class Test

class Test extends MediaSourceBase {
  
  /**
   * {@inheritdoc}
   */
  public function getMetadataAttributes() {
    // The metadata attributes are kept in state storage. This allows tests to
    // change the metadata attributes and makes it easier to test different
    // variations.
    $attributes = \Drupal::state()->get('media_source_test_attributes', [
      'attribute_1' => [
        'label' => $this->t('Attribute 1'),
        'value' => 'Value 1',
      ],
      'attribute_2' => [
        'label' => $this->t('Attribute 2'),
        'value' => 'Value 1',
      ],
    ]);
    return array_map(function ($item) {
      return $item['label'];
    }, $attributes);
  }
  
  /**
   * {@inheritdoc}
   */
  public function getMetadata(MediaInterface $media, $attribute_name) {
    $attributes = \Drupal::state()->get('media_source_test_attributes', [
      'attribute_1' => [
        'label' => $this->t('Attribute 1'),
        'value' => 'Value 1',
      ],
      'attribute_2' => [
        'label' => $this->t('Attribute 2'),
        'value' => 'Value 1',
      ],
    ]);
    if (in_array($attribute_name, array_keys($attributes))) {
      return $attributes[$attribute_name]['value'];
    }
    return parent::getMetadata($media, $attribute_name);
  }
  
  /**
   * {@inheritdoc}
   */
  public function getPluginDefinition() {
    return NestedArray::mergeDeep(parent::getPluginDefinition(), \Drupal::state()->get('media_source_test_definition', []));
  }
  
  /**
   * {@inheritdoc}
   */
  public function defaultConfiguration() {
    return parent::defaultConfiguration() + [
      'test_config_value' => 'This is default value.',
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
    $form = parent::buildConfigurationForm($form, $form_state);
    $form['test_config_value'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Test config value'),
      '#default_value' => $this->configuration['test_config_value'],
    ];
    return $form;
  }

}