Comments on C#er : IMage: ViewModel binding with the Managed Extensibility F...

Whoops, I designed a UserControl using this code (...

2012-02-04T05:50:40.538-05:00

Whoops, I designed a UserControl using this code (WPF with CompositionInitializer + .NET 4).

In runtime, it works OK, but when I try to add the UserControl the the parent's XAML it just states that it cannot creat an instance of the it (my UserControl has its ViewModel attached to it using your example).

It can send you the code, it's plain a simple (a few lines, only). Thanks!!

Hi Jeremy, when I played around with ViewModelLoc...

2010-12-11T14:30:10.937-05:00

Hi Jeremy,

when I played around with ViewModelLocaor I tried the same approach as you. While at runtime everything is fine, I wasn't able to get it running at design time. The problem is also described in the mef discussion board http://mef.codeplex.com/Thread/View.aspx?ThreadId=208306 When the designer tries to create an instance of ViewModelLocator as a recourse the Satisfy()method in the ctor fails.

I really wouldn't want to have MEF wiring my u...

2010-08-08T13:11:36.113-04:00

I really wouldn't want to have MEF wiring my unit tests. That's the point of having decoupled parts - when I'm testing "module A" then any dependencies are mocked, and I mock those directly, no need for MEF. It's only in production where it wires all together.

As for the composition initializer, it is hierarchical - anything that is referenced. In this case, when the locator is in XAML, the App doesn't have a direct reference, so the locator must also satisfy imports. If we imported it programmatically and set it, it would be fine.

Thanks for the swift reply! "while in unit t...

2010-08-08T12:39:45.287-04:00

Thanks for the swift reply!

"while in unit tests you can simply set them", you mean setting directly the properties or using MEF to inject them? If yes, any example on how to inject Mocks/Stubs to properties of ViewModels using MEF?

I tried to use the VMLocator on your example "RegionsWithMef - Part 1", It worked only when I kept the "CompositionContainer.SatisfyImports(this)". I thought in MEF, the "SatisfyImports" is hierarchical, in a sense, in the App.xaml.cs, there was a call to "SatisfyImports", hence Shell class was created (XAML was parsed). While XAML was parsed, the ViewModelLocator class was created and hence, its Imports should have been imported also, but seems not!

Regards
Bilal

Yes, I've moved completely away from importing...

2010-08-08T12:20:54.461-04:00

Yes, I've moved completely away from importing constructors and just expose the dependencies as properties. Functionally, this is easy and makes sense because MEF can import these, while in unit tests you can simply set them. From a purist's point of view it exposes more about the internals of the class than some may like, but I say .... SO? I'm abstracting via interfaces, that gives me all I need.

Hi Jeremy, The solution above solves controlling t...

2010-08-08T12:13:16.765-04:00

Hi Jeremy,
The solution above solves controlling the life time of the ViewModel, what about controlling importing a ViewModel with a specific constructor?

My analysis is, with the use of MEF, no need for specific constructors as we can IMPORT anything we need without the constructor, but again would like to hear your analysis on this.

Thanks for the helpful material on your blog.

Regards
Bilal

I'll see what I can put together for a future ...

2010-07-14T06:26:44.640-04:00

I'll see what I can put together for a future post. Thanks!

>>With a little clever magic you could also ...

2010-07-14T05:18:07.367-04:00

>>With a little clever magic you could also use ExportFactory and generate what you need with a WeakReference so when the control is done, it can go out of scope.
Do you have a blog entry how this little magic should look like? i would really want to see ExportFactory and WeakReference in action :)

That was it ! I was implicitly exporting the mode...

2010-07-13T15:44:03.839-04:00

That was it !

I was implicitly exporting the model for this viewmodel

[Export]
public class EstablishmentsModel : ModelBase, IEstablishmentsModel
{

Change it to explicitly declare the type and it worked

[Export(typeof(IEstablishmentsModel))]
public class EstablishmentsModel : ModelBase, IEstablishmentsModel
{

Thanks !!!

No, not necessarily. You shouldn't have to. Yo...

2010-07-13T13:59:38.916-04:00

No, not necessarily. You shouldn't have to. You are exporting explicitly. The question now becomes whether or not the Restaurant View Model has some dependency that isn't being satisfied, causing the export to be rejected.

