Merge pull request #493 from mor10/try/responsive-featured-images-ind…

…ex-archive

Add sizes attribute to featured images on index and archive pages.

inc/template-functions.php

@@ -112,6 +112,28 @@ function twentynineteen_image_filters_enabled() {
 	return true;
 }
 
+/**
+ * Add custom sizes attribute to responsive image functionality for post thumbnails.
+ *
+ * @origin Twenty Nineteen 1.0
+ *
+ * @param array $attr  Attributes for the image markup.
+ * @return string Value for use in post thumbnail 'sizes' attribute.
+ */
+function twentynineteen_post_thumbnail_sizes_attr( $attr ) {
+
+	if ( is_admin() ) {
+		return;
+	}
+
+	if ( ! is_singular() ) {
+		$attr['sizes'] = '(max-width: 34.9rem) calc(100vw - 2rem), (max-width: 53rem) calc(8 * (100vw / 12)), (min-width: 53rem) calc(6 * (100vw / 12)), 100vw';
+	}
+
+	return $attr;
+}
+add_filter( 'wp_get_attachment_image_attributes', 'twentynineteen_post_thumbnail_sizes_attr', 10, 1 );
+
 /**
  * Returns the size for avatars used in the theme.
  */