Symfony 4, get the root path of the project from a custom class (not a controller class)

In Symfony AppKernel class is handling the project root directory under method getProjectDir(). To get it in the controller you can do:

$projectRoot = $this->get('kernel')->getProjectDir();

it will return you a project root directory.

If you need the project root directory in one of your classes you have two choices which I will present to you. First is passing AppKernel as dependency:

class Foo 
{
    /** KernelInterface $appKernel */
    private $appKernel;

    public function __construct(KernelInterface $appKernel)
    {
        $this->appKernel = $appKernel;
    }
}

Thanks to Symfony 4 autowiring dependencies it will be autmomaticaly injeted into your class and you could access it by doing:

$this->appKernel->getProjectDir();

But please notice: I don't think it's a good idea, until you have real need and more to do with AppKernel class than getting the project root dir. Specially if you think later on creating about unit tests for your class. You would automatically increase complexity by having a need to create mock of AppKernel for example.

Second option and IMHO better would be to pass only a string with path to directory. You could achieve this by defining a service inside config/services.yaml like this:

services:
    (...)
    MyNamespace\Foo:
        arguments:
            - %kernel.project_dir%

and your constructor would look like:

class Foo 
{
    /** string $rootPath */
    private $rootPath;

    public function __construct(string $rootPath)
    {
        $this->rootPath = $rootPath;
    }
}

Without Kernel injection

config/services.yaml

services:
    _defaults:
        autowire: true
        autoconfigure: true
        bind:
            $projectDir: '%kernel.project_dir%'

....

class Foo
{
    private $projectDir;

    public function __construct(string $projectDir)
    {
        $this->projectDir = $projectDir;
    }
}

If your class is extending: Symfony\Bundle\FrameworkBundle\Controller\AbstractController, then you can get the root dir as

$projectRoot = $this->getParameter('kernel.project_dir');

or

Inject ContainerBagInterface to your controller

protected $projectRoot;

public function __construct(ContainerBagInterface $containerBag)
{
    $this->projectRoot = $containerBag->get('kernel.project_dir');;
}

or

Even better and the recommended approach

Inject the root_dir to your Foo class. Add the following to your config under services

services:
    foo:
        class:     App\Path\To\Foo
        arguments: ['%kernel.project_dir%']

The container should pass the argument to your class on resolving, the Foo class should look like this

<?php

namespace App\Path\To;

class Foo {

  private $projectDir;

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