Drupal 8 Services Example using PHP Traits and pass by reference

page--template-example.html.twig

<style>
.grid {
  display: grid;
  grid-gap: 10px;
  grid-template-columns: repeat(auto-fill, minmax(250px,1fr));
  grid-auto-rows: 40px;
}
</style>
<script>
  var site_url = '{{ base_url }}';
</script>
<!-- Fixed navbar -->
{% include directory ~ '/partials/header.html.twig' %}
{%
  set classes = [
    'paragraph',
    'paragraph--type--' ~ paragraph.bundle|clean_class,
    'paragraph-id--' ~ paragraph.id(),
    ]
%}

    {# page-banner #}
 <div class="container-fluid">
   <div class="row">
     <div class="col-md-12">
       <div id="page-title-bg" style="background-image: url('{{ file_url(node.field_image.entity.fileuri) }}')"></div>
     </div>
   </div>
 </div>
 {# End page-banner #}

<!-- contain -->
<div class="container" id="main-container">
  <!-- Main content -->
  <div class="row">
    {{ drupal_entity('block', 'cpv3_local_tasks') }}
    <div class="col-md-12" style="padding-left: 30px;">
      <h1>{{ node.label }}</h1>
    </div>
  </div>
 <!-- Left-side bar -->
<div class="row content-wrapper" id="community-plan-update">
    <div class="col-md-9">
      <div class="section-content clear">
        <div class="row ">
            {% if node.field_slider %}
              {% for parag_id in node.field_slider.value %}
                {{ bamboo_render_entity('paragraph', parag_id['target_id']) }}
              {% endfor %}
            {% endif %}
          </div>
        </div>
        {# menu #}
        <div class="row">
          <div class="col-md-12 dropdown">
            <div id="hollywood-tab">
    <ul class="nav nav-tabs" role="tablist">
          {% for item in downtown_menu %}
                  {% if loop.index == 0 %}
                    {% set active_class = 'active' %}
                    {% set class = 'padding-left0'  %}
                  {% else %}
                    {% set active_class = '' %}
                    {% set class = '' %}
                  {% endif %}

                    <li role="presentation" class="padding-top0 {{ (loop.last) ? '' : 'tab-line' }}{{ active_class }}">

                    <a href="#{{ item['name']|clean_id }}" tabindex="0" aria-controls="{{ item.counter }}" role="tab" data-toggle="tab" data-value="{{ item['name'] }}" class="{{ class }}">
                    {{item['name']}} </a>
                      {% for sub in item['sub'] %}
                       {% if sub is not empty and loop.first %}
                       <i class="fa fa-caret-down"></i>
                     {% endif %}
                       {% endfor %}

  <ul class="dropdown-submenu" role="tablist">
    {% for sub in item['sub'] %}
      {% if sub is not empty %}
                      <li role="presentation" class="childLevel-1 padding-top0 {{ active_class }}">

                        <a href="#{{ sub['name']|clean_id }}" tabindex="0" aria-controls="{{ item.counter }}" role="tab" data-toggle="tab" data-value="{{ sub['name'] }}" style="cursor: pointer;" class="{{ class }}">
                      {{sub['name']}}</a></li>
{% endif %}
{% endfor %}
</ul>
</li>
{% endfor %}
</ul>
</div>
</div>
</div>
<!-- End hollywood-tab dropdown menu, taxonomy terms-->

                <!-- Begin editor UI -->
    <div class="row" id="hollywood">
      <div class="tab-content">
        {% for item in downtown_menu %}
          {% if loop.index == 1 %}
            <div class="tab-pane active fade in row padding-left0" id="{{ item['name']|clean_class }}">
              {% endif %}
            {% endfor %}
          {% if node.field_tab_about_content_items %}
            {% for parag_id in node.field_tab_about_content_items.value %}
              {{ bamboo_render_entity('paragraph', parag_id['target_id']) }}
            {% endfor %}
          {% endif %}
    </div>
    {% for item in downtown_menu %}
             {% if loop.index == 1 %}
    {% for sub in item['sub'] %}
      {% if sub is not empty %}
      <div class="tab-pane fade in row" id="{{ sub['name']|clean_class }}">
{% set submenu_content = drupal_field('field_submenu_content_order_1', 'node') %}
         {% if loop.index == 1 %}
              <h3>{{ sub['name'] }}</h3>
         {{ submenu_content.0 }}
         {% endif %}
         {% if loop.index == 2 %}
              <h3>{{ sub['name'] }}</h3>
         {{ submenu_content.1 }}
         {% endif %}
         {% if loop.index == 3 %}
              <h3>{{ sub['name'] }}</h3>
         {{ submenu_content.2 }}
         {% endif %}
         {% if loop.index == 4 %}
              <h3>{{ sub['name'] }}</h3>
         {{ submenu_content.3 }}
         {% endif %}
         {% if loop.index == 5 %}
              <h3>{{ sub['name'] }}</h3>
         {{ submenu_content.4 }}
         {% endif %}
  </div>

