WordPress Plugin with Custom Post and Metabox using Singleton

class-wp-cpt-plugin-base-singleton.php

<?php

if ( !class_exists('Primped_Base_Singleton') ):

abstract class Primped_Base_Singleton {

    /**
     * Parsed options for module
     *
     * @var array
     */
    protected $options;

    /**
     * List of default options for module
     *
     * @var array
     */
    static protected $default_options = array();

    /**
     * Collection of modules objects
     *
     * @var array
     */
    static private $instances;

    static public function get_instance($options =array()) {
        $class = get_called_class();

        if ( !isset(self::$instances[$class]) ) {

            if( !is_array($options) || !is_array($class::$default_options) ) {
                throw new InvalidArgumentException('Module options have to be an Array.');
            }
            self::$instances[$class] = new $class($options);
        }
        return self::$instances[$class];
    }

    /**
     * Protected constructor
     * @param   array
     */
    protected function __construct($options =array()) {
        $this->options = $this->parseOptions($options);
    }

    /**
     * Parses options array for module
     * @param   array
     * @return  array
     */
    protected function parseOptions($options) {
        return $options;
    }


    final private function __clone() { }

}

endif;

class-wp-cpt-plugin.php

<?php

require_once( __DIR__ . '/class-wp-cpt-plugin-base-singleton.php' );

if ( !class_exists('WP_CPT_Plugin') {

    class WP_CPT_Plugin extends Primped_Base_Singleton {

        const PLUGIN_VERSION       = '1.0';
        const OPTIONS_PREFIX       = 'wp_cpt_plugin_';
        const CPT_SW_NAME          = 'wp_cpt_plugin';
        const CPT_SW_NAME_SINGULAR = 'WP CPT Plugin';
        const CPT_SW_NAME_PLURAL   = 'WP CPT Plugins';

        protected function __construct( $options = array() ) {
            parent::__construct( $options );

            // Registering CPT
            add_action( 'init', array($this, 'register_cpt_wpcptplugins') );

            //Saving a POST including Custom Fields
            add_action( 'save_post', array($this, 'save_post'), 10, 2 );
        }

        /**
         * Create Temporary Table for Property Photos Process
         */
        public static function do_install() {
            global $wpdb;

            update_option(self::OPTIONS_PREFIX . 'version', self::PLUGIN_VERSION);
        }

        /**
         * Register custom post type for wpcptplugins
         */
        function register_cpt_wpcptplugins() {

            $labels = array(
                'name' => _x( self::CPT_SW_NAME_PLURAL, self::CPT_SW_NAME ),
                'singular_name' => _x( self::CPT_SW_NAME_SINGULAR, self::CPT_SW_NAME ),
                'add_new' => _x( 'Add New', self::CPT_SW_NAME ),
                'add_new_item' => _x( 'Add New ', strtolower(self::CPT_SW_NAME_SINGULAR), self::CPT_SW_NAME ),
                'edit_item' => _x( 'Edit ' . self::CPT_SW_NAME_SINGULAR, self::CPT_SW_NAME ),
                'new_item' => _x( 'New ' . self::CPT_SW_NAME_SINGULAR, self::CPT_SW_NAME ),
                'view_item' => _x( 'View ' . self::CPT_SW_NAME_SINGULAR, self::CPT_SW_NAME ),
                'search_items' => _x( 'Search ' . self::CPT_SW_NAME_PLURAL, self::CPT_SW_NAME ),
                'not_found' => _x( 'No agencies found', self::CPT_SW_NAME ),
                'not_found_in_trash' => _x( 'No ' . strtolower(self::CPT_SW_NAME_PLURAL) . ' found in Trash', self::CPT_SW_NAME ),
                'parent_item_colon' => _x( 'Parent ' . self::CPT_SW_NAME_PLURAL, self::CPT_SW_NAME ),
                'menu_name' => _x( self::CPT_SW_NAME_PLURAL, self::CPT_SW_NAME ),
            );

