The container

The container is PSR-11 and its stated goal is speed: what a class needs is worked out once by reflection and remembered, rather than read again on every request.

$container = new Container();
$controller = $container->get(UserController::class);

It reads constructors and typed public properties, so a class asks for what it needs by declaring it:

final class UserController implements ControllerInterface
{
    public function __construct(
        private readonly UserResponse $response,
        private readonly Orm $orm,
    ) {
    }
}

Interfaces

An interface is bound to what should answer to it:

$container = new Container([
    LoggerInterface::class => Logger::class,
    CacheInterface::class => new FileCache($storage, __DIR__ . '/../var/cache'),
]);

A class asking for LoggerInterface then gets a Logger. An application passes these to App along with its route provider — that list is the container configuration.

Nothing requires it

No package in Quillstack needs a container to be used. Every one of them can be built by hand:

$factory = new UriFactory();
$client = new Client();
$serializer = new Serializer();

That is a rule rather than an accident. A component that cannot be constructed without first reading another component's documentation is not a simple component, whatever its own documentation says.

More: Container.