class MigrateSourcePluginManager

class MigrateSourcePluginManager extends MigratePluginManager {
    
    /**
     * MigrateSourcePluginManager constructor.
     *
     * @param string $type
     *   The type of the plugin: row, source, process, destination, entity_field,
     *   id_map.
     * @param \Traversable $namespaces
     *   An object that implements \Traversable which contains the root paths
     *   keyed by the corresponding namespace to look for plugin implementations.
     * @param \Drupal\Core\Cache\CacheBackendInterface $cache_backend
     *   Cache backend instance to use.
     * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
     *   The module handler to invoke the alter hook with.
     */
    public function __construct($type, \Traversable $namespaces, CacheBackendInterface $cache_backend, ModuleHandlerInterface $module_handler) {
        parent::__construct($type, $namespaces, $cache_backend, $module_handler, 'Drupal\\migrate\\Annotation\\MigrateSource');
    }
    
    /**
     * {@inheritdoc}
     */
    protected function getDiscovery() {
        if (!$this->discovery) {
            $discovery = new AnnotatedClassDiscoveryAutomatedProviders($this->subdir, $this->namespaces, $this->pluginDefinitionAnnotationName, $this->additionalAnnotationNamespaces);
            $this->discovery = new ContainerDerivativeDiscoveryDecorator($discovery);
        }
        return $this->discovery;
    }
    
    /**
     * Finds plugin definitions.
     *
     * @return array
     *   List of definitions to store in cache.
     *
     * @todo This is a temporary solution to the fact that migration source
     *   plugins have more than one provider. This functionality will be moved to
     *   core in https://www.drupal.org/node/2786355.
     */
    protected function findDefinitions() {
        $definitions = $this->getDiscovery()
            ->getDefinitions();
        foreach ($definitions as $plugin_id => &$definition) {
            $this->processDefinition($definition, $plugin_id);
        }
        $this->alterDefinitions($definitions);
        return ProviderFilterDecorator::filterDefinitions($definitions, function ($provider) {
            return $this->providerExists($provider);
        });
    }

}