class FormTestProgrammaticForm

class FormTestProgrammaticForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'form_test_programmatic_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['textfield'] = [
            '#title' => 'Textfield',
            '#type' => 'textfield',
        ];
        $form['checkboxes'] = [
            '#title' => t('Checkboxes'),
            '#type' => 'checkboxes',
            '#options' => [
                1 => 'First checkbox',
                2 => 'Second checkbox',
            ],
            // Both checkboxes are selected by default so that we can test the ability
            // of programmatic form submissions to uncheck them.
'#default_value' => [
                1,
                2,
            ],
        ];
        $form['field_to_validate'] = [
            '#type' => 'radios',
            '#title' => 'Field to validate (in the case of limited validation)',
            '#description' => 'If the form is submitted by clicking the "Submit with limited validation" button, then validation can be limited based on the value of this radio button.',
            '#options' => [
                'all' => 'Validate all fields',
                'textfield' => 'Validate the "Textfield" field',
                'field_to_validate' => 'Validate the "Field to validate" field',
            ],
            '#default_value' => 'all',
        ];
        $form['field_restricted'] = [
            '#type' => 'textfield',
            '#title' => 'Textfield (no access)',
            '#access' => FALSE,
        ];
        // The main submit button for the form.
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => 'Submit',
        ];
        // A secondary submit button that allows validation to be limited based on
        // the value of the above radio selector.
        $form['submit_limit_validation'] = [
            '#type' => 'submit',
            '#value' => 'Submit with limited validation',
            // Use the same submit handler for this button as for the form itself.
            // (This must be set explicitly or otherwise the form API will ignore the
            // #limit_validation_errors property.)
'#submit' => [
                '::submitForm',
            ],
        ];
        $user_input = $form_state->getUserInput();
        if (!empty($user_input['field_to_validate']) && $user_input['field_to_validate'] != 'all') {
            $form['submit_limit_validation']['#limit_validation_errors'] = [
                [
                    $user_input['field_to_validate'],
                ],
            ];
        }
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
        if ($form_state->isValueEmpty('textfield')) {
            $form_state->setErrorByName('textfield', t('Textfield is required.'));
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $form_state->set('programmatic_form_submit', $form_state->getValues());
    }

}