opencase/modules/opencase_cases/opencase_cases.module

<?php

/**
 * @file
 * Contains opencase_cases.module.
 */

use Drupal\Core\Routing\RouteMatchInterface;

/**
 * Implements hook_help().
 */
function opencase_cases_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    // Main module help for the opencase_cases module.
    case 'help.page.opencase_cases':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('Provides case entities and a required field on activities linking them to a case.') . '</p>';
      return $output;

    default:
  }
}

/**
 * Implements hook_theme().
 */
function opencase_cases_theme() {
  return [
    'opencase_cases' => [
      'render element' => 'children',
    ],
  ];
}

function opencase_cases_entity_base_field_info($entity_type) {
  $fields = array();
  
  // Add Case field to Activity 
  if ($entity_type->id() === 'oc_activity') {
    $fields['oc_case'] = BaseFieldDefinition::create('entity_reference')
      ->setLabel(t('Case'))
      ->setDescription(t('The case this activity belongs to.'))
      ->setSetting('target_type', 'oc_case')
      ->setSetting('handler', 'default')
      ->setTranslatable(TRUE)
      ->setCardinality(1)
      ->setDefaultValueCallback('opencase_cases_default_activity_case_value')
      ->setDisplayOptions('view', [
        'type' => 'string',
        'weight' => -3,
      ])
      ->setRequired(TRUE);
  }
  return $fields;
}

/**
 * When creating an activity, it sets the case id from the URL.
 */
public static function opencase_cases_default_activity_case_value() {
  return array(\Drupal::request()->query->get('case_id'));
}

/**
 * Implementation of hook_form_alter()
 * Changes what page is redirected to after deleting things
 */
function opencase_cases_form_alter(&$form, &$form_state, $form_id) {
  if (preg_match('/oc_actor_.*_delete_form/', $form_id) or (preg_match('/oc_case_.*_delete_form/', $form_id)) or (preg_match('/oc_organisation_.*_delete_form/', $form_id))) {
    $form['actions']['submit']['#submit'][] = '_opencase_cases_redirect_to_home';
    $form['actions']['cancel']['#url'] = $form_state->getFormObject()->getEntity()->toUrl();
  }
  if (preg_match('/oc_activity_.*_delete_form/', $form_id)) {
    $form['actions']['submit']['#submit'][] = '_opencase_cases_delete_activity_redirect';
    $form['actions']['cancel']['#url'] = $form_state->getFormObject()->getEntity()->toUrl();
  }
}

function _opencase_cases_redirect_to_home($form, &$form_state) {
  $form_state->setRedirect('<front>'); 
}
function _opencase_cases_delete_activity_redirect($form, &$form_state) {
  $case_id = $form_state->getFormObject()->getEntity()->oc_case->target_id;
  $form_state->setRedirect('entity.oc_case.canonical', ['oc_case' => $case_id]);
}