Interface default method use case

events.php

<?php

interface CarriesError
{
    public function setError(Error $error): static;

    public function getError(): ?Error;
}

interface CarriesResponse
{
    public function setResponse(ResponseInterface $response): static;

    public function getResponse(): ?ResponseInterface;
}

trait ErrorCarrier
{
    private Error $error;

    public function setError(Error $error): static
    {
        $this->error = $error;
        return $this;
    }

    public function getError(): ?Error
    {
        return $this->error ?? null;
    }
}

trait ResponseCarrier
{
    private ResponseInterface $response;

    public function setResponse(ResponseInterface $response): static
    {
        $this->response = $response;
        return $this;
    }

    public function getResponse(): ?ResponseInterface
    {
        return $this->response ?? null;
    }
}

class PreRouting implements StoppableEventInterface, CarriesResponse, CarriesError
{
    use ResponseCarrier;
    use ErrorCarrier;

    // @todo Here's where we want asymmetric visibility.
    public function __construct(
        private ServerRequestInterface $request,
    ) {}

    public function request(): ServerRequestInterface
    {
        return $this->request;
    }

    public function setRequest(ServerRequestInterface $request): static
    {
        $this->request = $request;
        return $this;
    }

    public function isPropagationStopped(): bool
    {
        return isset($this->response) || isset($this->error);
    }
}

class PostRouting implements StoppableEventInterface, CarriesResponse, CarriesError
{
    use ResponseCarrier;
    use ErrorCarrier;

    // @todo Here's where we want asymmetric visibility.
    public function __construct(
        private ServerRequestInterface $request,
    ) {}

    public function request(): ServerRequestInterface
    {
        return $this->request;
    }

    public function setRequest(ServerRequestInterface $request): static
    {
        $this->request = $request;
        return $this;
    }

    public function isPropagationStopped(): bool
    {
        return isset($this->response) || isset($this->error);
    }
}

class HandleResponse implements CarriesResponse
{
    public function __construct(
        private ResponseInterface $response,
        public readonly ServerRequestInterface $request,
    ) {}

    public function setResponse(ResponseInterface $response): static
    {
        $this->response = $response;
        return $this;
    }

    public function getResponse(): ResponseInterface
    {
        return $this->response;
    }
}