionChangedAction.Add:
                categories.Add(new CategoryViewModel(e.NewItems[0] as Category));
                break;
            case NotifyCollectionChangedAction.Remove:
                categories.Remove(new CategoryViewModel(e.OldItems[0] as Category));
                break;
            default:
                throw new NotImplementedException();
        }
    }

    //Some Properties and other non-related things

    public ReadOnlyObservableCollection<CategoryViewModel> Categories 
    {
        get { return new ReadOnlyObservableCollection<CategoryViewModel>(categories); }
    }

    public void AddCategory(CategoryViewModel category)
    {
        RecipeModel.AddCategory(category.CategoryModel);
    }

    public void RemoveCategory(CategoryViewModel category)
    {
        RecipeModel.RemoveCategory(category.CategoryModel);
    }

    public override bool Equals(object obj)
    {
        var comparedRecipe = obj as RecipeViewModel;
        if (comparedRecipe == null)
        { return false; }
        return RecipeModel == comparedRecipe.RecipeModel;
    }

    public override int GetHashCode()
    {
        return RecipeModel.GetHashCode();
    }
}

.

/// <summary>
/// A UI-friendly wrapper for a Category
/// </summary>
public class CategoryViewModel : ViewModelBase
{
    /// <summary>
    /// Gets the wrapped Category
    /// </summary>
    public Category CategoryModel { get; private set; }

    private CategoryViewModel parent;
    private ObservableCollection<RecipeViewModel> recipes = new ObservableCollection<RecipeViewModel>();

    /// <summary>
    /// Creates a new UI-friendly wrapper for a Category
    /// </summary>
    /// <param name="category"></param>
    public CategoryViewModel(Category category)
    {
        this.CategoryModel = category;
        (category.DirectRecipes as INotifyCollectionChanged).CollectionChanged += baseCategoryDirectRecipesCollectionChanged;

        foreach (var item in category.DirectRecipes)
        {
            var recipeVM = new RecipeViewModel(item); //Causes infinite loop
            recipes.AddIfNewAndNotNull(recipeVM);
        }
    }

    /// <summary>
    /// Adds a recipe to this category
    /// </summary>
    /// <param name="recipe"></param>
    public void AddRecipe(RecipeViewModel recipe)
    {
        CategoryModel.AddRecipe(recipe.RecipeModel);
    }

    /// <summary>
    /// Removes a recipe from this category
    /// </summary>
    /// <param name="recipe"></param>
    public void RemoveRecipe(RecipeViewModel recipe)
    {
        CategoryModel.RemoveRecipe(recipe.RecipeModel);
    }

    /// <summary>
    /// A read-only collection of this category's recipes
    /// </summary>
    public ReadOnlyObservableCollection<RecipeViewModel> Recipes
    {
        get { return new ReadOnlyObservableCollection<RecipeViewModel>(recipes); }
    }


    private void baseCategoryDirectRecipesCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        switch (e.Action)
        {
            case NotifyCollectionChangedAction.Add:
                var recipeVM = new RecipeViewModel((Recipe)e.NewItems[0], this);
                recipes.AddIfNewAndNotNull(recipeVM);
                break;
            case NotifyCollectionChangedAction.Remove:
                recipes.Remove(new RecipeViewModel((Recipe)e.OldItems[0]));
                break;
            default:
                throw new NotImplementedException();
        }
    }

    /// <summary>
    /// Compares whether this object wraps the same Category as the parameter
    /// </summary>
    /// <param name="obj">The object to compare equality with</param>
    /// <returns>True if they wrap the same Category</returns>
    public override bool Equals(object obj)
    {
        var comparedCat = obj as CategoryViewModel;
        if(comparedCat == null)
        {return false;}
        return CategoryModel == comparedCat.CategoryModel;
    }

    /// <summary>
    /// Gets the hashcode of the wrapped Categry
    /// </summary>
    /// <returns>The hashcode</returns>
    public override int GetHashCode()
    {
        return CategoryModel.GetHashCode();
    }
}

