A DD4T.net Implementation - Dependency Injection

As promised in a previous post, I'll talk a bit about the dependency injection, and namely about using Ninject with DD4T .net.

First, I'm not going to attempt to compete with Rob's post about Using Ninject with DD4T. On the contrary, I'm completely piggy-backing on Rob's implementation with a twist of my own. So, in case you wonder how to setup Ninject in DD4T, please check his blog post.

In my implementation, I'm using the Factory pattern a lot. I have many interfaces that have their own implementing classes, then for each such interface, I provide a factory that builds and returns the right object.

Even further, I'm creating interfaces for these factories and then I bind them using Ninject. And Ninject does a great job in helping out here. These factories need to be singletons, in order to save resources and maximize performance. A factory does not keep state; instead, it takes all its settings from the parameters sent to its method. Ninject makes it extremely easy to define singletons, rather than just implement the singleton pattern on your own.

Let's take the example of the ModelFactory that was discussed earlier in this post. Initially I defined the interface IModelFactory:

    public interface IModelFactory

    {

        T GetModel<T>(string componentUri, string viewOrTemplateUri) where T : ModelBase;

        T GetModel<T>(IComponent component) where T : ModelBase;

        T ResolveModel<T>(T model, string viewOrTemplateUri) where T : ModelBase;

        IList<T> ResolveModels<T>(IList<T> models, string viewOrTemplateUri) where T : ModelBase;

    }

Then I created the implementing class, the ModelFactory itself:

    public class ModelFactory : IModelFactory

    {

        public ModelFactory()

        {

            ...

        }

        public T GetModel<T>(string componentUri, string viewOrTemplateUri) where T : ModelBase

        {

            ...

        }

        public T GetModel<T>(IComponent component) where T : ModelBase

        {

            ...

        }

        public T ResolveModel<T>(T model, string viewOrTemplateUri) where T : ModelBase

        {

            ...

        }

        public IList<T> ResolveModels<T>(IList<T> models, string viewOrTemplateUri) where T : ModelBase

        {

            ...

        }

    }

Since the ModelFactory should be a singleton, I decided to bind it to its interface using Ninject:

    public class DD4TNinjectModule : NinjectModule

    {

        public override void Load()

        {

            ...

            Bind<IModelFactory>().To<ModelFactory>().InSingletonScope();

        }

    }

This gives me the assurance that the ModelFactory is a singleton that I can access from anywhere in my application by using the following construct:

    _modelFactory = DependencyResolver.Current.GetService<IModelFactory>();

which works great in a lazy initialisation property:

    private IModelFactory _modelFactory;

    public IModelFactory ModelFactory

    {

        get

        {

            if (_modelFactory == null)

            {

                _modelFactory = DependencyResolver.Current.GetService<IModelFactory>();

            }

            return _modelFactory;

        }

        set { _modelFactory = value; }

    }

But the thing that I like the most about Ninject is the way one can specify Injection Patterns, namely the Initialization Methods and Property Setter Injection. Let's assume the ModelFactory class has some properties that should be 'injected' just after the ModelFactory singleton instance is created. Ninject can handle this elegantly, by simply specifying the annotation attribute [Ninject] before the property declaration.

    [Inject]

    public virtual IComponentFactory ComponentFactory { get; set; }

Property ComponentFactory of class ModelFactory is required during model construction. Since class ModelFactory is instantiated by Ninject, we need Ninject to also set the appropriate ComponentFactory instance. The code above does just that, simply, elegantly and looking a lot like the Spring's @Autowired annotation.

However, in order for this [Inject]-ed property to have a value, we must first define a binding in for interface IComponentFactory. We do that by the following line in DD4TNinjectModule:

    Bind<IComponentFactory>().To<CachingComponentFactory>().InSingletonScope();