class BatchTestSimpleForm

class BatchTestSimpleForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'batch_test_simple_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['batch'] = [
            '#type' => 'select',
            '#title' => 'Choose batch',
            '#options' => [
                'batch_0' => 'batch 0',
                'batch_1' => 'batch 1',
                'batch_2' => 'batch 2',
                'batch_3' => 'batch 3',
                'batch_4' => 'batch 4',
                'batch_6' => 'batch 6',
                'batch_7' => 'batch 7',
            ],
            '#multiple' => TRUE,
        ];
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => 'Submit',
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        batch_test_stack(NULL, TRUE);
        foreach ($form_state->getValue('batch') as $batch) {
            $function = '_batch_test_' . $batch;
            batch_set($function());
        }
        $form_state->setRedirect('batch_test.redirect');
    }

}