Add check to ensure site is using WordPress 4.8 (#1319)

gutenberg.php

@@ -13,11 +13,14 @@
 define( 'GUTENBERG_DEVELOPMENT_MODE', true );
 ### END AUTO-GENERATED DEFINES
 
-// Load API functions.
-require_once dirname( __FILE__ ) . '/lib/blocks.php';
-require_once dirname( __FILE__ ) . '/lib/client-assets.php';
-require_once dirname( __FILE__ ) . '/lib/i18n.php';
-require_once dirname( __FILE__ ) . '/lib/register.php';
+require_once dirname( __FILE__ ) . '/lib/init-checks.php';
+if ( gutenberg_can_init() ) {
+	// Load API functions, register scripts and actions, etc.
+	require_once dirname( __FILE__ ) . '/lib/blocks.php';
+	require_once dirname( __FILE__ ) . '/lib/client-assets.php';
+	require_once dirname( __FILE__ ) . '/lib/i18n.php';
+	require_once dirname( __FILE__ ) . '/lib/register.php';
 
-// Register server-side code for individual blocks.
-require_once dirname( __FILE__ ) . '/lib/blocks/latest-posts.php';
+	// Register server-side code for individual blocks.
+	require_once dirname( __FILE__ ) . '/lib/blocks/latest-posts.php';
+}

lib/init-checks.php

@@ -0,0 +1,40 @@
+<?php
+/**
+ * Verify that we can initialize the Gutenberg editor plugin.
+ *
+ * @package gutenberg
+ */
+
+if ( ! defined( 'ABSPATH' ) ) {
+	die( 'Silence is golden.' );
+}
+
+/**
+ * Display a notice and deactivate the Gutenberg plugin.
+ *
+ * @since 0.1.0
+ */
+function gutenberg_wordpress_version_notice() {
+	echo '<div class="error"><p>';
+	echo __( 'Gutenberg requires WordPress 4.8 or later to function properly. Please upgrade WordPress before activating Gutenberg.', 'gutenberg' );
+	echo '</p></div>';
+
+	deactivate_plugins( array( 'gutenberg/gutenberg.php' ) );
+}
+
+/**
+ * Verify that we can initialize the Gutenberg editor plugin.
+ *
+ * @since 0.1.0
+ * @return bool Whether we can initialize the plugin.
+ */
+function gutenberg_can_init() {
+	global $wp_version;
+
+	if ( version_compare( $wp_version, '4.8', '<' ) ) {
+		add_action( 'admin_notices', 'gutenberg_wordpress_version_notice' );
+		return false;
+	}
+
+	return true;
+}