Refactoring multiple interfaces to a common interface using MVVM, MEF and Silverlight4
- by Brian
I am just learning MVVM with MEF and already see the benefits but I am a little confused about some implementation details. The app I am building has several Models that do the same with with different entities (WCF RIA Services exposing a Entity framework object) and I would like to avoid implementing a similar interface/model for each view I need and the following is what I have come up with though it currently doesn't work.
The common interface has a new completed event for each model that implements the base model, this was the easiest way I could implement a common class as the compiler did not like casting from a child to the base type.
The code as it currently sits compiles and runs but the is a null IModel being passed into the [ImportingConstructor] for the FaqViewModel class.
I have a common interface (simplified for posting) defined as follows, this should look familiar to those who have seen Shawn Wildermuth's RIAXboxGames sample.
public interface IModel
{
void GetItemsAsync();
event EventHandler<EntityResultsArgs<faq>> GetFaqsComplete;
}
A base method that implements the interface
public class ModelBase : IModel
{
public virtual void GetItemsAsync() { }
public virtual event EventHandler<EntityResultsArgs<faq>> GetFaqsComplete;
protected void PerformQuery<T>(EntityQuery<T> qry, EventHandler<EntityResultsArgs<T>> evt) where T : Entity
{
Context.Load(qry, r =>
{
if (evt == null) return;
try
{
if (r.HasError)
{
evt(this, new EntityResultsArgs<T>(r.Error));
}
else if (r.Entities.Count() > 0)
{
evt(this, new EntityResultsArgs<T>(r.Entities));
}
}
catch (Exception ex)
{
evt(this, new EntityResultsArgs<T>(ex));
}
}, null);
}
private DomainContext _domainContext;
protected DomainContext Context
{
get
{
if (_domainContext == null)
{
_domainContext = new DomainContext();
_domainContext.PropertyChanged += DomainContext_PropertyChanged;
}
return _domainContext;
}
}
void DomainContext_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
{
switch (e.PropertyName)
{
case "IsLoading":
AppMessages.IsBusyMessage.Send(_domainContext.IsLoading);
break;
case "IsSubmitting":
AppMessages.IsBusyMessage.Send(_domainContext.IsSubmitting);
break;
}
}
}
A model that implements the base model
[Export(ViewModelTypes.FaqViewModel, typeof(IModel))]
public class FaqModel : ModelBase
{
public override void GetItemsAsync()
{
PerformQuery(Context.GetFaqsQuery(), GetFaqsComplete);
}
public override event EventHandler<EntityResultsArgs<faq>> GetFaqsComplete;
}
A view model
[PartCreationPolicy(CreationPolicy.NonShared)]
[Export(ViewModelTypes.FaqViewModel)]
public class FaqViewModel : MyViewModelBase
{
private readonly IModel _model;
[ImportingConstructor]
public FaqViewModel(IModel model)
{
_model = model;
_model.GetFaqsComplete += Model_GetFaqsComplete;
_model.GetItemsAsync(); // Load FAQS on creation
}
private IEnumerable<faq> _faqs;
public IEnumerable<faq> Faqs
{
get { return _faqs; }
private set
{
if (value == _faqs) return;
_faqs = value;
RaisePropertyChanged("Faqs");
}
}
private faq _currentFaq;
public faq CurrentFaq
{
get { return _currentFaq; }
set
{
if (value == _currentFaq) return;
_currentFaq = value;
RaisePropertyChanged("CurrentFaq");
}
}
public void GetFaqsAsync()
{
_model.GetItemsAsync();
}
void Model_GetFaqsComplete(object sender, EntityResultsArgs<faq> e)
{
if (e.Error != null)
{
ErrorMessage = e.Error.Message;
}
else
{
Faqs = e.Results;
}
}
}
And then finally the Silverlight view itself
public partial class FrequentlyAskedQuestions
{
public FrequentlyAskedQuestions()
{
InitializeComponent();
if (!ViewModelBase.IsInDesignModeStatic)
{
// Use MEF To load the View Model
CompositionInitializer.SatisfyImports(this);
}
}
[Import(ViewModelTypes.FaqViewModel)]
public object ViewModel
{
set
{
DataContext = value;
}
}
}
