Add fonts preloading support

1. +++ b/bs_base.theme
   @@ -22,6 +22,16 @@ function bs_base_theme_suggestions_page_alter(array &$suggestions, array $variab
   +  $res = \Drupal::theme()->getActiveTheme();
   

   $res seems a strange variable name for that, maybe $active_theme?

2. +++ b/bs_base.theme
   @@ -112,3 +122,35 @@ function bs_base_preprocess_maintenance_page(&$variables) {
   +    if (strpos($font, '@') === 0) {
   +      $theme_name = substr($font, 1, strpos($font, '/') - 1);
   

   Overall looks good. You could also do this with a regex, less fiddling around with string functions and +1/-1 stuff:

   https://www.phpliveregex.com/p/CB2

   So that would be:

   if (preg_match('/^\@([^\/]+)(\/.+)$/', $font, $match)) {
     $href = '/' . drupal_get_path('theme', $match[1]) . $match[2];
   }
   

   Also, I think you want to check that the first character is a /, if not, then we could support it being in the active theme, so you can specify just "font/myfont.woff2", so I'd add an elseif for that.

3. +++ b/bs_base.theme
   @@ -112,3 +122,35 @@ function bs_base_preprocess_maintenance_page(&$variables) {
   +      'type' => 'font/' . substr($font, strpos($font, ".") + 1),
   

   again, not a big deail, you could also use pathinfo() for this: pathinfo($font, PATHINFO_EXTENSION)

4. +++ b/bs_base.theme
   @@ -112,3 +122,35 @@ function bs_base_preprocess_maintenance_page(&$variables) {
   +    ], TRUE];
   

   I think the second argument is expected to be a machine name, so I'd use something like 'bs_bse_preload_font'.

   it's a weird syntax and IIRC meant to be used to identify certain things to alter them.

+++ b/bs_base.theme
@@ -112,3 +122,40 @@ function bs_base_preprocess_maintenance_page(&$variables) {
+    // Support twig expansion syntax like @custom_theme/fonts/font1.woff.
+    if (preg_match('/^\@([^\/]+)(\/.+)$/', $font, $match)) {
+      $href = '/' . drupal_get_path('theme', $match[1]) . $match[2];
+    }
+    // Support fonts loading from the current active theme.
+    elseif (strpos($font, '/') !== 0) {
+      $active_theme = \Drupal::theme()->getActiveTheme();
+      $href = '/' . $active_theme->getPath() . '/' . $font;
+    }

Thinking more about this, I think you want to use something like file_url_transform_relative(file_create_url(drupal_get_path('module', 'ckeditor_test') . '/ckeditor_test.css')). file_create_url() has hooks that support things like separate domains for assets.

Found this while I was researching the header links, and crossorigin is technicaly only required if the font is from a different domain, but if we combine it with file_create_url() then that might happen on some sites, so that makes sense and doesn't hurt.

+++ b/bs_base.theme
@@ -112,3 +122,40 @@ function bs_base_preprocess_maintenance_page(&$variables) {
+  foreach ($fonts as $font) {
+    $href = $font;
+    // Support twig expansion syntax like @custom_theme/fonts/font1.woff.
+    if (preg_match('/^\@([^\/]+)(\/.+)$/', $font, $match)) {
...
+    }
+    // Support fonts loading from the current active theme.
+    elseif (strpos($font, '/') !== 0) {
+      $active_theme = \Drupal::theme()->getActiveTheme();
+      $href = file_url_transform_relative(file_create_url(drupal_get_path('theme', $active_theme->getPath()) . '/' . $font));
+    }

note that one case is left now that doesn't go through file_create_url(), which would be something like '/libraries/something/....woff2'.

One option would be to rename $href to $uri, then you could change this if/elseif to if/elseif/else and the else would then basically to a $uri = ltrim($font, '/'), if and elseif is just $uri = drupal_get_path(...

and then you can do a single $href = file_url_transform_relative(file_create_url($uri)) below, for all cases.