Issue #3440278 + bonus : various bugfixes to allow correct nesting, correct source plugins context filtering. super nice

Closes #3440278

modules/ui_patterns_field_formatters/src/Plugin/Derivative/FieldPropertiesSourceDeriverBase.php

@@ -112,11 +112,14 @@ abstract class FieldPropertiesSourceDeriverBase extends DeriverBase implements C
     $field_name_context = new ContextDefinition('string');
     $field_name_context
       ->setRequired()
+      ->setLabel("field_name")
       ->setDefaultValue($field_name)
       ->addConstraint('AllowedValues', [$field_name]);
     $bundle_context = new ContextDefinition('string');
     $bundle_context
       ->setRequired()
+      ->setLabel("bundle")
+      // @todo check exactly why there is "" in the allowed values.
       ->addConstraint('AllowedValues', array_merge(array_values($bundles), [""]));
     $derivative_for_this_field = array_merge($base_plugin_definition, [
       // The base id for the derivatives.
@@ -165,6 +168,13 @@ abstract class FieldPropertiesSourceDeriverBase extends DeriverBase implements C
   public function getDerivativeDefinitionsForField(array $derivative_for_this_field, FieldStorageDefinitionInterface $field_storage_definition) {
     // Generate one plugin for each field property,
     // compatible only for the corresponding bundles.
+    $ui_patterns_component_per_item_context = new ContextDefinition('string');
+    $ui_patterns_component_per_item_context
+      ->setRequired(TRUE)
+      ->setLabel("ui_patterns_entity_field_granularity")
+      ->setDefaultValue("item")
+      // This line is fundamental, and does what we want to achieve.
+      ->addConstraint("AllowedValues", ["item"]);
     foreach ($field_storage_definition->getPropertyDefinitions() as $property_id => $property_definition) {
       // Skip entity reference
       // Description could have been more precise,
@@ -184,6 +194,8 @@ abstract class FieldPropertiesSourceDeriverBase extends DeriverBase implements C
           "prop_types" => $this->getPropTypesDeriver($property_definition),
         ]
       );
+      $derivative['context_definitions'] = array_merge(
+        $derivative['context_definitions'], ["ui_patterns_entity_field_granularity" => $ui_patterns_component_per_item_context]);
       $derivative['id'] = implode(PluginBase::DERIVATIVE_SEPARATOR, [
         $derivative['id'],
         $property_id,
@@ -191,7 +203,11 @@ abstract class FieldPropertiesSourceDeriverBase extends DeriverBase implements C
       $derivative['ui_patterns_field_formatter'] = array_merge($derivative['ui_patterns_field_formatter'], [
         'property' => $property_id,
       ]);
-      $this->derivatives[$derivative['id']] = $derivative;
+      // If no compatible prop types were inferred, we discard the plugin.
+      if (count($derivative["prop_types"]) > 0) {
+        $this->derivatives[$derivative['id']] = $derivative;
+      }
+
     }
   }