       {% endif %}
       {% endfor %}
       {% endif %}
       {% endfor %}

        {% for item in downtown_menu %}
          {% if loop.index == 2 %}
            <div class="tab-pane fade in row padding-left0" id="{{ item['name']|clean_class }}">
            {% endif %}
          {% endfor %}
          {% if node.field_tab_plan_content_items %}
            {% for parag_id in node.field_tab_plan_content_items.value %}
              {{ bamboo_render_entity('paragraph', parag_id['target_id']) }}
            {% endfor %}
          {% endif %}
        </div>
        {% for item in downtown_menu %}
                 {% if loop.index == 2 %}
        {% for sub in item['sub'] %}
        {% if sub is not empty %}
        <div class="tab-pane fade in row" id="{{ sub['name']|clean_class }}">
  {% set submenu_content = drupal_field('field_submenu_content_order_2', 'node') %}
           {% if loop.index == 1 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.0 }}
           {% endif %}
           {% if loop.index == 2 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.1 }}
           {% endif %}
           {% if loop.index == 3 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.2 }}
           {% endif %}
           {% if loop.index == 4 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.3 }}
           {% endif %}
           {% if loop.index == 5 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.4 }}
           {% endif %}
    </div>

         {% endif %}
         {% endfor %}
         {% endif %}
         {% endfor %}

        {% for item in downtown_menu %}
                 {% if loop.index == 3 %}
                     <div class="tab-pane fade in row padding-left0" id="{{ item['name']|clean_class }}">
                 {% endif %}
                   {% endfor %}
                     {% if node.field_tab_map_content_items %}
                       {% for parag_id in node.field_tab_map_content_items.value %}
                         {{ bamboo_render_entity('paragraph', parag_id['target_id']) }}
                         {% endfor %}
                         {% endif %}
</div>
{% for item in downtown_menu %}
         {% if loop.index == 3 %}
{% for sub in item['sub'] %}
{% if sub is not empty %}
<div class="tab-pane fade in row" id="{{ sub['name']|clean_class }}">
{% set submenu_content = drupal_field('field_submenu_content_order_3', 'node') %}
   {% if loop.index == 1 %}
        <h3>{{ sub['name'] }}</h3>
   {{ submenu_content.0 }}
   {% endif %}
   {% if loop.index == 2 %}
        <h3>{{ sub['name'] }}</h3>
   {{ submenu_content.1 }}
   {% endif %}
   {% if loop.index == 3 %}
        <h3>{{ sub['name'] }}</h3>
   {{ submenu_content.2 }}
   {% endif %}
   {% if loop.index == 4 %}
        <h3>{{ sub['name'] }}</h3>
   {{ submenu_content.3 }}
   {% endif %}
   {% if loop.index == 5 %}
        <h3>{{ sub['name'] }}</h3>
   {{ submenu_content.4 }}
   {% endif %}
</div>

 {% endif %}
 {% endfor %}
 {% endif %}
 {% endfor %}

        {% for item in downtown_menu %}
          {% if loop.index == 4 %}
            <div class="tab-pane fade in row padding-left0" id="{{ item['name']|clean_class }}">
            {% endif %}
          {% endfor %}
          {% if node.field_tab_resources_content_item %}
            {% for parag_id in node.field_tab_resources_content_item.value %}
              {{ bamboo_render_entity('paragraph', parag_id['target_id']) }}
            {% endfor %}
          {% endif %}
        </div>
        {% for item in downtown_menu %}
                 {% if loop.index == 4 %}
        {% for sub in item['sub'] %}
        {% if sub is not empty %}
        <div class="tab-pane fade in row" id="{{ sub['name']|clean_class }}">
        {% set submenu_content = drupal_field('field_submenu_content_order_4', 'node') %}
           {% if loop.index == 1 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.0 }}
           {% endif %}
           {% if loop.index == 2 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.1 }}
           {% endif %}
           {% if loop.index == 3 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.2 }}
           {% endif %}
           {% if loop.index == 4 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.3 }}
           {% endif %}
           {% if loop.index == 5 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.4 }}
           {% endif %}
        </div>

