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;
    }

}