How to implement cash rounding (e.g. for Swiss Francs CHF) for Commerce 2

We evaluated this feature for 2.x back in 2015 with a number of our Swiss contributors, and the consensus was that the Commerce 1.x feature was not implemented correctly and shouldn't be ported over.

Rounding to the nearest 0.05 CHF is called cash rounding, or Swedish rounding.
By definition it should only be applied to the order total (19.93 -> 19.95), and it should only be done if the order will be paid in cash.

The Commerce 1.x approach is to do this via currency rounding, which means that each individual (per-order-item) VAT amount and discount is rounded to the nearest 0.05, and done regardless of the payment method used.

Assuming that our understanding is correct, the right approach would be to provide a custom adjustment type (called "Rounding" or something similar), and an order processor, which would calculate the required amount, and add it as an order-level adjustment (using the newly defined adjustment type). The order processor would also need to look at $order->payment_gateway to decide whether to kick-in. That would also allow sites to decide whether to always round, or to round for a specific gateway only (such as "cash on delivery").

I think that it would be best for this functionality to live in a contrib module (commerce_cash_rounding?), and not in Commerce core.

Improving the title.

Hi M_Z,
1. I'd suggest to look at commerce_shipping's order processor, ShipmentOrderProcessor. The ones in tax and promotion are a bit more complicated.
2. No, you don't need to define the adjustment type. You can just use any string you want for the type. See Bojan's answer below.

I would still define a custom adjustment type, to make it cleaner.

In yourmodule.commerce_adjustment_types.yml:

rounding:
  label: 'Rounding'
  singular_label: 'rounding adjustment'
  plural_label: 'rounding adjustments'
  has_ui: true
  weight: 20

And an example order processor in src/RoundingOrderProcessor.php:

<?php

namespace Drupal\yourmodule;

use Drupal\commerce_order\Adjustment;
use Drupal\commerce_order\Entity\OrderInterface;
use Drupal\commerce_order\OrderProcessorInterface;

/**
 * Processes the order's shipments.
 */
class RoundingOrderProcessor implements OrderProcessorInterface {

  /**
   * {@inheritdoc}
   */
  public function process(OrderInterface $order) {
    $payment_gateway = $order->get('payment_gateway')->entity;
    if (!$payment_gateway) {
      // The payment gateway hasn't been selected yet.
    }

    if ($payment_gateway->id() == 'your_gateway') {
      $rounder = \Drupal::service('commerce_price.rounder');
      $order_total = $order->getTotalPrice();
      // Rounds down 10.21 and 10.22 to 10.20, rounds up 10.23 and 10.24 to 10.25. 
      $rounded_total = $rounder->round($order_total->divide(5))->multiply(5);
      $difference = $rounded_total->subtract($order_total);

      $order->addAdjustment(new Adjustment([
        'type' => 'rounding',
        'label' => 'Rounding',
        'amount' => $difference,
      ]));
    }
  }

}

Don't forget the entry in yourmodule.services.yml:

  yourmodule.rounding_order_processor:
    class: Drupal\yourmodule\RoundingOrderProcessor
    tags:
      - { name: commerce_order.order_processor, priority: -10 }

Closing since an answer was given in #6.