         {% endif %}
         {% endfor %}
         {% endif %}
         {% endfor %}

        {% for item in downtown_menu %}
          {% if loop.index == 5 %}
            <div class="tab-pane fade in row padding-left0" id="{{ item['name']|clean_class }}">
            {% endif %}
          {% endfor %}
          {% if node.field_tab_event_content_items %}
            {% for parag_id in node.field_tab_event_content_items.value %}
              {{ bamboo_render_entity('paragraph', parag_id['target_id']) }}
            {% endfor %}
          {% endif %}
        </div>
        {% for item in downtown_menu %}
                 {% if loop.index == 5 %}
        {% for sub in item['sub'] %}
        {% if sub is not empty %}
        <div class="tab-pane fade in row" id="{{ sub['name']|clean_class }}">
        {% set submenu_content = drupal_field('field_submenu_content_order_5', 'node') %}
           {% if loop.index == 1 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.0 }}
           {% endif %}
           {% if loop.index == 2 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.1 }}
           {% endif %}
           {% if loop.index == 3 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.2 }}
           {% endif %}
           {% if loop.index == 4 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.3 }}
           {% endif %}
           {% if loop.index == 5 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.4 }}
           {% endif %}
        </div>

         {% endif %}
         {% endfor %}
         {% endif %}
         {% endfor %}


        {% for item in downtown_menu %}
          {% if loop.index == 6 %}
            <div class="tab-pane fade in row padding-left0" id="{{ item['name']|clean_class }}">
            {% endif %}
          {% endfor %}
          {% if node.field_tab_contact_content_items %}
            {% for parag_id in node.field_tab_contact_content_items.value %}
              {{ bamboo_render_entity('paragraph', parag_id['target_id']) }}
            {% endfor %}
          {% endif %}
        </div>
        {% for item in downtown_menu %}
                 {% if loop.index == 6 %}
        {% for sub in item['sub'] %}
        {% if sub is not empty %}
        <div class="tab-pane fade in row" id="{{ sub['name']|clean_class }}">
        {% set submenu_content = drupal_field('field_submenu_content_order_6', 'node') %}
           {% if loop.index == 1 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.0 }}
           {% endif %}
           {% if loop.index == 2 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.1 }}
           {% endif %}
           {% if loop.index == 3 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.2 }}
           {% endif %}
           {% if loop.index == 4 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.3 }}
           {% endif %}
           {% if loop.index == 5 %}
                <h3>{{ sub['name'] }}</h3>
           {{ submenu_content.4 }}
           {% endif %}
        </div>

         {% endif %}
         {% endfor %}
         {% endif %}
         {% endfor %}
          </div>
            </div>
          </div>

    <!-- //End Left-side bar -->

    <!-- Right-side bar -->

    <div class="col-md-3 sidebar-menu hidden-sm hidden-xs">
      <div class="top-bar"></div>
      {{ page.side_menu }}
    </div>

    <!-- //End Right-side bar -->

    <!-- //End container -->
{# breakpoint() #}
    {% for item in full_width_data %}
      {% if item.flag %}
        <div class="clear full-width-content {% if item.para_id == 122 %} block-qoute-wrapper {% endif %}">
          {# full width content goes here #}
          {{ bamboo_render_entity('paragraph', item.para_id) }}
        </div>
      {% else %}
        {# small width content goes here #}
        <div class="container">
          <div class="row cms-fullwidth">
            <div class="section-content clear">
              {{ bamboo_render_entity('paragraph', item.para_id) }}
            </div>
          </div>

        {% endif %}
      {% endfor %}
    </div>
  </div>
</div>
{{ attach_library('cpv3/cpu') }}
<!-- footer section -->
{% include directory ~ '/partials/footer.html.twig' %}

