class FormTestCheckboxForm

class FormTestCheckboxForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return '_test_checkbox_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        // A required checkbox.
        $form['required_checkbox'] = [
            '#type' => 'checkbox',
            '#required' => TRUE,
            '#title' => 'required_checkbox',
        ];
        // A disabled checkbox should get its default value back.
        $form['disabled_checkbox_on'] = [
            '#type' => 'checkbox',
            '#disabled' => TRUE,
            '#return_value' => 'disabled_checkbox_on',
            '#default_value' => 'disabled_checkbox_on',
            '#title' => 'disabled_checkbox_on',
        ];
        $form['disabled_checkbox_off'] = [
            '#type' => 'checkbox',
            '#disabled' => TRUE,
            '#return_value' => 'disabled_checkbox_off',
            '#default_value' => NULL,
            '#title' => 'disabled_checkbox_off',
        ];
        // A checkbox is active when #default_value == #return_value.
        $form['checkbox_on'] = [
            '#type' => 'checkbox',
            '#return_value' => 'checkbox_on',
            '#default_value' => 'checkbox_on',
            '#title' => 'checkbox_on',
        ];
        // But inactive in any other case.
        $form['checkbox_off'] = [
            '#type' => 'checkbox',
            '#return_value' => 'checkbox_off',
            '#default_value' => 'checkbox_on',
            '#title' => 'checkbox_off',
        ];
        // Checkboxes with a #return_value of '0' are supported.
        $form['zero_checkbox_on'] = [
            '#type' => 'checkbox',
            '#return_value' => '0',
            '#default_value' => '0',
            '#title' => 'zero_checkbox_on',
        ];
        // In that case, passing a #default_value != '0'
        // means that the checkbox is off.
        $form['zero_checkbox_off'] = [
            '#type' => 'checkbox',
            '#return_value' => '0',
            '#default_value' => '1',
            '#title' => 'zero_checkbox_off',
        ];
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => t('Submit'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $form_state->setResponse(new JsonResponse($form_state->getValues()));
    }

}