Dependency injection
At the end of the chapter, we will cover one of the most interesting and controversial of the topics that come with, not only the MVC pattern, but OOP in general: dependency injection. We will show you why it is so important, and how to implement a solution that suits our specific application, even though there are quite a few different implementations that can cover different necessities.
Why is dependency injection necessary?
We still need to cover the way to unit test your code, hence you have not experienced it by yourself yet. But one of the signs of a potential source of problems is when you use the new statement in your code to create an instance of a class that does not belong to your code base—also known as a dependency. Using new to create a domain object like Book or Sale is fine. Using it to instantiate models is also acceptable. But manually instantiating, which something else, such as the template engine, the database connection, or the logger, is something...
