class AjaxAddRemoveElements

class AjaxAddRemoveElements extends FormBase {
    
    /**
     * Required by FormBase.
     */
    public function getFormId() {
        return 'form_api_example_ajaxaddremoveelements';
    }
    
    /**
     * Form with 'add more' and 'remove' buttons.
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['description'] = [
            '#type' => 'item',
            '#markup' => $this->t('This example shows an add-more button and a remove button for each specific element.'),
        ];
        // Get the number of names in the form already.
        $num_lines = $form_state->get('num_lines');
        // We have to ensure that there is at least one name field.
        if ($num_lines === NULL) {
            $form_state->set('num_lines', 1);
            $num_lines = $form_state->get('num_lines');
        }
        // Get a list of fields that were removed.
        $removed_fields = $form_state->get('removed_fields');
        // If no fields have been removed yet we use an empty array.
        if ($removed_fields === NULL) {
            $form_state->set('removed_fields', []);
            $removed_fields = $form_state->get('removed_fields');
        }
        $form['#tree'] = TRUE;
        $form['names_fieldset'] = [
            '#type' => 'fieldset',
            '#title' => $this->t('People coming to picnic'),
            '#prefix' => '<div id="names-fieldset-wrapper">',
            '#suffix' => '</div>',
        ];
        for ($i = 0; $i < $num_lines; $i++) {
            // Check if field was removed.
            if (in_array($i, $removed_fields)) {
                // Skip if field was removed and move to the next field.
                continue;
            }
            
            /* Create a new fieldset for each person
             * where we can add first and last name
             */
            // Fieldset title.
            $form['names_fieldset'][$i] = [
                '#type' => 'fieldset',
                '#title' => $this->t('Person') . ' ' . ($i + 1),
            ];
            // Date.
            $form['names_fieldset'][$i]['firstname'] = [
                '#type' => 'textfield',
                '#title' => $this->t('First name'),
            ];
            // Amount.
            $form['names_fieldset'][$i]['lastname'] = [
                '#type' => 'textfield',
                '#title' => $this->t('Last name'),
            ];
            $form['names_fieldset'][$i]['actions'] = [
                '#type' => 'submit',
                '#value' => $this->t('Remove'),
                '#name' => $i,
                '#submit' => [
                    '::removeCallback',
                ],
                '#ajax' => [
                    'callback' => '::addmoreCallback',
                    'wrapper' => 'names-fieldset-wrapper',
                ],
            ];
        }
        $form['names_fieldset']['actions'] = [
            '#type' => 'actions',
        ];
        $form['names_fieldset']['actions']['add_name'] = [
            '#type' => 'submit',
            '#value' => $this->t('Add one more'),
            '#submit' => [
                '::addOne',
            ],
            '#ajax' => [
                'callback' => '::addmoreCallback',
                'wrapper' => 'names-fieldset-wrapper',
            ],
        ];
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Submit'),
        ];
        return $form;
    }
    
    /**
     * Callback for both ajax-enabled buttons.
     *
     * Selects and returns the fieldset with the names in it.
     */
    public function addmoreCallback(array &$form, FormStateInterface $form_state) {
        return $form['names_fieldset'];
    }
    
    /**
     * Submit handler for the "add-one-more" button.
     *
     * Increments the max counter and causes a rebuild.
     */
    public function addOne(array &$form, FormStateInterface $form_state) {
        $num_field = $form_state->get('num_lines');
        $add_button = $num_field + 1;
        $form_state->set('num_lines', $add_button);
        $form_state->setRebuild();
    }
    
    /**
     * Submit handler for the "remove" button.
     *
     * Removes the corresponding line.
     */
    public function removeCallback(array &$form, FormStateInterface $form_state) {
        
        /*
         * We use the name of the remove button to find
         * the element we want to remove
         * Line 72: '#name' => $i,.
         */
        $trigger = $form_state->getTriggeringElement();
        $indexToRemove = $trigger['#name'];
        // Remove the fieldset from $form (the easy way)
        unset($form['names_fieldset'][$indexToRemove]);
        // Remove the fieldset from $form_state (the hard way)
        // First fetch the fieldset, then edit it, then set it again
        // Form API does not allow us to directly edit the field.
        $namesFieldset = $form_state->getValue('names_fieldset');
        unset($namesFieldset[$indexToRemove]);
        // $form_state->unsetValue('names_fieldset');
        $form_state->setValue('names_fieldset', $namesFieldset);
        // Keep track of removed fields so we can add new fields at the bottom
        // Without this they would be added where a value was removed.
        $removed_fields = $form_state->get('removed_fields');
        $removed_fields[] = $indexToRemove;
        $form_state->set('removed_fields', $removed_fields);
        // Rebuild form_state.
        $form_state->setRebuild();
    }
    
    /**
     * Required by FormBase.
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
    }
    
    /**
     * Required by FormBase.
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
    }

}