--- /dev/null
+<?php
+
+namespace Drupal\libraries\ExternalLibrary\Asset;
+
+use Drupal\libraries\ExternalLibrary\Dependency\DependentLibraryInterface;
+use Drupal\libraries\ExternalLibrary\LibraryInterface;
+use Drupal\libraries\ExternalLibrary\LibraryManagerInterface;
+use Drupal\libraries\ExternalLibrary\Version\VersionedLibraryInterface;
+
+/**
+ * Provides an interface for external asset libraries with a single library.
+ *
+ * Asset is the generic term for CSS and JavaScript files.
+ *
+ * In order to load assets of external libraries as part of a page request the
+ * assets must be registered with Drupal core's library system. Therefore,
+ * Libraries API makes all libraries that are required by the installed
+ * installation profile, modules, and themes available as core asset libraries
+ * with the identifier 'libraries/[machine_name]' where '[machine_name]' is
+ * the Libraries API machine name of the external library.
+ *
+ * Thus, assuming that the external library 'flexslider' has been declared as a
+ * dependency, for example, it can be attached to a render array in the $build
+ * variable with the following code:
+ * @code
+ * $build['#attached']['library'] = ['libraries/flexslider'];
+ * @endcode
+ *
+ * In some cases an external library may contain multiple components, that
+ * should be loadable independently from each other. In this case, implement
+ * MultipleAssetLibraryInterface instead.
+ *
+ * @see libraries_library_info_build()
+ * @see \Drupal\libraries\ExternalLibrary\Asset\AssetLibraryTrait
+ * @see \Drupal\libraries\ExternalLibrary\Asset\MultipleAssetLibraryInterface
+ *
+ * @todo Support loading of source or minified assets.
+ * @todo Document how library dependencies work.
+ *
+ * @ingroup libraries
+ */
+interface AssetLibraryInterface extends
+ LibraryInterface,
+ VersionedLibraryInterface,
+ DependentLibraryInterface
+{
+
+ /**
+ * Returns a core asset library array structure for this library.
+ *
+ * @param \Drupal\libraries\ExternalLibrary\LibraryManagerInterface $library_manager
+ * The library manager that can be used to fetch dependencies.
+ *
+ * @return array
+ *
+ * @see \Drupal\libraries\ExternalLibrary\Asset\SingleAssetLibraryTrait
+ *
+ * @throws \Drupal\libraries\ExternalLibrary\Exception\InvalidLibraryDependencyException
+ *
+ * @todo Document the return value.
+ * @todo Reconsider passing the library manager.
+ */
+ public function getAttachableAssetLibrary(LibraryManagerInterface $library_manager);
+
+}