MigrationConfigurationForm.php

<?php

namespace Drupal\foo_migrate\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\migrate_plus\Entity\MigrationGroup;

/**
 * Interactive configuration of the Foo migration process.
 */
class MigrationConfigurationForm extends FormBase {

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'foo_migrate_migration_configuration_form';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['#tree'] = TRUE;
    $group = MigrationGroup::load('food7');
    $shared_configuration = $group->get('shared_configuration');
    if (isset($shared_configuration['source']['database'])) {
      $database = $shared_configuration['source']['database'];
    }
    else {
      $database = [
        'database' => NULL,
        'username' => NULL,
        'host' => NULL,
        'port' => 3306,
      ];
    }
    $form['database']['database'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Database'),
      '#default_value' => $database['database'],
    ];
    $form['database']['username'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Username'),
      '#default_value' => $database['username'],
    ];
    $form['database']['password'] = [
      '#type' => 'password',
      '#title' => $this->t('Password'),
    ];
    $form['database']['host'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Host'),
      '#default_value' => $database['host'],
    ];
    $form['database']['port'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Port'),
      '#default_value' => $database['port'],
    ];

    $form['actions'] = ['#type' => 'actions'];
    $form['actions']['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Submit'),
      '#button_type' => 'primary',
    ];

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $group = MigrationGroup::load('food7');
    $shared_configuration = $group->get('shared_configuration');
    $shared_configuration['source']['key'] = 'foo7';
    $shared_configuration['source']['database'] = $form_state->getValue('database');
    $shared_configuration['source']['database']['driver'] = 'mysql';
    $shared_configuration['source']['database']['namespace'] = 'Drupal\\Core\\Database\\Driver\\mysql';
    $shared_configuration['source']['database']['prefix'] = '';
    $group->set('shared_configuration', $shared_configuration);
    $group->save();
    drupal_set_message($this->t('Database configuration saved.'));
  }

}