Monday, November 09, 2009

Magellan and IOC/DI Containers

by Paul Stovell via Paul Stovell on 11/9/2009 10:40:32 AM

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Magellan was designed to work without an Inversion of Control or Dependency Injection container, to keep it simple and accessible. However, as applications become more complicated, modern WPF applications can benefit immensely from IOC containers.

Magellan's extensibility points make using a container easy. Magellan resolves and instantiates two main types of objects - controllers and views. The out of the box implementation uses conventions, but they can be overridden.

To take control of resolving controllers, implement the IControllerFactory interface. Here is an example using Ninject:

public class NinjectControllerFactory : IControllerFactory
{
    private readonly IKernel _container;

    public NinjectControllerFactory(IKernel container)
    {
        _container = container;
    }

    public IController CreateController(NavigationRequest request, string controllerName)
    {
        var controller = _container.Get<IController>(controllerName);
        return new ControllerFactoryResult(controller);
    }
}

The ControllerFactoryResult has a second overload that also allows you to pass a callback that is invoked when the request has been processed - this can be used for disposing the controller if required.

Then assign it as the default controller factory:

var container = new StandardKernel();
container.Bind<IController>().To<HomeController>().Named("Home");
container.Bind<IController>().To<SettingsController>().Named("Settings");
ControllerBuilder.Current.SetControllerFactory(new NinjectControllerFactory(container));

When a navigation request is processed, the Navigator will consult the ControllerBuilder.Current for the controller factory. It will use this to ask the factory for a controller, then execute the controller. The ReleaseController method will be called as soon as the navigation request has completed.

Typically, each navigation will create a new controller - if you want to use the same controller, you could have your controller factory recycle the objects, or make them singletons.

The second object that Magellan creates is views. This is done through the IViewActivator implementation. The default implementation looks for a public, parameterless constructor.

Again, let's use Ninject to control view instantiation.

public class NinjectViewActivator : IViewActivator
{
    private readonly IKernel _container;

    public NinjectViewActivator(IKernel container)
    {
        _container = container;
    }

    public object Instantiate(Type viewType)
    {
        return _container.Get(viewType);        
    }
}

We then need to tell the view engines that it is available. Since the view engines are automatically registered, we need to manually re-register them:

var activator = new NinjectViewActivator(container);
ViewEngines.Engines.Clear();
ViewEngines.Engines.Add(new PageViewEngine(activator));
ViewEngines.Engines.Add(new WindowViewEngine(activator));

If you are using Magellan with Composite WPF, you will also need to register the region view engine:

ViewEngines.Engines.Add(new CompositeViewEngine(activator));

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