As you use object-oriented programming with WordPress more and more, you’re going to encounter new sets of problems. These problems aren’t as simple as the ones that you encountered when you started. But that’s also good news. It also means that you’re making progress.
One set of problems that you start to encounter as you advance in your use of object-oriented programming deal with scaling. As your code has more and more classes, it becomes a challenge to assemble these classes together. That’s why in the past we looked at how to design a class whose job was to do that.
But there are other problems that come with having a lot of classes in your code. It’s how much your classes depend on other classes. This is what we call coupling in programming. (Not just object-oriented programming.)
To help with coupling, a famous software engineer (Robert C. Martin who’s also known as Uncle Bob) created the dependency inversion principle. It helps you decouple your classes by making dependencies external to them. (There’s more to it than that. But this is one of the core elements of the dependency inversion principle.)
This reduces coupling but creates a new problem as well. That problem is that you can’t initialize objects inside your class anymore. You have to pass them to your own object through its constructor or some other method like a setter method.
This makes it even harder to assemble your classes together. This is why software engineers created dependency injection. It’s a programming technique that helps solve this problem.
Most web application frameworks use it in some form or another. But that’s not the case with WordPress since a lot of its code isn’t object-oriented. That said, if you want to build an object-oriented plugin for WordPress, it’s pretty much mandatory. Lucky for us, it’s not that hard to use dependency injection in your own code!