Dependency injection is a design pattern commonly used in object-oriented software architectures in order to support Inversion of Control. At a very high level it refers to the practice of passing existing objects (or services) into a class when it is instantiated, or via a setter method, rather than creating them inside the class itself. Doing so allows some other logic external to your code to determine how to initialize and configure the service object, and keeps your code more flexible.
In the case of Drupal this allows your code to state a need, such as, “I need to access the database. Please provide me with a database service,” without knowing anything about the specifics. Drupal then determines, based on other factors, what database type (MySQL, SQLite, etc.) is being used, and how to connect to it, how to authenticate, and any other requirements on your behalf.
Currently in Drupal, dependency injection is the preferred method for accessing and using services and should be used whenever possible. Rather than calling out to the global services container, services are instead passed as arguments to a constructor or injected via setter methods. This allows for a loose coupling of components, which makes things easier to test, and easier to replace with alternative implementations.
Example tasks
- Use dependency injection to make use of services in your module’s code
- Write custom code that is easier to test and refactor
Confidence
While relatively new to Drupal, dependency injection is a tried and true technique commonly used in object-oriented PHP applications. The implementation in Drupal is unlikely to change significantly, and therefore you should feel comfortable learning it just about any context and then applying it to Drupal.
Drupalize.Me resources
Sharpen your object-oriented skills by exploring the ideas and reasons behind dependency injection. This simple principle separates developers who write functional code from those that are able to build great, and maintanable applications. In this series, we'll see dependency injection in action, why it's important, and how it relates to services and service-oriented architecture. We'll also refactor our demo application to use a dependency injection container, using a fantastic—but simple—container called Pimple.
In this tutorial, we’ll be coding with a real example where we create a simple app to help people give their money away, we’re calling it SendMoneyToStrangers.com. We’ve already bootstrapped a small app, which you can download and use if you want to follow along. It uses an SQLite database, so if you don't have that set up, you can grab the database and find installation instructions on the SQLite site. Once you SQLite, you will install the app from the demo code we are providing. We will also be using Composer to make it easier to get Pimple when we need it. If you are not familiar with Composer, you can watch the short tutorial The Wonderful World of Composer to get up and running.
Additional resources
SQLite site
The Wonderful World of Composer tutorial
Pimple
To learn how to apply these concepts in Drupal 8 module development, check out the Module Development Essentials series, starting with Understand the Service Container.
In this tutorial, you will learn how to extend the ControllerBase class in Drupal and get services out of the container.
Additional resources
Injecting services in your D8 plugins (lullabot.com)
abstract class ControllerBase — api.drupal.org
PHP Service Container — Drupalize.Me
It's best practice to access any of the services provided by Drupal via the service container to ensure the decoupled nature of these systems is respected. In order to do so, you need to know what services exists, and then, where possible, use dependency injection to use them in your code.
This tutorial walks through the process of:
- Discovering existing services and learn their machine name
- Using the machine name of service to request a copy from the service container
Services
TopicServices are objects that encapsulate the code for performing specific tasks in a reusable and decoupled way.
External resources
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Services and dependency injection (Drupal.org)
- Examples of how to use dependency injection in a Drupal context.
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Services and Dependency Injection Container (api.drupal.org)
- A more technical look at how Drupal implements dependency injection and services
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Dependency Injection (phptherightway.com)
- A good overview of dependency injection and related concepts with a focus on PHP applications. Contains a well-curated list of links to other articles that discuss the various concepts in detail
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Late Static Binding (php.net)
- Drupal makes use of a PHP feature known as late static binding to handle injection of services in many cases. See
\Drupal\system\Plugin\Block\SystemBrandingBlock::create()for an example. This isn’t necessarily required knowledge, but if you’re curious how that code works you can read up on late static binding.
- Drupal makes use of a PHP feature known as late static binding to handle injection of services in many cases. See