SubTerms.php

<?php


// @file src/taxterm_subs/Service/SubTerms.php
namespace Drupal\taxterm_subs\Service;
//namespace Drupal\taxterm_subs\SubtermsTraits;
//namespace Drupal\taxterm_subs\Traits;
//require_once DRUPAL_ROOT . '/modules/custom/taxterm_subs/src/SubtermsTrait/SubtermsTrait.php';
//require_once 'SubtermsTrait.php';


/**
SubtermsTrait
*/
trait SubtermsTrait {
public function subterms(&$vid=('westside_community_plan_update')) {
$terms = \Drupal::entityTypeManager()->getStorage('taxonomy_term')->loadTree($vid, 0);
  if (!empty($terms)) {
      foreach ($terms as $term) {
          $children = \Drupal::entityTypeManager()->getStorage('taxonomy_term')->loadTree($vid, $term->tid, 2);
              if (empty($children)) {
              // no child
              if ($term->parents[0] == 0)
                  $tax_tree[$term->tid] = [
                      'tid' => $term->tid,
                      'name' => $term->name,
                  ];
          } else {
              $tax_tree[$term->tid] = [
                  'tid' => $term->tid,
                  'name' => $term->name,
              ];
      }
      foreach ($children as $child) {
              $tax_tree[$term->tid]['sub'][$child->tid] = [
                  'tid' => $child->tid,
                  'name' => $child->name,
              ];
            }
          }
        }
 return $tax_tree;
}
}

/**
 * The SubTerms service. Loads child taxonomy terms for submenus here to keep theme file tidy.
 */
class SubTerms {
  use SubtermsTrait;

public function createSubtermsBhes(&$terms=array(), &$tax_tree=array(), &$vid=('boyle_heights_cpu_menu_es')) {
  $this->subterms();
  /** place in theme file:
  * $variables['bhspan_menu'] = \Drupal::service('term_subs')->createSubtermsBhes();
  **/
  return $this->subterms($vid);
  }
public function createSubtermsBoyleHeights(&$terms=array(), &$tax_tree=array(), &$vid=('boyle_heights_cpu')) {
  $this->subterms();
  /** place in theme file:
  * $variables['boyleheights_menu'] = \Drupal::service('term_subs')->createSubtermsBoyleHeights();
  **/
  return $this->subterms($vid);
    }
public function createSubtermsHarbor(&$terms=array(), &$tax_tree=array(), &$vid=('harbor_menu')) {
  $this->subterms();
  /** place in theme file:
      * $variables['harborplans_menu'] = \Drupal::service('term_subs')->createSubtermsHarbor();
  **/
  return $this->subterms($vid);
        }
public function createSubtermsWestside(&$terms=array(), &$tax_tree=array(), &$vid=('westside_community_plan_update')) {
  $this->subterms();
  /** place in theme file:
      * $variables['westside_menu'] = \Drupal::service('term_subs')->createSubtermsWestside();
  **/
  return $this->subterms($vid);
                }
public function createSubtermsSouthEast(&$terms=array(), &$tax_tree=array(), &$vid=('southeast_valley_menu')) {
  $this->subterms();
  /** place in theme file:
      * $variables['sevplans_menu'] = \Drupal::service('term_subs')->createSubtermsSouthEast();
  **/
 return $this->subterms($vid);
    }
public function createSubtermsSouthWest(&$terms=array(), &$tax_tree=array(), &$vid=('southwest_valley_community_plan_')) {
      $this->subterms();
      /** place in theme file:
          * $variables['swvalley_menu'] = \Drupal::service('term_subs')->createSubtermsSouthWest();
      **/
     return $this->subterms($vid);
        }
public function createSubtermsDowntown(&$terms=array(), &$tax_tree=array(), &$vid=('downtown_community_plan_update')) {
    $this->subterms();
              /** place in theme file:
                  * $variables['downtown_menu'] = \Drupal::service('term_subs')->createSubtermsDowntown();
              **/
    return $this->subterms($vid);
                }
public function createSubtermsGeneral(&$terms=array(), &$tax_tree=array(), &$vid=('gerneral_plan_update')) {
    $this->subterms();
    /** place in theme file:
        * $variables['general_menu'] = \Drupal::service('term_subs')->createSubtermsGeneral();
    **/
    return $this->subterms($vid);
    }
  public function createSubtermsHollywood(&$terms=array(), &$tax_tree=array(), &$vid=('hollywood_community_plan')) {
    $this->subterms();
        /** place in theme file:
            * $variables['hollywood_menu'] = \Drupal::service('term_subs')->createSubtermsHollywood();
        **/
    return $this->subterms($vid);
  }
  public function createSubtermsVenice(&$terms=array(), &$tax_tree=array(), &$vid=('venice_local_coastal_program')) {
    $this->subterms();
    /** place in theme file:
        * $variables['venice_menu'] = \Drupal::service('term_subs')->createSubtermsVenice();
    **/
    return $this->subterms($vid);
  }
//  public function createSubtermsHousing(&$terms=array(), &$tax_tree=array(), &$vid=('housing_element_update')) {
  //  $this->subterms();
    /** place in theme file:
        * $variables['housing_menu'] = \Drupal::service('term_subs')->createSubtermsHousing();
    **/
  //  return $this->subterms($vid);
//  }
}