除非有要求，否则我不会费心展示模型(配方和类别)，但它们基本上负责业务逻辑(例如，将配方添加到类别也会添加链接的另一端，即如果类别包含一个食谱，然后该食谱也包含在该类别中)并且基本上决定了事情的进展。 ViewModels 为 WPF 数据绑定(bind)提供了一个很好的接口(interface)。这就是包装类的原因

由于无限循环在构造函数中并且它正在尝试创建新对象，我不能只设置一个 bool 标志来防止这种情况发生，因为两个对象都没有完成构造。

我在想的是(作为单例或传递给构造函数或两者)一个 Dictionary<Recipe, RecipeViewModel>和 Dictionary<Category, CategoryViewModel>这将延迟加载 View 模型，但如果已经存在则不会创建新 View 模型，但我还没有抽出时间尝试看看它是否会起作用，因为它已经很晚了，我有点厌倦了处理这个过去 6 个小时左右。

不能保证这里的代码会编译，因为我去掉了一堆与手头问题无关的东西。

Answer 1

回到您最初的问题(和代码)。如果您想要的是自动同步的多对多关系，请继续阅读。寻找处理这些情况的复杂代码的最佳位置是任何 ORM 框架的源代码，这对于这个工具领域来说是非常常见的问题。我会查看 nHibernate (https://nhibernate.svn.sourceforge.net/svnroot/nhibernate/trunk/nhibernate/) 的源代码，了解它如何实现处理 1-N 和 M-N 关系的集合。

您可以尝试的简单方法是创建您自己的小集合类来处理它。下面我删除了您原来的包装类并添加了一个 BiList 集合，该集合使用对象(集合的所有者)和属性另一端的名称进行初始化以保持同步(仅适用于 M-N，但 1- N 很容易添加)。当然，你会想要润色代码:

using System.Collections.Generic;

public interface IBiList
{
    // Need this interface only to have a 'generic' way to set the other side
    void Add(object value, bool addOtherSide);
}

public class BiList<T> : List<T>, IBiList
{
    private object owner;
    private string otherSideFieldName;

    public BiList(object owner, string otherSideFieldName) {
        this.owner = owner;
        this.otherSideFieldName = otherSideFieldName;
    }

    public new void Add(T value) {
        // add and set the other side as well
        this.Add(value, true);
    }

    void IBiList.Add(object value, bool addOtherSide) {
        this.Add((T)value, addOtherSide);
    }

    public void Add(T value, bool addOtherSide) {
        // note: may check if already in the list/collection
        if (this.Contains(value))
            return;
        // actuall add the object to the list/collection
        base.Add(value);
        // set the other side
        if (addOtherSide && value != null) {
            System.Reflection.FieldInfo x = value.GetType().GetField(this.otherSideFieldName);
            IBiList otherSide = (IBiList) x.GetValue(value);
            // do not set the other side
            otherSide.Add(this.owner, false);
        }
    }
}

class Foo
{
    public BiList<Bar> MyBars;
    public Foo() {
        MyBars = new BiList<Bar>(this, "MyFoos");
    }
}

class Bar
{
    public BiList<Foo> MyFoos;
    public Bar() {
        MyFoos = new BiList<Foo>(this, "MyBars");
    }
}



public class App
{
    public static void Main()
    {
        System.Console.WriteLine("setting...");

        Foo testFoo = new Foo();
        Bar testBar = new Bar();
        Bar testBar2 = new Bar();
        testFoo.MyBars.Add(testBar);
        testFoo.MyBars.Add(testBar2);
        //testBar.MyFoos.Add(testFoo); // do not set this side, we expect it to be set automatically, but doing so will do no harm
        System.Console.WriteLine("getting foos from Bar...");
        foreach (object x in testBar.MyFoos)
        {
            System.Console.WriteLine("  foo:" + x);
        }
        System.Console.WriteLine("getting baars from Foo...");
        foreach (object x in testFoo.MyBars)
        {
            System.Console.WriteLine("  bar:" + x);
        }
    }
}