code: public function getFieldInputSpecification($field)
      {
          if (!$this->hasField($field)) {
              return [];
          }
  
          if (!isset($this->fields[$field]['__filter__'])) {
              $this->fields[$field]['__filter__'] = $this->copyArrayValues(
                  $this->fields[$field],
                  [
                      'input_filter' => [],
                      'required',
                  ]
              );
          }
  
          return $this->fields[$field]['__filter__'];
      }
docs: Get input filter spec for a field.
  
  @param string $field
  
  @return array