The truth behind Inversion of Control – Part I – Dependency Injection

Note: this article series assumes you have already read my previous articles on IoC containers.

There is an evil truth behind the concept of Inversion of Control container. An unspoken code tragedy taking place everyday while passing unnoticed. Firm in my intentions to stop this shame, I decided to stand up and start writing this series of articles that will tell about the problem, the principles and the possible solutions.
I started to notice the symptoms of this “blasphemy” (against the code gods 🙂 ) quite a while ago, but I couldn’t pin down the reason of them. Nevertheless the problem was pretty clear: the IoC container solution was scarily used too often as alternative to the Singleton pattern to inject dependencies. With the code growing and the project evolving, many classes started to take the form of a blob of functions, with a common pitfall: dependencies, dependencies everywhere.
What means and what is wrong with using an IoC container as mere substitute of a Singleton, is something that I am going to describe the best I can with this series of articles. Don’t get me wrong, IoC containers are great tools, but if they are used in the wrong way, the can actually lead to major issues as well. I realised that IoC containers cannot be used without understanding how to use them, that’s why I started to look for a safer solution that could be adopted even by inexperienced coders. Before to look at this solution, let’s start taking some steps back and explain what Dependency Injection actually is:


What Dependency Injection is

Dependency Injection isn’t anything fancy. A dependency is just an interface of which a class is dependent on. Usually dependencies are solved in two ways: injecting them or passing them by Singletons. Singletons break encapsulation in the sense that, as a global variable, they can be used without a scope. Singletons awkwardly hide your dependencies: there is nothing in the class interface showing that the dependency is used internally. Singletons strongly couple your implementations, resulting eventually in too long and painful code refactoring. To be even more practical, Singletons, as all the global variables holding references, are also often source of memory leaks. For these reasons we use injection to solve dependencies.

Now, think for a moment if there wasn’t an IoC container in place. How would we inject our dependencies? For example, passing them as parameters in a constructor as shown in the example. Would you pass 10 parameters by constructor? I surely wouldn’t, at least just for how painful and inconvenient it is. Same reasoning applies when an IoC container is used. Just because it’s more convenient, it doesn’t mean you can take advantage of it. You are just making a mess with object coupling again. To be honest, if the design of the code would really follow the SOLID[1] principles, this problem wouldn’t arise, since the number of dependencies injected is directly linked to the number of responsibilities a class has got. One responsibility only should lead to a very few dependencies injected. However without a proper paradigm to follow, we all know that coders tend to break the Open/Close principle and add behaviours to existing classes instead to adopt a modular and extendible design. That’s when IoC containers start to be dangerous, since they actually help this process, making it less painful.

When dependencies are injected into an instance, where do these objects come from? If the dependencies are injected by constructor, they obviously come from the scope where the object, that needs the dependencies injected, has been created. On its turn, the class that is injecting the dependencies into the new object could need also dependencies injected, which therefore are passed by another class in the parent scope. This chain of dependency passages creates a waterfall of injections and the relationship between these objects is called Object Graph[1].

Albeit, where does this waterfall start from? It starts from the place where the initial objects are created. This place is called Composition Root. Root because is where the context is initialised, composition because is where the dependencies start to be created and injected and, therefore, the initial relations composed.

Now you can see what the real problem of the Unity framework is: the absence of a Composition Root. Unity doesn’t have a “main” class where the relations between objects can be composed. This is why the only way to create relationships with the bare Unity framework is using Singletons or static classes/methods.

Why are relationship between objects created? Mainly to let them communicate with each other. All forms of communications involve dependency injection. The only pattern that allows communication without dependency injection is the pattern called Event Bus[2] in the Java environment. The Event bus allows communications through events held by a Singleton, hence the Event Bus is one of the many anti pattern out there. Note that you could think to create something similar to an Event Bus without using a singleton (therefore injecting it). That’s an example of what I define to use injection as mere substitute of a Singleton.

Object Communication and Dependency Injection

Communication can couple or not couple objects, but in all the cases involves injection. There are several ways to let objects communicate:

  • Interface injection: usually A is injected in B, B is coupled with A [e.g.: Inside a B method A is used, B.something() { A.something());]
  • Events: usually B is injected in A, A is coupled with B [e.g.:Inside A, B is injected to expose the event, B.onSomething += A.Something]
  • Commands: B and A are uncoupled, B could call a command that calls a method of A. Commands are great to encapsulate business logic that could potentially change often. A Command Factory is usually injected in B.
  • Mediators: usually B and A do not know each other, but know their mediator. B and A pass themselves in the mediator and the mediator wires the communication between them (i.e.: through events or interfaces). Alternatively B and A are passed to the mediator outside B and A themselves, totally removing the dependency to the Mediator itself. This is my favourite flavour and the closest to dependency-less possible.
  • Various other patterns like: Observers, Event Queue[3] and so on.

How to pick up the correct one? If we don’t have guidelines it looks like one or another is the same. That’s why, some times, we end up using, randomly, one of those. Remember the first two patterns are the worst because they couple interfaces that could change over time.

We can anyway introduce the first sound practice of our guideline for our code design: our solution must minimize the number of dependencies.

Of course the second sound practice is about the concept of Single Responsibility Principles[4] (and Interface Segregation Principle[5]). One of the 5 principles of SOLID (ISP is another fundamental one), but the only one that must be actually taken as a rule. Your class MUST have one responsibility only. Communicating could be considered a responsibility, therefore it’s better to delegate it.

How we are going to achieve SRP and solve the dependencies blob problem is something I am going to explain in the next articles of this series.

bibliography

[1] SOLID (object-oriented design) http://guillemette.org/uqam/mgl7361/assets/documents/SOLID_principles.pdf

[2] Event Bus http://doc.akka.io/docs/akka/snapshot/java/event-bus.html

[3] Event Queue http://gameprogrammingpatterns.com/event-queue.html

[4] Single Responsibility Principle http://en.wikipedia.org/wiki/Single_responsibility_principle

[5] Interface Segregation Principle http://en.wikipedia.org/wiki/Interface_segregation_principle

I strongly suggest to read all my articles on the topic:

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