Simple Commerce Adjuster implementation

TestAdjuster.php

<?php
/**
 * Example Adjuster.
 * Place in: `/myplugin/adjusters/Test.php` (and rename as such)
 *
 * @link      https://oof.studio/
 * @copyright Copyright (c) 2018 oof. Studio
 */
namespace oofbar\test\adjusters;

use Craft;
use craft\base\Component;

use craft\commerce\base\AdjusterInterface;
use craft\commerce\elements\Order;
use craft\commerce\models\OrderAdjustment;

use oofbar\test\Test;

/**
 * Example Adjuster
 * 
 * 5% discount on all line items.
 *
 * @author oof. Studio <[email protected]>
 * @since 0.1
 */

class Test extends Component implements AdjusterInterface
{
    // Constants
    // =========================================================================

    /**
     * The discount adjustment type.
     */
    const ADJUSTMENT_TYPE = 'test-example';

    // Public Methods
    // =========================================================================

    /**
     * @inheritdoc
     */
    public function adjust(Order $order): array
    {
        $adjustments = [];

        foreach ($order->lineItems as $lineItem)
        {
            $adjustment = $this->_getEmptyAdjustment($order);
            $adjustment->setLineItem($lineItem);

            // Calculate Price diff:
            $basePrice = $lineItem->getSubtotal();
            $discountAmount = $basePrice * -0.05;
            $adjustment->amount = $discountAmount;

            $adjustments[] = $adjustment;
        }
      
        return $adjustments;
    }

    // Private Methods
    // =========================================================================

    private function _getEmptyAdjustment(Order $order)
    {
        $adjustment = new OrderAdjustment();
        $adjustment->type = self::ADJUSTMENT_TYPE;
        $adjustment->name = '5% Discount';
        $adjustment->orderId = $order->id;
        $adjustment->description = 'Discount for nice people';
        $adjustment->sourceSnapshot = [
            'privateProp' => 'Criteria you want to make sure you have access to, in case you have to recalculate, later!'
        ];

        return $adjustment;
    }
}

TestPlugin.php

<?php
/**
 * Example Adjuster plugin.
 * Place in: `/myplugin/Test.php` (and rename as such)
 *
 * @link      https://oof.studio/
 * @copyright Copyright (c) 2018 oof. Studio
 */
namespace oofbar\test;

use oofbar\test\adjusters\Test as TestAdjuster;

use Craft;
use craft\base\Plugin as BasePlugin;
use craft\events\RegisterComponentTypesEvent;

use craft\commerce\services\OrderAdjustments;

use yii\base\Event;

/**
 * Test Plugin
 *
 * @author    oof. Studio
 * @package   Test
 * @since     0.1
 */
class Test extends BasePlugin
{
    // Static Properties
    // =========================================================================

    /**
     * Static property that is an instance of this plugin class so that it can be accessed via
     * Test::$plugin
     *
     * @var Plugin
     */
    public static $plugin;

    // Public Methods
    // =========================================================================

    /**
     * Set our $plugin static property to this class so that it can be accessed via
     * Test::$plugin
     *
     * Called after the plugin class is instantiated; do any one-time initialization
     * here such as hooks and events.
     *
     * If you have a '/vendor/autoload.php' file, it will be loaded for you automatically;
     * you do not need to load it in your init() method.
     *
     */
    public function init()
    {
        parent::init();
        self::$plugin = $this;

        // Add our custom Adjusters, pushing them onto the front of the queue (so they take precedence over Tax/Discounts/Shipping):
        Event::on(
            OrderAdjustments::class,
            OrderAdjustments::EVENT_REGISTER_ORDER_ADJUSTERS,
            function(RegisterComponentTypesEvent $event) {
                array_unshift(
                    $event->types,
                    TestAdjuster::class
                );
                return $event->types;
            });
    }
}