class ViewsDataHelper

class ViewsDataHelper {
    
    /**
     * The views data object, containing the cached information.
     *
     * @var \Drupal\views\ViewsData
     */
    protected $data;
    
    /**
     * A prepared list of all fields, keyed by base_table and handler type.
     *
     * @var array
     */
    protected $fields;
    
    /**
     * Constructs a ViewsData object.
     *
     * @param \Drupal\views\ViewsData $views_data
     *   The views data object, containing the cached table information.
     */
    public function __construct(ViewsData $views_data) {
        $this->data = $views_data;
    }
    
    /**
     * Fetches a list of all fields available for a given base type.
     *
     * @param array|string $base
     *   A list or a single base_table, for example node.
     * @param string $type
     *   The handler type, for example field or filter.
     * @param bool $grouping
     *   Should the result grouping by its 'group' label.
     * @param string $sub_type
     *   An optional sub type. E.g. Allows making an area plugin available for
     *   header only, instead of header, footer, and empty regions.
     *
     * @return array
     *   A keyed array of in the form of 'base_table' => 'Description'.
     */
    public function fetchFields($base, $type, $grouping = FALSE, $sub_type = NULL) {
        if (!$this->fields) {
            $data = $this->data
                ->getAll();
            // This constructs this ginormous multi dimensional array to
            // collect the important data about fields. In the end,
            // the structure looks a bit like this (using nid as an example)
            // $strings['nid']['filter']['title'] = 'string'.
            //
            // This is constructed this way because the above referenced strings
            // can appear in different places in the actual data structure so that
            // the data doesn't have to be repeated a lot. This essentially lets
            // each field have a cheap kind of inheritance.
            foreach ($data as $table => $table_data) {
                $bases = [];
                $strings = [];
                $skip_bases = [];
                foreach ($table_data as $field => $info) {
                    // Collect table data from this table
                    if ($field == 'table') {
                        // calculate what tables this table can join to.
                        if (!empty($info['join'])) {
                            $bases = array_keys($info['join']);
                        }
                        // And it obviously joins to itself.
                        $bases[] = $table;
                        continue;
                    }
                    foreach ([
                        'field',
                        'sort',
                        'filter',
                        'argument',
                        'relationship',
                        'area',
                    ] as $key) {
                        if (!empty($info[$key])) {
                            if ($grouping && !empty($info[$key]['no group by'])) {
                                continue;
                            }
                            if ($sub_type && isset($info[$key]['sub_type']) && !in_array($sub_type, (array) $info[$key]['sub_type'])) {
                                continue;
                            }
                            if (!empty($info[$key]['skip base'])) {
                                foreach ((array) $info[$key]['skip base'] as $base_name) {
                                    $skip_bases[$field][$key][$base_name] = TRUE;
                                }
                            }
                            elseif (!empty($info['skip base'])) {
                                foreach ((array) $info['skip base'] as $base_name) {
                                    $skip_bases[$field][$key][$base_name] = TRUE;
                                }
                            }
                            foreach ([
                                'title',
                                'group',
                                'help',
                                'base',
                                'aliases',
                            ] as $string) {
                                // First, try the lowest possible level
                                if (!empty($info[$key][$string])) {
                                    $strings[$field][$key][$string] = $info[$key][$string];
                                }
                                elseif (!empty($info[$string])) {
                                    $strings[$field][$key][$string] = $info[$string];
                                }
                                elseif (!empty($table_data['table'][$string])) {
                                    $strings[$field][$key][$string] = $table_data['table'][$string];
                                }
                                elseif ($string == 'help') {
                                    $strings[$field][$key][$string] = '';
                                }
                                else {
                                    if ($string != 'base') {
                                        $strings[$field][$key][$string] = new FormattableMarkup("Error: missing @component", [
                                            '@component' => $string,
                                        ]);
                                    }
                                }
                            }
                        }
                    }
                }
                foreach ($bases as $base_name) {
                    foreach ($strings as $field => $field_strings) {
                        foreach ($field_strings as $type_name => $type_strings) {
                            if (empty($skip_bases[$field][$type_name][$base_name])) {
                                $this->fields[$base_name][$type_name]["{$table}.{$field}"] = $type_strings;
                            }
                        }
                    }
                }
            }
        }
        // If we have an array of base tables available, go through them
        // all and add them together. Duplicate keys will be lost and that's
        // Just Fine.
        if (is_array($base)) {
            $strings = [];
            foreach ($base as $base_table) {
                if (isset($this->fields[$base_table][$type])) {
                    $strings += $this->fields[$base_table][$type];
                }
            }
            uasort($strings, [
                'self',
                'fetchedFieldSort',
            ]);
            return $strings;
        }
        if (isset($this->fields[$base][$type])) {
            uasort($this->fields[$base][$type], [
                $this,
                'fetchedFieldSort',
            ]);
            return $this->fields[$base][$type];
        }
        return [];
    }
    
    /**
     * Sort function for fetched fields.
     *
     * @param array $a
     *   First item for comparison. The compared items should be associative arrays
     *   that include a 'group' and a 'title' key.
     * @param array $b
     *   Second item for comparison.
     *
     * @return int
     *   Returns -1 if $a comes before $b, 1 other way round and 0 if it cannot be
     *   decided.
     */
    protected static function fetchedFieldSort($a, $b) {
        $a_group = mb_strtolower($a['group']);
        $b_group = mb_strtolower($b['group']);
        if ($a_group != $b_group) {
            return $a_group < $b_group ? -1 : 1;
        }
        $a_title = mb_strtolower($a['title']);
        $b_title = mb_strtolower($b['title']);
        if ($a_title != $b_title) {
            return $a_title < $b_title ? -1 : 1;
        }
        return 0;
    }

}