IObservableCollection<ViewModel<T>> wrapping ObservableCollection<T>

Question

In C#/WPF, what is the best way to provide a class which implements IObservableCollection<ViewModel<T>> and wraps around ObservableCollection<T> from the model?

The model can add/remove T elements to the underlying collection, or the view could add/remove ViewModel elements from the view-model layer.

Is there anything fishy about this from the standpoint of the MVVM pattern? Are there libraries containing this, or a similar class?

Answer 1

I wrote a class below that you can use for single threaded purposes. I've only implemented Add and Remove, so you'd have to implement any other actions that you may use:...

Usage:

var viewModelWrapper = new ObservableCollectionViewModel<ItemViewModel, Model>(Model.List, item => new ItemViewModel(item));

Generic class - observable collection view model wrapper:

public class ObservableCollectionViewModel<TItemViewModel, TModel> :
    ObservableCollection<TItemViewModel>, IDisposable
    where TItemViewModel : BaseViewModel<TModel>
    where TModel : new()
{
    private readonly ObservableCollection<TModel> models;
    private readonly Func<TModel, TItemViewModel> viewModelConstructor;

    public ObservableCollectionViewModel(ObservableCollection<TModel> models,
        Func<TModel, TItemViewModel> viewModelConstructor)
    {
        this.models = models;
        this.viewModelConstructor = viewModelConstructor;
        CreateViewModelCollection();
        models.CollectionChanged += models_CollectionChanged;
        CollectionChanged += viewModels_CollectionChanged;
    }

    private void viewModels_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        models.CollectionChanged -= models_CollectionChanged;
        try
        {
            switch (e.Action)
            {
                case NotifyCollectionChangedAction.Add:
                    {
                        foreach (var newItem in e.NewItems)
                        {
                            models.Add(((TItemViewModel)newItem).Model);
                        }
                        break;
                    }
                case NotifyCollectionChangedAction.Remove:
                    {
                        foreach (var oldItem in e.OldItems)
                        {
                            models.Remove(((TItemViewModel)oldItem).Model);
                        }
                        break;
                    }
                // TODO: Add missing actions
                default: throw new NotImplementedException();
            }
        }
        finally
        {
            models.CollectionChanged += models_CollectionChanged;
        }
    }

    private void models_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        CollectionChanged -= viewModels_CollectionChanged;
        try
        {
            switch (e.Action)
            {
                case NotifyCollectionChangedAction.Add:
                    {
                        foreach (var newItem in e.NewItems)
                        {
                            Add(viewModelConstructor((TModel)newItem));
                        }
                        break;
                    }
                case NotifyCollectionChangedAction.Remove:
                    {
                        var viewModels = this.Where(viewModel => e.OldItems.Contains(viewModel.Model)).ToList();
                        foreach (var viewModel in viewModels)
                        {
                            Remove(viewModel);
                        }
                        break;
                    }
                // TODO: Add missing actions
                default: throw new NotImplementedException();
            }
        }
        finally
        {
            CollectionChanged += viewModels_CollectionChanged;
        }
    }

    /// <summary>
    /// Only called once, by constructor
    /// </summary>
    private void CreateViewModelCollection()
    {
        foreach (var model in models)
        {
            Add(viewModelConstructor(model));
        }
    }

    public void Dispose()
    {
        models.CollectionChanged -= models_CollectionChanged;
    }
}