How to Access composition host from UserControl

Oct 6, 2011 at 10:04 PM

My app mostly Silverlight + PRISM and I'm using MEF as container.

When I use PRISM's navigation and regions - all my services imported/exported properly.

Now I created my own UserControl that I put on a View with XAML. So, it's "disconnected" from container in a way that it is not created by PRISM.

Is there any way to use EventAggregator and so on in this control? I guess I can expose DependencyProperty and Bind to objects I need (expose them from ViewModel) but it seems like PRISM world "locked in" and it is not possible to tap into it's stuff from outside. Or do I miss something? Maybe PRISM has some kind of Singleton I can use Application-wide?

Thank you!

Developer
Oct 7, 2011 at 1:38 PM

Hi,

In case you wish to access the container from within a class that had not been instantiated using the container, you might find the ServiceLocator class useful.

The ServiceLocator provides an abstraction over the container used in your application (whether it's Unity, MEF or other), and can be accessed through a static property of a static class (so you don't need to have a reference to the container in the pieces of code where you wish to use it). To achieve this, the Bootstrapper sets the service locator provider (which is an adapter between the container-specific interface and the service locator interface).

As an example, to retrieve a type from the container using the ServiceLocator, you could do the following:

ISomeService service = ServiceLocator.Current.GetInstance<ISomeService>();

Please note that you need to reference the Microsoft.Practices.ServiceLocation project in order to use the ServiceLocator.

I hope you find this useful

Agustin Adami
http://blogs.southworks.net/aadami

Oct 7, 2011 at 3:05 PM

Thank you! This is exactly what I need.

Is it a good practice to use ServiceLocator? I figured I can pass ISomeService to my UserControl by binding from ViewModel which was instantiated using container. If I use ServiceLocator - Code will be much more clear but I wonder if that's a good design, etc..

Also, how do I remove copies of objects from Service Locator? Does it manage garbage collection automatically (when objects go out of scope)? I'm asking about objects where PartCreation == NonShared

Developer
Oct 7, 2011 at 5:52 PM

Hi,

In my opinion using the ServiceLocator isn't a bad practice. However, note that if you will unit test your component, you will have an additional dependency to mock (i.e. the service locator), which would add unnecessary complexity to your tests.

On the other hand note that the ServiceLocator is just an abstraction over IoC (Inversion of Control) containers, which  is used to obtain instances of registered components and not for registration. Therefore the lifetime of your instances will only deppend on how they were registered into the container; in your case, if you exported a class with the PartCreationPolicy set to NonShared, instances retrieved from the container will be kept alive as long as they are referenced by other classes.

<!--[if gte mso 9]><xml> <o:OfficeDocumentSettings> <o:RelyOnVML /> <o:AllowPNG /> </o:OfficeDocumentSettings> </xml><![endif]-->You'll find more information regarding that in Chapter 3 of the Prism documentation:

Based on the aforementioned, you could consider whether using the ServiceLocator applies to your scenario. In case it doesn't, you could consider an alternative design approach to avoid placing your view inside the xaml, so you can instantiate it through the container.

I hope you find this helpful

Agustin Adami
http://blogs.southworks.net/aadami

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Oct 11, 2011 at 3:38 AM

I think you can use the mefcontrib here. You can inject your services into a unity container and register the aggregatecatalog in it. So you can simply inject the interfaces in the constructors where you need. Something like this in your bootstrapper

 var unityContainer = new UnityContainer();
unityContainer.RegisterType<ISomeService, SomeService>();
 unityContainer.RegisterCatalog(AggregateCatalog);

Container = unityContainer.Resolve<CompositionContainer>();

http://mefcontrib.codeplex.com/

Cheers!