Dump Drupal listeners, including subscribers

drush.services.yml

# Sample services file for a "rdcm" module holding the commands.
services:
  rdcm.kernel_commands:
    class: Drupal\rdcm\Commands\KernelCommands
    arguments:
      - '@event_dispatcher'
    tags:
      - { name: drush.command }

KernelCommands.php

<?php
// Place in (module)/src/Commands

namespace Drupal\rdcm\Commands;

use Drush\Commands\DrushCommands;
use Robo\Common\OutputAwareTrait;
use Symfony\Component\EventDispatcher\EventDispatcherInterface;
use Symfony\Component\Yaml\Yaml;

/**
 * Class KernelCommands contains generic Drupal commands.
 */
class KernelCommands extends DrushCommands {

  use OutputAwareTrait;

  /**
   * The event_dispatcher service.
   *
   * @var \Symfony\Component\EventDispatcher\EventDispatcherInterface
   */
  protected $dispatcher;

  /**
   * KernelCommands constructor.
   *
   * @param \Symfony\Component\EventDispatcher\EventDispatcherInterface $dispatcher
   *   The event_dispatcher service.
   */
  public function __construct(EventDispatcherInterface $dispatcher) {
    parent::__construct();
    $this->dispatcher = $dispatcher;
  }

  /**
   * Show listeners command.
   *
   * @command dispatcher:listeners
   */
  public function showListeners() {
    $rp = new \ReflectionProperty(get_class($this->dispatcher), 'listeners');
    $rp->setAccessible(TRUE);
    $listeners = $rp->getValue($this->dispatcher);
    ksort($listeners);
    $dump = Yaml::dump($listeners, 3, 6);
    $this->output()->writeln($dump);
  }

}