Introduce WordPress_Admin_Page class.

src/Admin_Page/WordPress_Admin_Page.php

@@ -0,0 +1,93 @@
+<?php
+/**
+ * WordPress_Admin_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 basic WordPress admin page.
+ *
+ * @since 1.0.0
+ */
+class WordPress_Admin_Page extends Abstract_Admin_Page {
+
+	const CAPABILITY = 'capability';
+
+	/**
+	 * Gets the URL of the admin page.
+	 *
+	 * @since 1.0.0
+	 *
+	 * @return string Admin page URL.
+	 */
+	public function get_url() : string {
+		$parent_file = $this->get_parent_file();
+
+		return add_query_arg( 'page', $this->getConfigKey( self::SLUG ), admin_url( $parent_file ) );
+	}
+
+	/**
+	 * Registers the admin page within the environment.
+	 *
+	 * @since 1.0.0
+	 */
+	public function register() {
+		$callback = $this->get_register_hook_callback();
+
+		if ( doing_action( 'admin_menu' ) ) {
+			call_user_func( $callback );
+		} else {
+			add_action( 'admin_menu', $callback );
+		}
+	}
+
+	/**
+	 * 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 );
+
+			$hook_suffix = add_submenu_page( null, $title, $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() {
+		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::CAPABILITY;
+
+		return $required_keys;
+	}
+}