<?php
/**
 * @file
 * Displays children pages as a block
 */

namespace Drupalchild_pages_displayPluginBlock;
use DrupalCoreBlockBlockBase;
use DrupalCoreUrl;
use DrupalCorePath;
use DrupalCoreMenu;
/**
 * Provides a 'Next Previous' block.
 *
 * @Block(
 *   id = "child_page_block",
 *   admin_label = @Translation("Child Page Block"),
 *   category = @Translation("Blocks")
 * )
 */
class ChildPageBlock extends BlockBase {
  public function build(){
    $menu_tree = Drupal::menuTree();
    $menu_name = 'main';

    // Build the typical default set of menu tree parameters.
    $parameters = $menu_tree->getCurrentRouteMenuTreeParameters($menu_name);
    $expandedParents = $parameters->expandedParents;
    reset($expandedParents);
    $root = current($expandedParents);
    $parameters = new DrupalCoreMenuMenuTreeParameters();
    $parameters->setRoot($root);
    $parameters->setMaxDepth(3);
    $parameters->setMinDepth(1);

    // Load the tree based on this set of parameters.
    $tree = $menu_tree->load($menu_name, $parameters);

    //Set Cache for block
    $cache['max-age'] = 3600;
    $cache['contexts'][] = 'url.path';
    $cache['tags'][] = $root;

    //Get node IDs from menu
    $nids = array();
    foreach($tree as $item){
      $nids[] = $item->link->getUrlObject()->getRouteParameters()['node'];
    }

    //Load nodes & generate content from nodes
    $nodes = (!empty($nids)) ? entity_load_multiple('node', $nids) : array();
    if (!empty($nodes) && is_array($nodes)){
      foreach($nodes as $key=>$val) {
        $nodeContent = array(
          '#prefix' => '<div class="block-child-page-menu">',
          '#suffix' => '</div>'
        );
        $url = Drupal::service('path.alias_manager')->getAliasByPath('/node/'.$key);

        //Get image thumbnail from content & use specified image style
        if(null !== $val->get('field_thumbnail')->entity){
          $imgPath = ($val->get('field_thumbnail')->entity->uri->value);
          $imgCroppedPath = DrupalimageEntityImageStyle::load('basic_page_thumbnail')->buildUrl($imgPath);
          $img = ($imgPath != '') ? '<img src="' . $imgCroppedPath . '" />' : '';
        }else{
          $img = "";
        }
        $nodeContent['#markup'] = '<a href="'.$url.'">' .
            '<h2>' . $val->getTitle() . '</h2>' .
            $img .
          '</a>';
        $content[] = $nodeContent;
      }
    }else{
      //Menu has no nodes
      $content['#markup'] = '';
    }
    //Add cache & css library to block
    $content['#cache'] = $cache;
    $content["#attached"] = array('library' => array('child_pages_display/child_pages_display_css'));
    return $content;
  }
}