Added a dependency check for Gutenberg (#5718)

Co-authored-by: Jeff Ong <jonger4@gmail.com>

livro/functions.php

@@ -64,3 +64,5 @@ add_action( 'wp_enqueue_scripts', 'livro_styles' );
 
 // Add block patterns
 require get_template_directory() . '/inc/block-patterns.php';
+
+require get_template_directory() . '/inc/gutenberg-dependency-check.php';

livro/inc/gutenberg-dependency-check.php

@@ -0,0 +1,45 @@
+<?php
+
+add_action( 'admin_notices', 'show_admin_messages' );
+
+function print_admin_message( $version ) {
+	echo '<div id="message" class="error"><p><strong>';
+	printf( __( 'The installed theme requires <a href="https://wordpress.org/plugins/gutenberg/">Gutenberg</a> version %s or higher.', 'livro' ), $version );
+	echo '</strong></p></div>';
+}
+
+function show_admin_messages() {
+	$metadata = file_get_contents( get_stylesheet_directory() . '/style.css' );
+	preg_match( '/(?<=Requires Gutenberg:).+/', $metadata, $match );
+	if ( 0 === sizeof( $match ) ) {
+		return; // Gutenberg is not required
+	}
+	$version = trim( $match[0] );
+	if ( ! defined( 'IS_GUTENBERG_PLUGIN' ) ) {
+		print_admin_message( $version ); // Gutenberg is not activated
+		return;
+	}
+
+	// Determine Gutenberg version from defined constant
+	if ( defined( 'GUTENBERG_VERSION' ) ) {
+		if ( version_compare( GUTENBERG_VERSION, $version ) < 0 ) {
+			print_admin_message( $version );
+		}
+		return;
+	}
+
+	// We have confirmed that Gutenberg is installed and activated, however we cannot use the GUTENBERG_VERSION constant
+	// (probably because we are in development mode)
+	// we'll use the metadata from get_plugins() to determine the version of Gutenberg
+	$plugins = get_plugins();
+	foreach ( $plugins as $plugin ) {
+		if ( 'Gutenberg' === $plugin['Name'] ) {
+			if ( version_compare( trim( $plugin['Version'] ), $version ) < 0 ) {
+				print_admin_message( $version );
+			}
+			return;
+		}
+	}
+
+	// We weren't able to confirm the version, however we do know that it's installed and activated so we'll just hope for the best!
+}

livro/style.css

@@ -7,6 +7,7 @@ Description: Livro is a simple theme designed to evoke the calm feeling you get
 Requires at least: 5.8
 Tested up to: 5.8.3
 Requires PHP: 5.6
+Requires Gutenberg: 12.8
 Version: 1.0.9
 License: GNU General Public License v2 or later
 License URI: http://www.gnu.org/licenses/gpl-2.0.html