TutsPlus.com has continued their series covering object-oriented development practices in WordPress (plugins) with this third tutorial. In previous parts they set up the environment and introduced some of the basic concepts of OOP programming and getting the first classes and files defined.
In the last tutorial, we reviewed the original state of our autoloader and then went through a process of object-oriented analysis and design. The purpose of doing this is so that we can tie together everything that we've covered in this series and the introductory series.
Secondly, the purpose of doing this in its own tutorial is so we can spend the rest of this time walking through our class, seeing how each part fits together, implementing it in our plugin, and then seeing how applying object-oriented programming and the single responsibility principle can lead to a more focused, maintainable solution.
They start with a brief review of what they've covered so far and begin to build on the changes suggested in the previous part of the series. They've already broken it down into the different functional classes (according to the single-responsibility principle) and take the next step of including them and calling some example code to prove all is working as expected.