So you are taking the plunge into learning Drupal 8 for the purpose of developing sites, modules, themes, etc. You're a great Drupal 7 developer, familiar with many drupalisms but you don't have tons of experience with modern PHP frameworks, principles and practices. Well, Drupal 8 still includes many of the old drupalisms but still attempts to keep in line with times and modernise itself.

In this article I want to outline 6 steps I believe you should take to get started learning how to develop custom modules and/or themes in Drupal 8. On top of these 6 builds everything else.

The first three are PHP related in a more general fashion, while the last three target aspects of Drupal 8 itself.

1. Learn Object Oriented Programming

One of the biggest difference between Drupal 7 and 8 for developers is the way code is written. It's still PHP but it's now much more object oriented. Global procedural functions are still in place but most of the logic happens in classes.

In case you don't have much experience with Object Oriented Programming in PHP, this is the first thing you need to learn, brush upon or revise (depending on your level). There are many resources available out there, all scattered as hyperlinks in this section. There are also courses you can take, both free and paid.

Without quite a solid OOP foundation, you won't be able to understand much of how Drupal 8 modules are built.

2. Learn how to use Composer

One of the consequences of modernizing PHP has been the introduction of the Composer package manager. Projects are no longer built without it as it does a great job of installing, updating and managing in general the external libraries and dependencies of your project. Not to mention its very helpful autoloader.

Drupal 8 uses Composer to manage external PHP libraries and dependencies (such as Symfony components, Guzzle, etc) and there is talk about the ability to handle also contrib modules. So make sure you know how Composer works and even start using it in non-Drupal projects.

3. Get familiar with Symfony

One of the main points of contention (back then) in the Drupal 8 development cycle was the introduction of Symfony components. Some people didn't really agree with this great shift from the Drupal way, but others embraced it wholeheartedly. I am in the latter group as I love Symfony and used it even before developing anything in Drupal 8.

Drupal 7 developers are often being told that knowing Symfony is not required in order to develop in Drupal 8. While technically true, you still end up learning a lot of it through the Drupal experience. That being said, I strongly recommend learning some Symfony before. It is a great modular framework and knowing its concepts will really help you out in understanding how Drupal 8 is built (for the components it uses I mean). Once you can build a small website in Symfony, you'll enjoy developing in Drupal 8 that much more because concepts will be similar a lot of the time. Not too mention that you can use Symfony to build apps on its own.

4. Routing and controllers

Just like with Drupal 7, when starting to learn Drupal 8 you need to create the obligatory hello world module (creating a page with a parameter in the URL( usually world) that displays the text Hello + parameter). This example introduces you to many important things:

• Module folder structure
• Routing (no more hook_menu) through routing.yml files that map to Controller methods
• Controller classes which have various methods that can be mapped to routes
• Access arguments for these routes
• Rendering markup to the page inside the Controller methods
• etc

So I really recommend giving this a go.

5. Plugins

Another important concept you'll need to get familiar with is Plugins. Admittedly, this is not the easiest to grasp, but it is super important because it's everywhere. Not to worry though, it's not rocket science.

Many old Drupal 7 implementations of various concepts have been transformed to plugins: info hooks, blocks, field formatters, views handlers, etc. Understanding plugins is therefore very important for being able to extend default Drupal functionality.

6. Dependency injection and the service container

The last step I am going to mention here is dependency injection. Drupal 8 uses the Symfony dependency injection container to manage service instantiation and injection into classes that need them. This helps decouple functionality and increases testability.

However, many people are scared of this concept, mainly because they don't grasp it. I wasn't super comfortable either before understanding it. But you should definitely learn what it means, why we use it and how we use it. Because it is a very simple concept that is used all the time in procedural code as well.

You can already find many tutorials out there on Drupal 8 that load services statically through the \Drupal class. It is much faster to write so people (me included) prefer it when writing about D8. I usually also tend to make a note that using dependency injection is preferred in theses cases.

Not understanding what the service container and dependency injection is will easily let you fall into the habit of just statically requesting services and coupling your code like it was procedural. Once you are clear on this point, this will hopefully not happen any more and the sight of a \Drupal::get('some_service') will make you think twice.

Conclusion

There you have it. What I think are the first 6 steps you should take when learning Drupal 8 for the first time. Of course there are many other important things to learn and do but I believe they build on top of this foundation. Of course, this is me writing so others may have different opinions (very much welcomed in the comments). So let's discuss.