Here's what I'm using for view model collections:
Preface:
Your view model objects can be weakly typed. Give IModel
a property object Value {get;}
and expose that in a ModelViewModel : ViewModel<IModel>
that you use for all IModel
objects (see my ViewModel<T>
implementation below). If you have various combinations of ObservableCollection<IModel>
, ICollection<Model<T>>
, etc., the framework shown here is a lifesaver. If you still need generic view model, you can derive a ModelViewModel<T> : ModelViewModel
that takes a Model<T>
in its constructor. The logic to create the appropriate type would go in the converter passed to ViewModelCollection.Create
below. Do be warned that this design will impose a performance penalty.
ModelViewModel CreateModelViewModel(IModel model)
{
Type viewModelType = typeof(ModelViewModel<>).MakeGenericType(model.Type);
ModelViewModel viewModel = Activator.CreateInstance(viewModelType, model);
return viewModel;
}
Example usage:
public class CatalogViewModel : ViewModel<ICatalog>
{
public CatalogViewModel(ICatalog catalog)
: base(catalog)
{
Func<ICatalogProduct, ProductViewModel> viewModelFactory = CreateProductViewModel;
this.Products = ViewModelCollection.Create(catalog.Products, viewModelFactory);
}
public ICollection<ProductViewModel> Products
{
get;
private set;
}
private ProductViewModel CreateProductViewModel(ICatalogProduct product)
{
return new ProductViewModel(product, this);
}
}
Benefits:
- Uses lazy implementations to allow for efficient and even recursive bindings in trees.
- The view model collections only implement
INotifyCollectionChanged
if the underlying model collection implements INotifyCollectionChanged
.
Overview of the classes (full implementations linked to github):
ViewModel<TModel>
: Base class for my view model classes. Exposes a Model
property that I use in the view model's backing code.
ObservableViewModelCollection<TViewModel, TModel>
: Lazy (actually not currently, but definitely should be), observable mapping from a model to a view model. Implements INotifyCollectionChanged
.
ViewModelCollection<TViewModel, TModel>
: Lazy mapping from a collection of TModel to a collection of TViewModel
.
ViewModelCollection
: Static helper - returns an ICollection<TViewModel>
, using ObservableViewModelCollection<TViewModel, TModel>
when the source collection implements INotifyCollectionChanged
, otherwise using ViewModelCollection<TViewModel, TModel>
.
A few extra types that might be useful for your view model collections:
ConcatCollection
: Like ViewModelCollection, this includes a static helper to automatically choose an appropriate implementation. The ConcatCollection concatenates collections by binding directly to the source collection(s).
Here is an example of how I used this type to expose a Children
property to the view while maintaining my observable collections all the way to back to the original source.
public class ProductViewModel : ViewModel<IProduct>
{
public ProductViewModel(IProduct product)
: base(product)
{
Func<IProduct, ProductViewModel> productViewModelFactory = CreateProductViewModel;
Func<IRelease, ReleaseViewModel> releaseViewModelFactory = CreateReleaseViewModel;
this.Products = ViewModelCollection.Create(product.Products, productViewModelFactory);
this.Releases = ViewModelCollection.Create(product.Releases, releaseViewModelFactory);
this.Children = ConcatCollection.Create<object>((ICollection)this.Products, (ICollection)this.Releases);
}
public IList<ProductViewModel> Products
{
get;
private set;
}
public IList<ReleaseViewModel> Releases
{
get;
private set;
}
public IEnumerable<object> Children
{
get;
private set;
}
private ProductViewModel CreateProductViewModel(IProduct product)
{
return new ProductViewModel(product);
}
private ReleaseViewModel CreateReleaseViewModel(IRelease release)
{
return new ReleaseViewModel(release);
}
}