updated version for 2019 ;)

legacy/class-helper.php

+<?php
+
+if ( ! defined( 'ABSPATH' ) ) exit; // Exit if accessed directly
+
+if ( ! class_exists( 'WPDesk_Helper_Plugin' ) ) {
+
+	class WPDesk_Helper_Plugin {
+
+		protected $plugin_data;
+
+		protected $text_domain;
+
+		protected $ame_activated_key;
+
+		protected $ame_activation_tab_key;
+
+		function __construct( $plugin_data ) {
+			global $wpdesk_helper_plugins;
+
+			$this->plugin_data = $plugin_data;
+			if ( ! isset( $wpdesk_helper_plugins ) ) $wpdesk_helper_plugins = array();
+			$plugin_data['helper_plugin'] = $this;
+			$wpdesk_helper_plugins[] = $plugin_data;
+
+			$this->ame_activated_key = 'api_' . dirname($plugin_data['plugin']) . '_activated';
+			$this->ame_activation_tab_key = 'api_' . dirname($plugin_data['plugin']) . '_dashboard';
+
+		}
+
+		public function inactive_notice() { ?>
+			<?php if ( ! current_user_can( 'manage_options' ) ) return; ?>
+			<?php if ( 1==1 && isset( $_GET['page'] ) && $this->ame_activation_tab_key == $_GET['page'] ) return; ?>
+            <div class="update-nag">
+				<?php printf( __( 'The %s%s%s License Key has not been activated, so the plugin is inactive! %sClick here%s to activate the license key and the plugin.', 'wpdesk-plugin' ), '<strong>', $this->plugin_data['product_id'], '</strong>', '<a href="' . esc_url( admin_url( 'admin.php?page='.$this->ame_activation_tab_key ) ) . '">', '</a>' ); ?>
+            </div>
+			<?php
+		}
+
+
+		function is_active( $add_notice = false ) {
+			if ( get_option( $this->ame_activated_key, '0' ) != 'Activated' ) {
+				if ( $add_notice ) {
+					add_action( 'admin_notices', array( $this, 'inactive_notice' ) );
+				}
+				return false;
+			}
+			else {
+				return true;
+			}
+		}
+
+	}
+
+}