class ReferenceBase

abstract class ReferenceBase extends FieldPluginBase {
    
    /**
     * Gets the plugin ID for the reference type migration.
     *
     * The reference type migration will be added as a required dependency.
     *
     * @return string
     *   The plugin id.
     */
    protected abstract function getEntityTypeMigrationId();
    
    /**
     * Gets the name of the field property which holds the entity ID.
     *
     * @return string
     *   The entity id.
     */
    protected abstract function entityId();
    
    /**
     * {@inheritdoc}
     */
    public function alterFieldInstanceMigration(MigrationInterface $migration) {
        parent::alterFieldInstanceMigration($migration);
        // Add the reference migration as a required dependency to this migration.
        $migration_dependencies = $migration->getMigrationDependencies(TRUE);
        array_push($migration_dependencies['required'], $this->getEntityTypeMigrationId());
        $migration_dependencies['required'] = array_unique($migration_dependencies['required']);
        $migration->set('migration_dependencies', $migration_dependencies);
    }
    
    /**
     * {@inheritdoc}
     */
    public function defineValueProcessPipeline(MigrationInterface $migration, $field_name, $data) {
        $process = [
            'plugin' => 'sub_process',
            'source' => $field_name,
            'process' => [
                'target_id' => $this->entityId(),
            ],
        ];
        $migration->setProcessOfProperty($field_name, $process);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFieldWidgetMap() {
        return [
            $this->pluginId . '_select' => 'options_select',
            $this->pluginId . '_buttons' => 'options_buttons',
            $this->pluginId . '_autocomplete' => 'entity_reference_autocomplete_tags',
        ];
    }

}