--- /dev/null
+<?php
+
+namespace Drupal\libraries\ExternalLibrary\Definition;
+
+use Drupal\libraries\ExternalLibrary\Exception\LibraryDefinitionNotFoundException;
+
+/**
+ * Provides a definition discovery that checks a list of other discoveries.
+ *
+ * The discoveries are checked sequentially. If the definition was not present
+ * in some discoveries but is found in a later discovery the definition will be
+ * written to the earlier discoveries if they implement
+ * WritableDefinitionDiscoveryInterface.
+ *
+ * @see \Drupal\libraries\ExternalLibrary\Definition\WritableDefinitionDiscoveryInterface
+ */
+class ChainDefinitionDiscovery implements DefinitionDiscoveryInterface {
+
+ /**
+ * The list of definition discoveries that will be checked.
+ *
+ * @var \Drupal\libraries\ExternalLibrary\Definition\DefinitionDiscoveryInterface[]
+ */
+ protected $discoveries = [];
+
+ /**
+ * {@inheritdoc}
+ */
+ public function hasDefinition($id) {
+ foreach ($this->discoveries as $discovery) {
+ if ($discovery->hasDefinition($id)) {
+ return TRUE;
+ }
+ }
+
+ return FALSE;
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function getDefinition($id) {
+ /** @var \Drupal\libraries\ExternalLibrary\Definition\WritableDefinitionDiscoveryInterface[] $discoveries_to_write */
+ $discoveries_to_write = [];
+ foreach ($this->discoveries as $discovery) {
+ if ($discovery->hasDefinition($id)) {
+ $definition = $discovery->getDefinition($id);
+ break;
+ }
+ elseif ($discovery instanceof WritableDefinitionDiscoveryInterface) {
+ $discoveries_to_write[] = $discovery;
+ }
+ }
+
+ if (!isset($definition)) {
+ throw new LibraryDefinitionNotFoundException($id);
+ }
+
+ foreach ($discoveries_to_write as $discovery_to_write) {
+ $discovery_to_write->writeDefinition($id, $definition);
+ }
+
+ return $definition;
+ }
+
+ /**
+ * Adds a definition discovery to the list to check.
+ *
+ * @param \Drupal\libraries\ExternalLibrary\Definition\DefinitionDiscoveryInterface $discovery
+ * The definition discovery to add.
+ *
+ * @return $this
+ */
+ public function addDiscovery(DefinitionDiscoveryInterface $discovery) {
+ $this->discoveries[] = $discovery;
+ return $this;
+ }
+
+}