Laravel dependency injection
============================

:Published at: March 23, 2020
:Author: Erlend ter Maat
:Tags: Laravel, Dependency injection

----

I wrote about `dependency injection in Drupal </blog/2020/03/10/dependency-injection.html>`_ before. I also mentioned
that other frameworks can make it more easy! At Drupal one still has to write some extra code to make dependency
injection work. The creators of Laravel made the system to write cleaner code even more clean.

It is that simple!
------------------

Inject services in the constructor
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Given an example service:

.. code-block:: php

 class ExampleDependency {

 }

 class ExampleService {

     public function __construct(ExampleDependency $dependency) {

     }

 }

 $serviceInstance = resolve(ExampleService::class);

At laravel every class can be used as a service without any configuration!

Inject services in member methods
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: php

 class CurrentUser {

     public $name = 'Bob';

 }

 class ExampleService {

     public function sayHi(CurrentUser $account) {
         return "Hi {$account->name}!";
     }

 }

 app()->call([resolve(ExampleService::class), 'sahHi']);

How?
----

Under the hood Laravel uses `reflection <https://www.php.net/manual/en/intro.reflection.php>`_ to find out what a class
needs for constructing. If the class resolve mechanism finds a class definition it tries to resolve it as a service,
until all requirements are resolved. It then creates a new instance of the class.

Service container binding
-------------------------

Often you would like to look at services in a more abstract way.

.. code-block:: php

 interface SomeContract {

 }

 class ExampleDependency implements SomeContract {

 }

 class ExampleService {

      public function __construct(SomeContract $someContract) {

      }

 }

To make this resolve one has to specify the implementation of the contract in a service container:

.. code-block:: php

 class AppServiceContainer {

     public function register() {
         $app->bind(SomeContract::class, function () {
             return new ExampleDependency;
         }
     }

 }

 // Resolve by contract would now result in the
 // registered implementation of the contract.
 resolve(SomeContract::class);


Conclusion
----------

Laravel allows you to use any class as a service without any extra configuration or custom code. Of course your are not
required to only instantiate all classes using the service mechanism, but this system does make it easy for you to work
with dependency injection, and therefore it encourages you to write clean and maintainable code.