code: public function prepareResponse()
      {
          if (!$this->writer) {
              $this->writer = $this->writerFactory->getWriterFor(
                  $this->request->getFormats()
              );
          }
          $this->preparedResponse = $this->writer->format($this->getBody(), $this->responseName);
          return $this;
      }
docs: Prepare the response.
  
  This selects a writer from the writer factory and prepares serialises
  the data into a string. This is done early to allow exceptions and
  errors to be caught and handled earlier.
  
  @return $this