            $args = array(
                'labels' => $labels,
                'hierarchical' => false,
                'description' => 'Switcher for PRIMPED',
                'supports' => array( 'title', 'thumbnail', 'custom-fields' ),
                'public' => false,
                'menu_icon' => 'dashicons-format-gallery',
                'show_ui' => true,
                'show_in_menu' => true,
                'menu_position' => 5,
                'show_in_nav_menus' => true,
                'publicly_queryable' => true,
                'exclude_from_search' => true,
                'has_archive' => false,
                'query_var' => true,
                'can_export' => true,
                'rewrite' => true,
                'capability_type' => 'page',
                'register_meta_box_cb' => 'WP_CPT_Plugin::wpcptplugins_add_meta_box'
            );

            register_post_type( self::CPT_SW_NAME, $args );
        }


        public static function wpcptplugins_add_meta_box() {

            add_meta_box(
                'wpcptplugin-metabox',
                'Settings',
                __CLASS__ . '::markup_meta_boxes',
                self::CPT_SW_NAME,
                'normal',
                'core'
            );
        }

        /**
         * Builds the markup for all meta boxes
         *
         * @param object $post
         * @param array  $box
         */
        public static function markup_meta_boxes( $post, $box ) {
            $variables = array();

            switch ( $box['id'] ) {
                case 'wpcptplugin-metabox':
                    $variables['exampleBoxField'] = get_post_meta( $post->ID, 'wpcptplugin-metabox-field', true );
                    echo '<p>Meta box introduction / user instructions</p>

                            <table id="wpcptplugin-metabox" class="form-table">
                                <tbody>
                                    <tr>
                                        <th>
                                            <label for="wpcptplugin-metabox-field">Caption</label>
                                        </th>
                                        <td>
                                            <input id="wpcptplugin-metabox-field" name="wpcptplugin-metabox-field" type="text" class="regular-text" value="' . esc_attr( $variables['exampleBoxField'] ) . '" />
                                        </td>
                                    </tr>
                                </tbody>
                            </table>';
                    break;

                /*
                case 'wpps_some-other-box':
                    $variables['someOtherField'] = get_post_meta( $post->ID, 'wpps_some-other-field', true );
                    $view                        = 'wpps-cpt-example/metabox-another-box.php';
                    break;
                */

                default:
                    $view = false;
                    break;
            }
        }

        /**
         * Saves values of the the custom post type's extra fields
         *
         * @param int    $post_id
         * @param object $post
         */
        public static function save_post( $post_id, $revision ) {
            global $post;

            $ignored_actions = array( 'trash', 'untrash', 'restore' );

            if ( isset( $_GET['action'] ) && in_array( $_GET['action'], $ignored_actions ) ) {
                return;
            }

            if ( ! $post || $post->post_type != self::CPT_SW_NAME ) {
                return;
            }

            self::save_custom_fields( $post_id, $_POST );
        }

        /**
         * Validates and saves values of the the custom post type's extra fields
         *
         * @param int   $post_id
         * @param array $new_values
         */
        protected static function save_custom_fields( $post_id, $new_values ) {
            if ( isset( $new_values[ 'wpcptplugin-metabox-field' ] ) ) {
                if ( true ) { // some business logic check
                    update_post_meta( $post_id, 'wpcptplugin-metabox-field', $new_values[ 'wpcptplugin-metabox-field' ] );
                } else {
                    echo( 'Example of failing validation');
                }
            }
        }

    }
}

wp-cpt-plugin.php

<?php
/*
Plugin Name: WP CPT Plugin
Plugin URI: #
Description: Example of CPT Plugin with Metabox
Version: 1.0
Author: Leo Caseiro
Author URI: http://leocaseiro.com.br/
*/

require_once( __DIR__ . '/class-wp-cpt-plugin.php' );

register_activation_hook(__FILE__, array('WP_CPT_Plugin', 'do_install') );

WP_CPT_Plugin::get_instance();