taxterm_subs.info.yml

name: 'taxterm_subs'
type: module
description: 'code for generating child terms as dropdown menu'
core: 8.x
package: 'Custom'

taxterm_subs.services.yml

services:
  term_subs:
    class: '\Drupal\taxterm_subs\Service\SubTerms'

theme.theme

<?php
use Drupal\node\Entity\Node;
use Drupal\image\Entity\ImageStyle;
use Drupal\taxonomy\Entity\Term;
use Drupal\taxterm_subs\SubTerms;

function theme_preprocess_page(&$variables)
{

// Boyle Heights
       cpuMenuBH($variables);
      // Harbor
       cpuMenuHarbor($variables);
      // Westside
       cpuMenuWestside($variables);
      // SouthEast
       cpuMenuSouthEast($variables);
      // SouthWest Menu
       cpuMenuSouthWest($variables);
      // Downtown
       cpuMenuDowntown($variables);
       // General Plan
       cpuMenuGeneral($variables);
       // hollywood_menu
       cpuMenuHollywood($variables);
       // Venice
       cpuMenuVenice($variables);
       // housing_menu
       //cpuMenuHousing($variables);

      // CPU SPANISH TABBED MENUS //
      // Boyle Heights
       cpuMenuBHES($variables);
       }
       
       
/** CPU SubMenus Drupal Service**/

/** Boyle Heights Spanish  **/
function cpuMenuBHES(&$variables) {
  $variables['bhspan_menu'] = \Drupal::service('term_subs')->createSubtermsBhes();
}
/** Boyle Heights English  **/
function cpuMenuBH(&$variables) {
  $variables['boyleheights_menu'] = \Drupal::service('term_subs')->createSubtermsBoyleHeights();
}
/** Harbor  **/
function cpuMenuHarbor(&$variables) {
  $variables['harborplans_menu'] = \Drupal::service('term_subs')->createSubtermsHarbor();
}
/** Westside  **/
function cpuMenuWestside(&$variables) {
  $variables['westside_menu'] = \Drupal::service('term_subs')->createSubtermsWestside();
}
/** SouthEAST  **/
function cpuMenuSouthEast(&$variables) {
  $variables['sevplans_menu'] = \Drupal::service('term_subs')->createSubtermsSouthEast();
}
/** SouthWEST  **/
function cpuMenuSouthWest(&$variables) {
  $variables['swvalley_menu'] = \Drupal::service('term_subs')->createSubtermsSouthWest();
}
/** Downtown **/
function cpuMenuDowntown(&$variables) {
$variables['downtown_menu'] = \Drupal::service('term_subs')->createSubtermsDowntown();
}
/** General **/
function cpuMenuGeneral(&$variables) {
  $variables['general_menu'] = \Drupal::service('term_subs')->createSubtermsGeneral();
}
/** Hollywood **/
function cpuMenuHollywood(&$variables) {
$variables['hollywood_menu'] = \Drupal::service('term_subs')->createSubtermsHollywood();
}
/** Venice **/
function cpuMenuVenice(&$variables) {
  $variables['venice_menu'] = \Drupal::service('term_subs')->createSubtermsVenice();
}
       
       

This service takes a taxonomy hierarchy (parent > child) and makes a dropdown menu from the child terms. It uses php traits to re-purpose code and passes by reference variables to populate 10 different vocabularies for 10 different 'mini' sites within a larger site.