class ExampleConfigurableTextBlock

class ExampleConfigurableTextBlock extends BlockBase {
    
    /**
     * {@inheritdoc}
     *
     * This method sets the block default configuration. This configuration
     * determines the block's behavior when a block is initially placed in a
     * region. Default values for the block configuration form should be added to
     * the configuration array. System default configurations are assembled in
     * BlockBase::__construct() e.g. cache setting and block title visibility.
     *
     * @see \Drupal\block\BlockBase::__construct()
     */
    public function defaultConfiguration() {
        return [
            'block_example_string' => $this->t('A default value. This block was created at %time', [
                '%time' => date('c'),
            ]),
        ];
    }
    
    /**
     * {@inheritdoc}
     *
     * This method defines form elements for custom block configuration. Standard
     * block configuration fields are added by BlockBase::buildConfigurationForm()
     * (block title and title visibility) and BlockFormController::form() (block
     * visibility settings).
     *
     * @see \Drupal\block\BlockBase::buildConfigurationForm()
     * @see \Drupal\block\BlockFormController::form()
     */
    public function blockForm($form, FormStateInterface $form_state) {
        $form['block_example_string_text'] = [
            '#type' => 'textarea',
            '#title' => $this->t('Block contents'),
            '#description' => $this->t('This text will appear in the example block.'),
            '#default_value' => $this->configuration['block_example_string'],
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     *
     * This method processes the blockForm() form fields when the block
     * configuration form is submitted.
     *
     * The blockValidate() method can be used to validate the form submission.
     */
    public function blockSubmit($form, FormStateInterface $form_state) {
        $this->configuration['block_example_string'] = $form_state->getValue('block_example_string_text');
    }
    
    /**
     * {@inheritdoc}
     */
    public function build() {
        return [
            '#markup' => $this->configuration['block_example_string'],
        ];
    }

}