In this chapter, one of the things we will brush up is the usage of the Dependency Inversion Principle, that is, the D in SOLID. Let's start with the first principle: the S in SOLID of Single Responsibility Principle, which states that a method or a class should only have one reason to change and only have one responsibility. With this, we can't have our units take on more than one responsibility and need help from collaborators to do the entire job. These collaborators are things we now depend on and we should represent these dependencies clearly to our units so that anyone or anything instantiating it knows what we are depending on.

We have now flipped around the way in which we get dependencies. Instead of the unit trying to instantiate everything itself, we now clearly state what we need as collaborators, opening up for the calling code to decide what implementations of these dependencies you want to pass on. Also, this is an important aspect; typically, you...