Zend\Expressive\Emitter\EmitterStack::push PHP Method

    public function push($emitter)
    {
        $this->validateEmitter($emitter);
        parent::push($emitter);
    }

 /**
  * Create and return an Application instance.
  *
  * Will inject the instance with the container and/or router when provided;
  * otherwise, it will use a ZF2 ServiceManager instance and the FastRoute
  * router bridge.
  *
  * The factory also injects the Application with an Emitter\EmitterStack that
  * composes a SapiEmitter at the bottom of the stack (i.e., will execute last
  * when the stack is iterated).
  *
  * @param null|ContainerInterface $container IoC container from which to
  *     fetch middleware defined as services; defaults to a ServiceManager
  *     instance
  * @param null|Router\RouterInterface $router Router implementation to use;
  *     defaults to the FastRoute router bridge.
  * @return Application
  */
 public static function create(ContainerInterface $container = null, Router\RouterInterface $router = null)
 {
     $container = $container ?: new ServiceManager();
     $router = $router ?: new Router\FastRouteRouter();
     $emitter = new Emitter\EmitterStack();
     $emitter->push(new SapiEmitter());
     return new Application($router, $container, null, $emitter);
 }