Issue #5: Improve attaching plugin libraries.

ckeditor5.module

@@ -44,7 +44,7 @@ function ckeditor5_editor_info() {
     ),
     'file' => 'ckeditor5.admin.inc',
     'settings callback' => 'ckeditor5_settings_form',
-    'js settings callback' => 'ckeditor5_get_config',
+    'js settings callback' => '_ckeditor5_js_settings',
   );
 
   return $editors;
@@ -85,15 +85,27 @@ function ckeditor5_library_info() {
       array('filter', 'filter'),
       array('system', 'backdrop.ajax'),
       array('ckeditor5', 'ckeditor5'),
-      // CKEditor 5 does not provide its own plugin loader, so any added
-      // potentially needed plugins are added as dependencies.
-      // @todo: Improve loading to use libraries only when needed.
-      array('ckeditor5', 'backdrop.ckeditor5.backdrop-basic-styles'),
-      array('ckeditor5', 'backdrop.ckeditor5.backdrop-image'),
-      array('ckeditor5', 'backdrop.ckeditor5.backdrop-link'),
     ),
   );
 
+  // CKEditor 5 does not have a built-in script loader. Any needed dependencies
+  // should be present on the page when CKEditor loads. Loop through all text
+  // formats to determine the combined set of dependencies for all formats.
+  $text_formats = filter_formats();
+  $plugin_libraries = array();
+  foreach ($text_formats as $text_format) {
+    if ($text_format->editor === 'ckeditor5') {
+      $ckeditor5_text_format_libraries = _ckeditor5_get_libraries($text_format);
+      foreach ($ckeditor5_text_format_libraries as $library) {
+        $plugin_libraries[$library[0] . '--' . $library[1]] = $library;
+      }
+    }
+  }
+  $libraries['backdrop.ckeditor5']['dependencies'] = array_merge(
+    $libraries['backdrop.ckeditor5']['dependencies'],
+    array_values($plugin_libraries)
+  );
+
   $libraries['backdrop.ckeditor5.backdrop-basic-styles'] = array(
     'title' => 'CKEditor plugin to convert basic HTML tags to preferred tags.',
     'version' => $info['version'],
@@ -794,7 +806,7 @@ function _ckeditor5_link_image_plugin_check($format, $plugin_name) {
 }
 
 /**
- * Editor JS settings callback; Add CKEditor config to the page for a format.
+ * Get the full CKEditor 5 configuration for a given text format.
  *
  * Note that this function uses the term "config" to match that of CKEditor's
  * own terminology. It is not related to Backdrop's configuration system.
@@ -803,10 +815,15 @@ function _ckeditor5_link_image_plugin_check($format, $plugin_name) {
  *
  * @param object $format
  *   The filter format object for which CKEditor is adding its config.
- * @param array $existing_settings
- *   Settings that have already been added to the page by filters.
  */
-function ckeditor5_get_config($format, array $existing_settings) {
+function ckeditor5_get_config($format) {
+  // Static cache the configuration per format.
+  $ckeditor5_configs = &backdrop_static(__FUNCTION__, array());
+  $format_id = $format->name;
+  if (isset($ckeditor5_configs[$format_id])) {
+    return $ckeditor5_configs[$format_id];
+  }
+
   global $language;
 
   // Loop through all available plugins and check to see if it has been
@@ -858,6 +875,9 @@ function ckeditor5_get_config($format, array $existing_settings) {
       if (isset($all_buttons[$button_name]['plugin']['name'])) {
         $plugin_name = $all_buttons[$button_name]['plugin']['name'];
         $plugins[] = $plugin_name;
+        if (isset($plugin_info[$plugin_name]['library'])) {
+          $libraries[] = $plugin_info[$plugin_name]['library'];
+        }
         if (isset($plugin_info[$plugin_name]['plugin_dependencies'])) {
           $plugins = array_merge($plugin_info[$plugin_name]['plugin_dependencies'], $plugins);
         }
@@ -903,6 +923,10 @@ function ckeditor5_get_config($format, array $existing_settings) {
     // file. Exclude the pseudo-plugins that are configuration-only.
     'pluginList' => array_values(array_diff($plugins, $pseudo_plugins)),
 
+    // The library list also not a CKEditor configuration value. It is used by
+    // Backdrop to attach needed libraries for CKEditor.
+    'libraries' => $libraries,
+
     // 'contentsCss' => array_values($css),
   );
 
@@ -1004,7 +1028,9 @@ function ckeditor5_get_config($format, array $existing_settings) {
 
   backdrop_alter('ckeditor5_config', $config, $format);
 
-  return $config;
+  // Save the configuration into the static variable.
+  $ckeditor5_configs[$format_id] = $config;
+  return $ckeditor5_configs[$format_id];
 }
 
 /**
@@ -1203,6 +1229,38 @@ function ckeditor5_get_generic_html_settings($format) {
   return array($allowed, $disallowed);
 }
 
+/**
+ * Editor JS settings callback; Add CKEditor config to the page for a format.
+ *
+ * @param object $format
+ *    The filter format object for which CKEditor is adding its config.
+ * @param array $existing_settings
+ *    Settings that have already been added to the page by filters.
+ *
+ * @return void
+ */
+function _ckeditor5_js_settings($format, $existing_settings) {
+  $config = ckeditor5_get_config($format);
+  // Remove the Backdrop-specific libraries key.
+  unset($config['libraries']);
+  return $config;
+}
+
+/**
+ * Retrieve a list of CKEditor-related libraries used by a text format.
+ *
+ * @param object $format
+ *   The filter format object for which CKEditor is adding its config.
+ *
+ * @return array
+ *   An array of library keys.
+ */
+function _ckeditor5_get_libraries($format) {
+  $config = ckeditor5_get_config($format);
+  // Return the Backdrop-specific libraries key.;
+  return isset($config['libraries']) ? $config['libraries'] : array();
+}
+
 /**
  * Retrieves the default theme's CKEditor stylesheets defined in the .info file.
  *