How to create a form using block module in drupal 8?

Question

I want build a form using a block module in Drupal 8. I am aware of building the forms in Drupal 7 but the same seems to be different in Drupal 8.

Answer 1

To build a form using block module, you can easily use Webform module where you can add a form and display as a block.

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If you mean to create a form programatically in the custom block, you can achieve that by creating two files shown below:

Form file (src/Form/DemoForm.php):

 'email',
      '#title' => $this->t('Your .com email address.')
    );
    $form['show'] = array(
      '#type' => 'submit',
      '#value' => $this->t('Submit'),
    );

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, array &$form_state) {
    $values = $form_state->getValues();
    if (strpos($values['email'], '.com') === FALSE ) {
      $form_state->setErrorByName('email', t('This is not a .com email address.'));
    } 
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, array &$form_state) {
    drupal_set_message($this->t('Your email address is @email', array('@email' => $form_state['values']['email'])));
  }

}

^{Source: Building a Drupal 8 Module: Blocks and Forms.}

Block file (src/Plugin/Block/HelloBlock.php):

getForm('\Drupal\mymodule\Form\HelloBlock');
    //$form['#attached']['js'][] = drupal_get_path('module', 'example') .  '/js/example.js';
    //$form['#markup'] = $this->t('Custom text');
    return $form;
  }

}

^{Source: Create a custom block.}

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To add a form to the Block Configuration, see: Add a Form to the Block Configuration.