It feels like there must be some semi-simple solution to this, but I just can't figure it out.
Edit: The previous example showed the infinite loop more clearly, but this gives a bit more context. Check out the pre-edit for a quick overview of the problem.
The following 2 classes represent the View-Models of the Model View View-Model (MVVM) pattern.
/// <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();
}
}
I won't bother showing the Models (Recipe and Category) unless requested, but they basically take care of the business logic (for instance adding a recipe to a category will also add the other end of the link, i.e. if a category contains a recipe, then the recipe is also contained in that category) and basically dictate how things go. The ViewModels provide a nice interface for WPF databinding. That's the reason for the wrapper classes
Since the infinite loop is in the constructor and it's trying to create new objects, I can't just set a boolean flag to prevent this because neither object ever gets finished being constructed.
What I'm thinking is having (either as a singleton or passed in to the constructor or both) a Dictionary<Recipe, RecipeViewModel> and Dictionary<Category, CategoryViewModel> that will lazy-load the view models, but not create a new one if one already exists, but I haven't gotten around to trying to see if it'll work since it's getting late and I'm kinda tired of dealing with this for the past 6 hours or so.
No guarantee the code here will compile since I took a bunch of stuff out that was unrelated to the problem at hand.