MVVM with aggregated model classes - how to wrap i

问题:

I'm currently trying to create a small application using the MVVM pattern. However I don't really know how to correctly wrap up aggregated Model classes in my ViewModel. From what little I know about MVVM, you're not supposed to expose Models in your ViewModel as properties or else you could directly bind to the Model from your View. So it seems I have to wrap the nested Model in another ViewModel, but this imposes some problems while synching Model and ViewModel later on.

So how do you do that efficiently?

I'll give a short example. Let's suppose I have the following model classes:

public class Bar
{
    public string Name { get; set; }
}

public class Foo
{
    public Bar NestedBar { get; set; }
}

Now I create two ViewModel classes accordingly, wrapping the Models, but run into problems with the FooViewModel:

public class BarViewModel
{
    private Bar _bar;
    public string Name 
    { 
        get { return _bar.Name; }
        set { _bar.Name = value; }
    }
}

public class FooViewModel
{
    private Foo _foo;
    public BarViewModel Bar
    {
        get { return ???; }
        set { ??? = value; }
    }
}

Now what do I do with the Bar-property of FooViewModel? For "get" to work I need to return a BarViewModel instance. Do I create a new field of that type in FooViewModel and just wrap the _foo.NestedBar object in there? Changes to that field's properties should propagate down to the underlying Bar instance, right?

What if I need to assign another BarViewModel instance to that property, like so:

foo.Bar = new BarViewModel();

Now that won't propagate down to the model, which still holds the old instance of type Bar. I'd need to create a new Bar object based on the new BarViewModel and assing it to _foo, but how do you do that elegantly? It's pretty trivial in this sample, but if Bar is much more complex with lots of properties, that'll be a lot of typing... not to mention it'd be very prone to errors, if you forget to set one of the properties.

回答1:

My above answer only makes sense if you are doing DDD - if you are not - you can solve your problem like this - simply 'flattening' the model:

public class FooViewModel
{
    private Foo _foo;
    public string Name
    {
        get { return _foo.Name; }
        set { _foo.Name = value; }
    }
    public string BarProperty
    {
        get { return _foo.Bar.Property; }
        set { _foo.Bar.Property = value; }
    }
}

Or you could do like I showed in the prior example - just ignore everything about Aggregates... should still work.

回答2:

@Goblin

There are some flaws with your code: e.g. what if I get a list of Foo objects from database and I want to wrap each of them in an ObservableCollection?

then your Constructor of FooViewModel should accept the Foo model as parameter and not create it inside the Constructor!

Normally you do this to wrap a model into a viewmodel and put it the same time into a bindable Collection:

IEnumerable<Foo> foos = fooRepository.GetFoos();
foos.Select( m => viewmodelCollection.Add(new ViewModel(m,e.g.Service)));

The models properties are not copied to the ViewModel hell no!!! The ViewModel does delegate its properties to the model properties like:

public class FooViewModel
{
   private Foo _foo;

   public FooViewModel(Foo foo,IService service)
   {
      _foo = foo;

   }

   public string FoosName
   { 
      get{return _foo.Name };
      set
      {
         if(_foo.Name == value)
            return;

         _foo.Name = value;
         this.NotifyPropertyChanged("FoosName");
      }
   }

}

And like Goblin said all UI-Specific interfaces like:

IDataErrorInfo
INotifyPropertyChanged
IEditableObject

etc...

are implemented the by the ViewModel ONLY.

回答3:

Okay - first things first - using the term Aggregate implies you are adhering to DDD? If you are - you are doing an encapsulation no-no :-). One Aggregate should never be allowed to edit another Aggregate. If what you have is that both are really Aggregate they would become associated (which is perfectly 'legal' in a DDD-sense - but then your propety on the FooViewModel wouldn't be of type BarViewModel, but rather type Bar. That way Bar would (as it should) be responsible for updating itself - and we only maintain the link in FooViewModel.

However, if what you are doing is AggregateRoot with a ValueType child - then here is what you could do given a slightly modified domain model:

public class Foo
{
    public string SomeProperty { get; set; }
    public Bar Bar { get; set; }
    public void Save()
    {
        //Magically saves to persistent storage...
    }
}
public class Bar
{
    public Bar(string someOtherProperty)
    {
        SomeOtherProperty = someOtherProperty;
    }
    public string SomeOtherProperty { get; private set; }
}

And then for the ViewModels:

public class FooViewModel
{
    private Foo _foo;
    public FooViewModel()
    {
        Bar = new BarViewModel();
    }
    public BarViewModel Bar { get; private set; }
    public void SetFoo(Foo foo)
    {
        _foo = foo;
        SomeProperty = foo.SomeProperty;
        Bar.SetBar(foo.Bar);
    }
    public string SomeProperty { get; set; }
    public void SaveChanges()
    {
        _foo.SomeProperty = SomeProperty;
        _foo.Bar = Bar.CreateUpdatedBar();
        _foo.Save();
    }
}
public class BarViewModel
{
    public string SomeOtherProperty { get; set; }
    public void SetBar(Bar bar)
    {
        SomeOtherProperty = bar.SomeOtherProperty;
    }
    public Bar CreateUpdatedBar()
    {
        return new Bar(SomeOtherProperty);
    }
}

This way - the FooViewModel is now capable of controlling the BarViewModel (which does nothing but accept a valuetype - and create a new one when asked). This also solves a common UI-problem ('How do we edit an object that has no setters?' - answer: 'We don't - we create a new one'). A lot of fleshing out is missing (INotifyPropertyChanged, dirty-tracking etc., but those are easy if you get through this leap of thinking :-).

I hope this makes a wee bit of sense :-) Otherwise, I'll be happy to elaborate.