Symfony AutoBind Parameter is Dead, Long live Constant Parameters

2020-09-21

In the past, we used YAML to define parameters for Symfony application:

parameters:
    location: Prague

How do we get parameter in a service? Via name auto binding: the parameter name = the param name in __construct:

final class SomeService
{
    private string $location;

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

        dump($location); // "Prague"
    }
}

This allows us to have a typed parameter right in the constructor. We know it's a string.

YAML Without Knowledge

• But where is the location param defined?
• Where are all the places the parameter is used in?
• Is the string $location in the __construct really a constructor parameter? What if that's a value object?

$valuesObjects = [
    new SomeService('Prague'),
    new SomeService('Berlin'),
    new SomeService('London'),
];

We don't know. We can be either anxious about it or not care about this detail at all.

Or we could quickly know with 1 click in PHPStorm. How?

Welcome Constant Parameters

With Symfony configs in *.php format, this is easypick:

use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;

return function (ContainerConfigurator $containerConfigurator): void {
    $parameters = $containerConfigurator->parameters();
    $parameters->set('location', 'Prague');
};

Wait, this was just a string. We want constant parameters:

use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;
use App\Configuration\Option;

return function (ContainerConfigurator $containerConfigurator): void {
    $parameters = $containerConfigurator->parameters();
    $parameters->set(Option::LOCATION, 'Prague');
};

When Constant Parameter and Service Parameter Meet

We still want to avoid service configuration... we have 2 options.

• A. Use Symfony parameter bag:

use App\Configuration\Option;
use Symfony\Component\DependencyInjection\ParameterBag\ParameterBagInterface;

final class SomeService
{
    private string $location;

    public function __construct(ParameterBagInterface $parameterBag)
    {
        // re-type to (string) is needed, because "get()" return mixed type
        $this->location = (string) $parameterBag->get(Option::LOCATION);
    }
}

• B. Use Symplify ParameterProvider:

use App\Configuration\Option;
use Symplify\PackageBuilder\Parameter\ParameterProvider;

final class SomeService
{
    private string $location;

    public function __construct(ParameterProvider $parameterProvider)
    {
        // no re-type needed + parameter type validation included inside the ParameterProvider service
        $this->location = $parameterProvider->provideStringParameter(Option::LOCATION);
    }
}

Both services work to find first is out of the box, second is mutable and useful for testing or post-container configurations. Pick the one that suits your project better.

Are you unsure? Use the Symfony ParameterBagInterface.

One More Thing...

We now have a clean design and constant parameters without any name <=> name hacking. That's nice.

But the best is yet to come. In the previous version, we were missing essential information about the parameter:

Where are the places the parameter is used in?

Well, why not just click on the constant in your IDE?

Happy coding!

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