特质访问类依赖是一个坏主意吗？

Question

I have seen an example on Stackexchange (please note the trait accessing the class property):

trait CheckPermissionTrait
{
    protected function checkPermission($object_id)
    {
        $judge = $this->container->get('acme_judge');

        $user  = $this->container->get('security.context')->getToken()->getUser();

        if( !$judge->isPermitted($user, $object_id) ) {
            throw $this->createAccessDeniedException("Brabbel");
        }
    }
}

And read one of the repliers comments:

Your trait, then is not a valid use-case: all of its users are required, by definition, to add a $this->container property to its dependencies, which will of course have an impact on that class' contract and the contract of its children.

Why is the author claiming that being a bad use case, if that might be what someone needs? Like if someone has few classes that all have the required dependency and the same logic reccures in all of them, should they just keep the code duplicated?

Answer 1

Indeed trait used in this way - a bad idea. If someone decide to use this trait in your code - he must ensure the existence of "container" attribute. "container" should be the correct type (include the methods used) - otherwise it will get an error. In fact this code can not be reused, and this potentially bug. In addition, it violates a rule DIP (dependency inversion principle) of the rules of SOLID.

It is possible to get around this:

interface ExampleContainerInterface{

}
trait CheckPermissionTrait
{
    protected $container;
    public function __construct(ExampleContainerInterface $container)
    {
        $this->container = $container;
    }

    protected function checkPermission($object_id)
    {
        $judge = $this->container->get('acme_judge');
        $user  = $this->container->get('security.context')->getToken()->getUser();
        if( !$judge->isPermitted($user, $object_id) ) {
            throw $this->createAccessDeniedException("Brabbel");
        }
    }
}

class ExampleClassA
{
    use CheckPermissionTrait;
}
class ExampleClassB
{
    use CheckPermissionTrait;
}

or like this (php7):

interface ExampleContainerInterface{

}
trait CheckPermissionTrait
{
    abstract public function getContainer():ExampleContainerInterface;
    protected function checkPermission($object_id)
    {
        $container = $this->getContainer();
        $judge = $container->get('acme_judge');
        $user  = $container->get('security.context')->getToken()->getUser();
        if( !$judge->isPermitted($user, $object_id) ) {
            throw $this->createAccessDeniedException("Brabbel");
        }
    }
}

class ExampleClassA
{
    use CheckPermissionTrait;
    protected $container;
    public function getContainer():ExampleContainerInterface
    {
        return $this->container;
    }
}
class ExampleClassB
{
    use CheckPermissionTrait;

    protected $container;
    public function getContainer():ExampleContainerInterface
    {
        return $this->container;
    }
}