Introduce Admin_Page_Collection interface.

src/Admin_Page_Collection.php

@@ -0,0 +1,48 @@
+<?php
+/**
+ * Admin_Page_Collection interface
+ *
+ * @package Leaves_And_Love\OOP_Admin_Pages
+ * @since 1.0.0
+ */
+
+namespace Leaves_And_Love\OOP_Admin_Pages;
+
+use Leaves_And_Love\OOP_Admin_Pages\Exception\Admin_Page_Not_Found_Exception;
+
+/**
+ * Interface to manage admin page instances.
+ *
+ * @since 1.0.0
+ */
+interface Admin_Page_Collection {
+
+	/**
+	 * Registers the admin pages in the collection within the environment.
+	 *
+	 * @since 1.0.0
+	 */
+	public function register();
+
+	/**
+	 * Checks whether an admin page is part of the collection.
+	 *
+	 * @since 1.0.0
+	 *
+	 * @param string $slug Identifier of the admin page to check for.
+	 * @return bool True if the admin page is part of the collection, false otherwise.
+	 */
+	public function has_page( string $slug ) : bool;
+
+	/**
+	 * Gets an admin page instance that is part of the collection.
+	 *
+	 * @since 1.0.0
+	 *
+	 * @param string $slug Identifier of the admin page to get.
+	 * @return Admin_Page Admin page instance.
+	 *
+	 * @throws Admin_Page_Not_Found_Exception Thrown when the admin page is not found.
+	 */
+	public function get_page( string $slug ) : Admin_Page;
+}

src/Exception/Admin_Page_Not_Found_Exception.php

@@ -0,0 +1,20 @@
+<?php
+/**
+ * Admin_Page_Not_Found_Exception class
+ *
+ * @package Leaves_And_Love\OOP_Admin_Pages\Exception
+ * @since 1.0.0
+ */
+
+namespace Leaves_And_Love\OOP_Admin_Pages\Exception;
+
+use Exception;
+
+/**
+ * Exception thrown when an admin page is not found in a collection.
+ *
+ * @since 1.0.0
+ */
+class Admin_Page_Not_Found_Exception extends Exception {
+
+}