Maybe I need to decorate the interface ? [Inherit...

2010-07-13T13:53:21.523-04:00

Maybe I need to decorate the interface ?

[InheritedExport]
public interface IRestaurantSearchViewModel { };

public class ViewModelLocator { public ViewMo...

2010-07-13T09:41:49.278-04:00

public class ViewModelLocator
{
public ViewModelLocator()
{
CompositionInitializer.SatisfyImports(this);
}

[Import]
public IRestaurantSearchViewModel RestaurantSearchModelInstance { get; set; }
}

}

How are you importing it? It looks like you are im...

2010-07-13T05:48:16.794-04:00

How are you importing it? It looks like you are importing a "RestarauntSearchViewModel" instead of a "IRestaurantSearchViewModel" ...

Jeremy, Can you tell me what this message means ?...

2010-07-12T20:22:51.586-04:00

Jeremy,

Can you tell me what this message means ?

1) No valid exports were found that match the constraint '((exportDefinition.ContractName == "mvvmLight.ViewModels.ViewModel.RestaurantSearchViewModel") AndAlso (exportDefinition.Metadata.ContainsKey("ExportTypeIdentity") AndAlso "mvvmLight.ViewModels.ViewModel.RestaurantSearchViewModel".Equals(exportDefinition.Metadata.get_Item("ExportTypeIdentity"))))', invalid exports may have been rejected.

I am exporting like so:

public interface IRestaurantSearchViewModel { };

[Export(typeof(IRestaurantSearchViewModel))]
public class RestaurantSearchViewModel : ViewModelBase , IRestaurantSearchViewModel
{

What am I doing wrong ?

Thanks,
Greg

Two ways I have done that in the past, Sacha (than...

2010-04-15T08:21:46.243-04:00

Two ways I have done that in the past, Sacha (thanks for asking ... great question ... interested to know more about how Cinch handles this as well).

First, I've based the view model on, say, BaseViewModel, with an exposed View property. On the view, which can also be based (ViewBase : UserControl, IView), the ViewBase hooks Loaded and then sets ((BaseViewModel)DataContext).View = this;

Second, I've done it with attached behaviors. Using a behavior ala Blend, for example, you can simply hook OnAttached and if both the VM and View have a well defined interface, connect the two there.

Do you have an article you'd like linked that explains how Cinch does this?

What about if you simply had to pass some conext a...

2010-04-15T02:57:07.414-04:00

What about if you simply had to pass some conext about the view (say through an IView) such as editing a Customer and the View had an ID property, and you wanted to pass that to VM, how do you see this working???

Thanks for the reply. 2. depending on what button ...

2010-03-18T17:12:08.726-04:00

Thanks for the reply.
2. depending on what button a user clicked the contentcontrol is going to change its content. so either
1. create the instance of the view using exportfactory or
2. have an import in the viewmodel(for each type of the view I want to show
right?

1. We could, but we don't have to. For example...

2010-03-18T16:57:45.133-04:00

1. We could, but we don't have to. For example, I may have a "main view model" used throughout that is a shared part. No need for lazy there ... we're not concerned about garbage collection because it persists throughout the lifetime of the application and the import in the locator simply becomes a reference to the singleton. For short-lived views, absolutely. Make it lazy, and only have it come into scope if/when needed. With a little clever magic you could also use ExportFactory and generate what you need with a WeakReference so when the control is done, it can go out of scope.

2. Not sure I catch this. Swapping the views in a content control can have many implications. You might use a viewmodel-first approach that uses a locator similar to what Caliburn does and fires the view in, or you might use a region management pattern, or any other means.

Hi, I have few questions 1. In the locator class, ...

2010-03-18T16:52:11.671-04:00

Hi,
I have few questions
1. In the locator class, Are we going to have all the viewmodels exposed as Lazy
2. Lets say, I have to change the views displayed in a contentcontrol, am I going to properties of all the views as properties
like below in the ViewModel

[Import]
public Lazy View1{get;set;}
[Import]
public Lazy View2{get;set;}

This compares well with the ViewModelLocator class...

2010-03-18T04:16:57.864-04:00

This compares well with the ViewModelLocator class in Laurent Bugnion's MVVM Lite framework. Your implementation using MEF requires less code, and you get loose coupling of ViewModel implementations.