--- /dev/null
+<?php
+
+namespace Drupal\block_test\Plugin\Block;
+
+use Drupal\Core\Block\BlockBase;
+use Drupal\Core\Form\FormStateInterface;
+
+/**
+ * Provides a test settings validation block.
+ *
+ * @Block(
+ * id = "test_settings_validation",
+ * admin_label = @Translation("Test settings validation block"),
+ * )
+ */
+class TestSettingsValidationBlock extends BlockBase {
+
+ /**
+ * {@inheritdoc}
+ */
+ public function blockForm($form, FormStateInterface $form_state) {
+ return ['digits' => ['#type' => 'textfield']] + $form;
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function blockValidate($form, FormStateInterface $form_state) {
+ if (!ctype_digit($form_state->getValue('digits'))) {
+ $form_state->setErrorByName('digits', $this->t('Only digits are allowed'));
+ }
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function build() {
+ return ['#markup' => 'foo'];
+ }
+
+}