Notifier

config.yml

Injector:
  SlackNotifier:
    factory: 'SlackNotificationFactory'

SlackNotificationFactory.php

<?php

use SilverStripe\Framework\Injector\Factory;

/**
 * Creates a SlackNotifier from constants if they're set. If not, then it will
 * return a NullNotifier.
 *
 * This means we don't have to have constant "defined" checks anywhere we need to use slack.
 */
class SlackNotificationFactory implements Factory {

	/**
	 * {@inheritdoc}
	 *
	 * @return Notifier
	 */
	public function create($service, array $params = []) {
    
		if(defined('DEPLOYMENT_BACKEND_ALERT_URI') && defined('DEPLOYMENT_BACKEND_ALERT_CHANNEL')) {
      
			$username = defined('DEPLOYMENT_BACKEND_USERNAME') ? DEPLOYMENT_BACKEND_USERNAME : 'Deploynaut';

			return new SlackNotifier(
				DEPLOYMENT_BACKEND_ALERT_URI,
				DEPLOYMENT_BACKEND_ALERT_CHANNEL,
				$username
			);
      
		}
    
    		return new NullNotifier();
	}

}

Usage.php

<?php

// SlackNotifer and NullNotifier both implement a comment 'Notifier' interface
//
// In live or in testing, wherever slack constants are defined, we will get
// an instance of SlackNotifier which will notify to the specified channel.
//
// In dev, we can omit these constants which means we get an instance of 
// NullNotifier which notifies into nothing.

$notifier = SlackNotificerFactory::create();

// or...

$notifier = Injector::inst()->get('SlackNotifier');