MiddlewareInterface

Middlewares must take a MiddlewareInterface as their first constructor argument {Google\ApiCore\Middleware\ResponseMetadataMiddleware}, which represents the next middleware in the chain. This next middleware MUST be invoked by this MiddlewareInterface, and the result must be returned as part of the __invoke method implementation.

To create your own middleware, first implement the interface, as well as pass the handler in through the constructor:

use Google\ApiCore\Call;
use Google\ApiCore\Middleware\MiddlewareInterface;

class MyTestMiddleware implements MiddlewareInterface
{
    public function __construct(MiddlewareInterface $handler)
     {
.         $this->handler = $handler;
     }
     public function __invoke(Call $call, array $options)
     {
         echo "Logging info about the call: " . $call->getMethod();
         return ($this->handler)($call, $options);
     }
}

Next, add the middleware to any class implementing GapicClientTrait by passing in a callable which returns the new middleware:

$client = new ExampleGoogleApiServiceClient();
$client->addMiddleware(function (MiddlewareInterface $handler) {
    return new MyTestMiddleware($handler);
});