code: public function field($model, $attribute, $options = [])
      {
          $config = $this->fieldConfig;
          if ($config instanceof \Closure) {
              $config = call_user_func($config, $model, $attribute);
          }
          if (!isset($config['class'])) {
              $config['class'] = $this->fieldClass;
          }
  
          return Yii::createObject(ArrayHelper::merge($config, $options, [
              'model' => $model,
              'attribute' => $attribute,
              'form' => $this,
          ]));
      }
docs: Generates a form field.
  A form field is associated with a model and an attribute. It contains a label, an input and an error message
  and use them to interact with end users to collect their inputs for the attribute.
  @param Model $model the data model.
  @param string $attribute the attribute name or expression. See [[Html::getAttributeName()]] for the format
  about attribute expression.
  @param array $options the additional configurations for the field object. These are properties of [[ActiveField]]
  or a subclass, depending on the value of [[fieldClass]].
  @return ActiveField the created ActiveField object.
  @see fieldConfig