Laravel Dependency Injection: When do you have to? When can you mock Facades? Advantages of either method?

Question

I\'ve been using Laravel for a while now and I have been reading a lot about Dependency Injection an testable code. I\'ve come to a point of confusion when talking about Fac

Answer 1

Although you use dependency injection on Option #1, your controller is still coupled with the Eloquent ORM. (Note that i avoid to use the term Model here because in MVC the Model is not just a class or an object but a layer. It's your business logic.).

Dependency Injection allows for Dependency Inversion but they are not the same thing. According to the Dependency Inversion principle both high and low level code should depend on abstractions. In your case the high level code is your controller and the low level code is the Eloquent ORM that fetches data from MySQL, but as you can see none of them depends on abstractions.

As a consequence, you are not able to change your data access layer without affecting your controller. How would you go about changing for example from MySQL to MongoDB or to the File System? To do this you have to use repositories (or whatever you want to call it).

So create a repositories interface that all your concrete repository implementations (MySQL, MongoDB , File System etc.) should implement.

interface PostRepositoriesInterface {

    public function getAll();
}

and then create your concrete implementation e.g. for MySQL

class DbPostRepository implements PostRepositoriesInterface {

    public function getAll()
    {

        return Post::all()->toArray();

        /* Why toArray()? This is the L (Liskov Substitution) in SOLID. 
           Any implementation of an abstraction (interface) should be substitutable
           in any place that the abstraction is accepted. But if you just return 
           Post:all() how would you handle the situation where another concrete 
           implementation would return another data type? Probably you would use an if
           statement in the controller to determine the data type but that's far from 
           ideal. In PHP you cannot force the return data type so this is something
           that you have to keep in mind.*/
    }
}

Now your controller must type hint the interface and not the concrete implementation. This is what "Code on an interface an not on implementation" is all about. This is Dependency Inversion.

class HomeController extends BaseController {

    public function __construct(PostRepositoriesInterface $repo)
    {
        $this->repo= $repo;
    }

    public function index()
    {
        $posts = $this->repo->getAll();

        return View::make( 'posts' , compact( $posts ) );
    }

}

This way your controller is decoupled from your data layer. It's open for extension but closed for modification. You can switch to MongoDB or to the File System by creating a new concrete implementation of PostRepositoriesInterface (e.g. MongoPostRepository) and change only the binding from (Note that i don't use any namespaces here):

App:bind('PostRepositoriesInterface','DbPostRepository');

to

App:bind('PostRepositoriesInterface','MongoPostRepository');

In an ideal situation your controller should contain only application and not business logic. If you ever find yourself wanting to call a controller from another controller its a sign that you've done something wrong. In this case your controllers contain too much logic.

This also makes testing easier. Now you are able to test your controller without actually hitting the database. Note that a controller test must test only if the controller functions properly which means that the controller calls the right method, gets the results and pass it to the view. At this point you are not testing the validity of the results. This is not controller's responsibility.

public function testIndexActionBindsPostsFromRepository()
{ 

    $repository = Mockery::mock('PostRepositoriesInterface');

    $repository->shouldReceive('all')->once()->andReturn(array('foo'));

    App::instance('PostRepositoriesInterface', $repository);

    $response = $this->action('GET', 'HomeController@index'); 

    $this->assertResponseOk(); 

    $this->assertViewHas('posts', array('foo')); 
}

EDIT

If you choose to go with option #1 you can test it like this

class HomeControllerTest extends TestCase {

  public function __construct()
  {
      $this->mock = Mockery::mock('Eloquent', 'Post');
  }

  public function tearDown()
  {
      Mockery::close();
  }

  public function testIndex()
  {
      $this->mock
           ->shouldReceive('all')
           ->once()
           ->andReturn('foo');

      $this->app->instance('Post', $this->mock);

      $this->call('GET', 'posts');

      $this->assertViewHas('posts');
  }

}