Dependency injection container

Dependency injection is a way of implementing inversion of control design pattern. It's used to handle the dependencies between multiple classes without the direct instantiation of one class in another - which causes tight coupling in the code, and makes it less testable and harder to maintain. Dependency injection is a way of implementing inversion of control design pattern. It's used to handle the dependencies between multiple classes without the direct instantiation of one class in another - which causes tight coupling in the code and makes it less testable and harder to maintain.

In general, if you instantiate another class like this inside your class

new SuperClass()

that should be avoided because it tightly couples two classes.

eightshift libs are using PHP-DI as an implementation of a dependency injection container. We'll explain the implementation by following the eightshift-boilerplate example.

In your main class, which is used to store all the dependencies in your code, and which extends the main eightshift-lib class (eightshift_Libs\Core\Main), you need to define a method called get_service_classes() which contains an array of service classes (those that have a way to register hooks in your WordPress project):

protected function get_service_classes() : array {
  return [

    // Config.
    Config::class,

    // Manifest.
    Lib_Manifest\Manifest::class => [ Config::class ],

    // I18n.
    Lib_I18n\I18n::class => [ Config::class ],

    // Enqueue.
    Lib_Enqueue\Enqueue_Admin::class => [ Config::class, Lib_Manifest\Manifest::class ],
    Lib_Enqueue\Enqueue_Theme::class => [ Config::class, Lib_Manifest\Manifest::class ],
    Lib_Enqueue\Enqueue_Blocks::class => [ Config::class, Lib_Manifest\Manifest::class ],

    // Login.
    Lib_Login\Login::class,

    // Media.
    Media\Media::class,

    // Admin.
    Admin\Modify_Admin_Appearance::class,

    // Menu.
    Menu\Menu::class,

    // Blocks.
    Lib_Blocks\Blocks::class => [ Config::class ],
  ];
}

Whenever a class has a dependency on another class, you'd add this dependency through a process of constructor injection. For instance, the Enqueue_Admin class depends on both the Config and Manifest classes. Looking at the implementation we have:

class Enqueue_Admin implements Service {
  public function __construct( Config_Data $config, Manifest_Data $manifest ) {
    $this->config   = $config;
    $this->manifest = $manifest;
  }
}

Notice that we could add Lib_Manifest\Manifest as a type hint in the constructor of the Enqueue_Admin class and Lib_Manifest\Config as well, but instead, we used their interfaces: Config_Data and Manifest_Data. If we used concrete classes, we wouldn't have to add these in the get_service_classes() method array, because the DI container lib would be able to figure out what class the Enqueue_Admin class uses as a dependency through a process called reflection.

This, however, creates a tight coupling - the Enqueue_Admin class now always depends on these concrete classes, and we cannot easily replace them if we want to create automated tests. This is why we use interfaces since interface only mandates the contract our class needs to honor - not the implementation of that contract.

By adding the interfaces we need to say to our DI container what the classes, that implement these interfaces, actually are. This is why we needed to define:

Lib_Enqueue\Enqueue_Admin::class => [ Config::class, Lib_Manifest\Manifest::class ]

in our get_service_classes() method.

The beauty of this method is that we can easily add mocks in our automated tests:

protected function get_service_classes() : array {
  $services = [
    Lib_Enqueue\Enqueue_Admin::class => [ Config::class, Lib_Manifest\Manifest::class ],
    // Other classes go here.
  ];

  if ( getenv('TEST') ) {
    $services[ Lib_Enqueue\Enqueue_Admin::class ] => [ Fixture\Config::class, Fixture\Manifest::class ];
  }

  return $services;
}

If we had an environment variable TEST defined in our phpunit.xml defined, this means that the Lib_Enqueue\Enqueue_Admin class will get overwritten during the test run, and we can pass the fake classes from the Fixture namespace.

This is just a simple example, but one that could prove useful when writing testable code in the future, and something to be mindful of.

Useful hint

Don't pass too many dependencies in your classes. Having more than three dependencies is usually a sign that your class may be doing too much, and that it should probably be broken down into more different classes.

Useful hint​

Useful hint