Understanding Constructor, $this keyword, and controller class in PHP/Laravel

Question

I know this has been covered in pieces before, but I am struggling with how to apply it to my code.

I am developing a PHP application in Laravel but want to make my code more modular and testable, which means pulling my logic away from my heavyset controllers and separating them out into separate files and calling them from within the controller.

In one such controller (ImageController), I am calling upon the logic in ImageRepository.php and it looks like this:

    <?php
namespace App\Http\Controllers;
use App\Logic\Image\ImageRepository;
use Illuminate\Support\Facades\Input;
class ImageController extends Controller
{
    protected $image;
    public function __construct(ImageRepository $imageRepository)
    {
        $this->image = $imageRepository;
    }
    public function getUpload()
    {
        return view('pages.upload');
    }
    public function postUpload()
    {
        $photo = Input::all();
        $response = $this->image->upload($photo);
        return $response;
    }
    /*public function deleteUpload()
    {
        $filename = Input::get('id');
        if(!$filename)
        {
            return 0;
        }
        $response = $this->image->delete( $filename );
        return $response;
    }
    */
}

My problem is I don't understand how this code works, as I got it from another source and want to understand it so I can replicate this architecture elsewhere in my code.

My route when uploading an image is this:

Route::post('upload_image', ['as' => 'upload-post', 'uses'     =>'ImageController@postUpload']);

So my first question is, I never call the construct function in my route. It goes right to postUpload(). Does this mean it has no purpose? Also why does the construct function not have a comma between ImageRepository and $imageRepository... from my understanding of the docs you only do that if one of them is a boolean?

Also why does $response = $this->image->upload($photo); mean anything in postUpload()? That function upload() comes from the Repository, is use enough so that it knows what to do? Why does $this->image mean anything, what does the $this refer to? The ImageController class? Is the image in $this->image derived from protected $image?

I guess I should have stuck with regular PHP before moving to the Laravel framework because while I can navigate through Laravel easily enough to make a working application, it seems to be hindering my ability to follow best practices / architecture. AFAIK a controller is mainly something to manipulate data and send it to the view or the database, I don't understand perhaps why it is a class?

Sorry for the multiple questions I am just very confused. I taught myself php on codeacademy but their class declarations and object instances are very easy to follow, this is not. If someone could explain the code to me that would be very helpful.

Thank you!

`$this` can only EVER refer to the same object/class that the code using `$this` is a member of. every object is its own `$this`, and `$this` can never point at some other foreign object. it's like "localhost" for networking - every machine is its own localhost, and localhost cannot point elsewhere. — Marc B, Jul 08 '16 at 20:35
@MarcB so *what is the object/class* that the code using $this is a member of? — Peter Kionga-Kamau, Nov 12 '21 at 00:50

score 2 · Accepted Answer · edited May 23 '17 at 12:08

I never call the construct function in my route. It goes right to postUpload(). Does this mean it has no purpose?

The constructor is automatically called when the controller object is created.

why does the construct function not have a comma between ImageRepository and $imageRepository... from my understanding of the docs you only do that if one of them is a boolean?

ImageRepository is not another argument. It is a type hint (or in PHP 7, a type declaration)

why does $response = $this->image->upload($photo); mean anything in postUpload()? That function upload() comes from the Repository, is use enough so that it knows what to do?

The use is necessary for your controller to be able to use ImageRepository. Once it is loaded into your object in the constructor with $this->image = $imageRepository;, your controller methods have access to its methods, (such as upload) via $this->image.

Why does $this->image mean anything, what does the $this refer to?

Yes, $this does refer to the ImageController class. The PHP manual states:

The pseudo-variable $this is available when a method is called from within an object context. $this is a reference to the calling object

Is the image in $this->image derived from protected $image?

protected $image sets the visibility of the controller object's $image property. It does not assign anything to the property; that is done in the constructor.

Okay my last question is what does the `protected $image` do? It is never assigned a value so why is it there? Thank you so much! — Summer Developer, Jul 08 '16 at 20:39

Understanding Constructor, $this keyword, and controller class in PHP/Laravel

1 Answers1

Linked