/**
 * {@inheritdoc}
 */
public function buildForm(array $form, FormStateInterface $form_state) {

    $form['#tree'] = true; //Make the form fields a hierachical array*

    $form['#prefix'] = '<div id="sponsorship-form-wrapper">';
    $form['#suffix'] = '</div>';

    $form['sponsor'] = [
        '#type' => 'fieldset',
        '#title' => $this->t('You (sponsor)'),
    ];
    $form['sponsor']['civility'] = [
        '#type' => 'select',
        '#title' => $this->t('Civility'),
        '#options' => array('Miss' => $this->t('Miss'), 'Mister' => $this->t('Mister')),
        '#size' => 1,
    /* .......... */
    $form['sponsor']['submit'] = [
        '#type' => 'submit',
        '#value' => t('Send'),
    ];


    $form['sponsee'] = [
        '#type' => 'fieldset',
        '#title' => $this->t('Your sponsee'),
        '#prefix' => '<div id="sponsees-fieldset-wrapper">',
        '#suffix' => '</div>',
    ];

    //Default value of nb_sponsees to 1
    if ($form_state->getValue('nb_sponsee') == null) {
        $form_state->setValue('nb_sponsee', 1);
    }

    for ($i = 0; $i < $form_state->getValue('nb_sponsee'); $i++) {
        $form['sponsee'][$i]['civility'] = [
            '#type' => 'select',
            '#title' => $this->t('Civility'),
            '#options' => array('Miss' => $this->t('Miss'), 'Mister' => $this->t('Mister')),
            '#size' => 1,
        ];
        $form['sponsee'][$i]['firstname'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Firstname'),
            '#maxlength' => 64,
            '#size' => 64,
        ];
        $form['sponsee'][$i]['name'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Name'),
            '#maxlength' => 64,
            '#size' => 64,
        ];
        $form['sponsee'][$i]['phone'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Phone'),
            '#maxlength' => 64,
            '#size' => 64,
        ];
        $form['sponsee'][$i]['e_mail'] = [
            '#type' => 'email',
            '#title' => $this->t('E-mail'),
        ];
    }

    //If less of 3 sponsee we add the button
    if ($form_state->getValue('nb_sponsee') < 3) {
        $form['sponsee']['add_sponsee'] = [
            '#type' => 'submit',
            '#name' => 'add_sponsee',
            '#submit' => [$this, 'ajaxHandleSponseesSubmit'],
            '#value' => t('Add a sponsee'),
            '#ajax' => array(
                'callback' => [$this, 'ajaxHandleSponseesCallback'],
                'wrapper' => 'sponsees-fieldset-wrapper',
                'event' => 'click',
                'effect' => 'fade',
                'progress' => array('message' => '', 'type' => 'throbber'),
            ),
        ];
    }

    return $form;
}

/**
 * {@inheritdoc}
 */
public function validateForm(array &$form, FormStateInterface $form_state) {
    parent::validateForm($form, $form_state);
}

/**
 * {@inheritdoc}
 */
public function submitForm(array &$form, FormStateInterface $form_state) {
    // Display result.
    foreach ($form_state->getValues() as $key => $value) {
        drupal_set_message($key . ': ' . $value);
    }
}

/*
 * Custom Submit to increment nb_sponsees
 */
function ajaxHandleSponseesSubmit(array &$form, FormStateInterface &$form_state) {
    $form_state->setValue('nb_sponsee', $form_state->getValue('nb_sponsee') + 1);
    $form_state->setRebuild(true);
}

/**
 * Ajax submit handler that will return the form structure
 */
public function ajaxHandleSponseesCallback(array &$form, FormStateInterface &$form_state) {
    return $form['sponsee'];
}