c# - MVVM Sync Collections

Question

Is there a standardized way to sync a collection of Model objects with a collection of matching ModelView objects in C# and WPF? I'm looking for some kind of class that would keep the following two collections synced up assuming I only have a few apples and I can keep them all in memory.

Another way to say it, I want to make sure if I add an Apple to the Apples collection I would like to have an AppleModelView added to the AppleModelViews collection. I could write my own by listening to each collections' CollectionChanged event. This seems like a common scenario that someone smarter than me has defined "the right way" to do it.

public class BasketModel
{
    public ObservableCollection<Apple> Apples { get; }
}

public class BasketModelView
{
    public ObservableCollection<AppleModelView> AppleModelViews { get; }
}

Answer 1

I use lazily constructed, auto-updating collections:

public class BasketModelView
{
    private readonly Lazy<ObservableCollection<AppleModelView>> _appleViews;

    public BasketModelView(BasketModel basket)
    {
        Func<AppleModel, AppleModelView> viewModelCreator = model => new AppleModelView(model);
        Func<ObservableCollection<AppleModelView>> collectionCreator =
            () => new ObservableViewModelCollection<AppleModelView, AppleModel>(basket.Apples, viewModelCreator);

        _appleViews = new Lazy<ObservableCollection<AppleModelView>>(collectionCreator);
    }

    public ObservableCollection<AppleModelView> Apples
    {
        get
        {
            return _appleViews.Value;
        }
    }
}

Using the following ObservableViewModelCollection<TViewModel, TModel>:

namespace Client.UI
{
    using System;
    using System.Collections.Generic;
    using System.Collections.ObjectModel;
    using System.Collections.Specialized;
    using System.Diagnostics.Contracts;
    using System.Linq;

    public class ObservableViewModelCollection<TViewModel, TModel> : ObservableCollection<TViewModel>
    {
        private readonly ObservableCollection<TModel> _source;
        private readonly Func<TModel, TViewModel> _viewModelFactory;

        public ObservableViewModelCollection(ObservableCollection<TModel> source, Func<TModel, TViewModel> viewModelFactory)
            : base(source.Select(model => viewModelFactory(model)))
        {
            Contract.Requires(source != null);
            Contract.Requires(viewModelFactory != null);

            this._source = source;
            this._viewModelFactory = viewModelFactory;
            this._source.CollectionChanged += OnSourceCollectionChanged;
        }

        protected virtual TViewModel CreateViewModel(TModel model)
        {
            return _viewModelFactory(model);
        }

        private void OnSourceCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
        {
            switch (e.Action)
            {
            case NotifyCollectionChangedAction.Add:
                for (int i = 0; i < e.NewItems.Count; i++)
                {
                    this.Insert(e.NewStartingIndex + i, CreateViewModel((TModel)e.NewItems[i]));
                }
                break;

            case NotifyCollectionChangedAction.Move:
                if (e.OldItems.Count == 1)
                {
                    this.Move(e.OldStartingIndex, e.NewStartingIndex);
                }
                else
                {
                    List<TViewModel> items = this.Skip(e.OldStartingIndex).Take(e.OldItems.Count).ToList();
                    for (int i = 0; i < e.OldItems.Count; i++)
                        this.RemoveAt(e.OldStartingIndex);

                    for (int i = 0; i < items.Count; i++)
                        this.Insert(e.NewStartingIndex + i, items[i]);
                }
                break;

            case NotifyCollectionChangedAction.Remove:
                for (int i = 0; i < e.OldItems.Count; i++)
                    this.RemoveAt(e.OldStartingIndex);
                break;

            case NotifyCollectionChangedAction.Replace:
                // remove
                for (int i = 0; i < e.OldItems.Count; i++)
                    this.RemoveAt(e.OldStartingIndex);

                // add
                goto case NotifyCollectionChangedAction.Add;

            case NotifyCollectionChangedAction.Reset:
                Clear();
                for (int i = 0; i < e.NewItems.Count; i++)
                    this.Add(CreateViewModel((TModel)e.NewItems[i]));
                break;

            default:
                break;
            }
        }
    }
}