Symfony 4.1: How to use Dependency Injection in UnitTest (Swift_Mailer) - unit-testing

Question

In my Symfony4.1-project i am trying to test a method, which should send mails using SwiftMailer, via a unit test.
My test class looks like this
namespace App\Tests;
use App\Controller\UserImageValidationController;
use Symfony\Bundle\FrameworkBundle\Tests\TestCase;
class UserImageValidationControllerTest extends TestCase
{
private $mailer = null;
public function __construct(\Swift_Mailer $testmySwiftMailer)
{
$this->mailer = $testmySwiftMailer;
}
public function testMail()
{
$controller = new UserImageValidationController();
$controller->notifyOfMissingImage(
'a',
'b',
'c',
'd',
$this->mailer
);
}
}
The problem is, that when i run ./bin/phpunit i get an exception saying
Uncaught ArgumentCountError: Too few arguments to function App\Tests\UserImageValidationControllerTest::__construct(), 0 [...] and exactly 1 expected [...]
It seemes like in the test environment DI isn't working.
So i added
bind:
$testmySwiftMailer: '#swiftmailer.mailer.default'
To my config/services_test.yaml but i still got the same error.
I also added autowiring: true to that file (just to try it) and it also doesn't work.
Also, i tried with a service-alias, like it states in the file's comments: still no success.
How do i get the swiftmailer injected into my test case constructor?

Answer 1

Tests are not part of the container and don't act as services so your solution is not valid. Extend Symfony\Bundle\FrameworkBundle\Test\KernelTestCase and do that instead (making sure your service is public first):
protected function setUp()
{
static::bootKernel();
$this->mailer = static::$kernel->getContainer()->get('mailer');
}
protected function tearDown()
{
$this->mailer = null;
}

Answer 2

The accepted answer will not work for services that are not defined as public. However after Symfony 4.1, to be able to access private services while on testing, you need to get the service from a special testing container.
From the Symfony documentation:
tests based on WebTestCase and KernelTestCase now access to a special container via the static::$container property that allows fetching non-removed private services
example:
namespace App\Tests;
use App\Controller\UserImageValidationController;
use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;
class UserImageValidationControllerTest extends WebTestCase
{
private $mailer = null;
protected function setUp()
{
self::bootKernel();
// gets the special container that allows fetching private services
$container = self::$container;
$this->mailer = $container->get('mailer');
}
public function testMail()
{
$controller = new UserImageValidationController();
$controller->notifyOfMissingImage(
'a',
'b',
'c',
'd',
$this->mailer
);
}
}