* events.
   */
  open fun onOpen(
    eventSource: EventSource,
    response: Response,
  ) {
  }

  /**
   * Invoked when a new event has been sent to the client.
   *
   * @param id The `id` line of the event, might be null.
   * @param type The `event` line of the event, might be null.
   * @param data The `data` line of the event.
   */
  open fun onEvent(

  // These properties are only accessed by the thread executing the call.

  /** Initialized in [callStart]. */
  private var callStackTrace: Any? = null

  /** Finds an exchange to send the next request and receive the next response. */
  private var exchangeFinder: ExchangeFinder? = null

  var connection: RealConnection? = null
    private set
  private var timeoutEarlyExit = false

  /**

          System.out.println("Vary: " + response.headers("Vary"));
        }
      }
    ```

### Posting a String ([.kt][PostStringKotlin], [.java][PostStringJava])

Use an HTTP POST to send a request body to a service. This example posts a markdown document to a web service that renders markdown as HTML. Because the entire request body is in memory simultaneously, avoid posting large (greater than 1 MiB) documents using this API.