1 files changed, 76 insertions, 0 deletions
diff --git a/themes/twentynineteen/inc/back-compat.php b/themes/twentynineteen/inc/back-compat.php
new file mode 100644
index 00000000..fd821987
--- /dev/null
+++ b/themes/twentynineteen/inc/back-compat.php
@@ -0,0 +1,76 @@
+<?php
+/**
+ * Twenty Nineteen back compat functionality
+ *
+ * Prevents Twenty Nineteen from running on WordPress versions prior to 4.7,
+ * since this theme is not meant to be backward compatible beyond that and
+ * relies on many newer functions and markup changes introduced in 4.7.
+ *
+ * @package WordPress
+ * @subpackage Twenty_Nineteen
+ * @since Twenty Nineteen 1.0.0
+ */
+
+/**
+ * Prevent switching to Twenty Nineteen on old versions of WordPress.
+ *
+ * Switches to the default theme.
+ *
+ * @since Twenty Nineteen 1.0.0
+ */
+function twentynineteen_switch_theme() {
+	switch_theme( WP_DEFAULT_THEME );
+	unset( $_GET['activated'] );
+	add_action( 'admin_notices', 'twentynineteen_upgrade_notice' );
+}
+add_action( 'after_switch_theme', 'twentynineteen_switch_theme' );
+
+/**
+ * Adds a message for unsuccessful theme switch.
+ *
+ * Prints an update nag after an unsuccessful attempt to switch to
+ * Twenty Nineteen on WordPress versions prior to 4.7.
+ *
+ * @since Twenty Nineteen 1.0.0
+ *
+ * @global string $wp_version WordPress version.
+ */
+function twentynineteen_upgrade_notice() {
+	$message = sprintf( __( 'Twenty Nineteen requires at least WordPress version 4.7. You are running version %s. Please upgrade and try again.', 'twentynineteen' ), $GLOBALS['wp_version'] );
+	printf( '<div class="error"><p>%s</p></div>', $message );
+}
+
+/**
+ * Prevents the Customizer from being loaded on WordPress versions prior to 4.7.
+ *
+ * @since Twenty Nineteen 1.0.0
+ *
+ * @global string $wp_version WordPress version.
+ */
+function twentynineteen_customize() {
+	wp_die(
+		sprintf(
+			__( 'Twenty Nineteen requires at least WordPress version 4.7. You are running version %s. Please upgrade and try again.', 'twentynineteen' ),
+			$GLOBALS['wp_version']
+		),
+		'',
+		array(
+			'back_link' => true,
+		)
+	);
+}
+add_action( 'load-customize.php', 'twentynineteen_customize' );
+
+/**
+ * Prevents the Theme Preview from being loaded on WordPress versions prior to 4.7.
+ *
+ * @since Twenty Nineteen 1.0.0
+ *
+ * @global string $wp_version WordPress version.
+ */
+function twentynineteen_preview() {
+	if ( isset( $_GET['preview'] ) ) {
+		wp_die( sprintf( __( 'Twenty Nineteen requires at least WordPress version 4.7. You are running version %s. Please upgrade and try again.', 'twentynineteen' ), $GLOBALS['wp_version'] ) );
+	}
+}
+add_action( 'template_redirect', 'twentynineteen_preview' );