Introduce WordPress_Admin_Submenu_Page class.

Author: Felix Arntz

src/Admin_Page/WordPress_Admin_Submenu_Page.php

@@ -0,0 +1,80 @@
+<?php
+/**
+ * WordPress_Admin_Submenu_Page class
+ *
+ * @package Leaves_And_Love\OOP_Admin_Pages\Admin_Page
+ * @since 1.0.0
+ */
+
+namespace Leaves_And_Love\OOP_Admin_Pages\Admin_Page;
+
+/**
+ * Class representing a WordPress admin page that also has a submenu entry.
+ *
+ * @since 1.0.0
+ */
+class WordPress_Admin_Submenu_Page extends WordPress_Admin_Page {
+
+	const PARENT_SLUG = 'parent_slug';
+	const MENU_TITLE  = 'menu_title';
+
+	/**
+	 * Gets the callback to use to register the admin page within WordPress.
+	 *
+	 * @since 1.0.0
+	 *
+	 * @return callable Callback to hook into the respective menu action.
+	 */
+	protected function get_register_hook_callback() {
+		return function() {
+			$slug        = $this->getConfigKey( self::SLUG );
+			$title       = $this->getConfigKey( self::TITLE );
+			$capability  = $this->getConfigKey( self::CAPABILITY );
+			$parent_slug = $this->getConfigKey( self::PARENT_SLUG );
+			$menu_title  = $this->hasConfigKey( self::MENU_TITLE ) ? $this->getConfigKey( self::MENU_TITLE ) : $title;
+
+			$hook_suffix = add_submenu_page( $parent_slug, $title, $menu_title, $capability, $slug, array( $this, 'render' ) );
+
+			add_action( "load-{$hook_suffix}", array( $this, 'initialize' ) );
+		};
+	}
+
+	/**
+	 * Gets the parent file for the admin page URL in WordPress.
+	 *
+	 * May contain query parameters.
+	 *
+	 * @since 1.0.0
+	 *
+	 * @return string Parent file for the URL.
+	 */
+	protected function get_parent_file() {
+		$parent_slug = $this->getConfigKey( self::PARENT_SLUG );
+
+		if ( false !== strpos( $parent_slug, '?' ) ) {
+			list( $base_slug, $query ) = explode( '?', $parent_slug, 2 );
+		} else {
+			$base_slug = $parent_slug;
+		}
+
+		if ( '.php' === substr( $base_slug, -4 ) ) {
+			return $parent_slug;
+		}
+
+		return 'admin.php';
+	}
+
+	/**
+	 * Gets the configuration keys that are required for an admin page.
+	 *
+	 * @since 1.0.0
+	 *
+	 * @return array Array of configuration keys.
+	 */
+	protected function get_required_config_keys() {
+		$required_keys   = parent::get_required_config_keys();
+		$required_keys[] = self::PARENT_SLUG;
+
+		return $required_keys;
+	}
+}