Evaluates its parameter of type Future[T], and once the Future has been completed, extracts its result as a value of type Try[T] and passes it to the inner route.


def onComplete[T](future:  Future[T])(implicit ec: ExecutionContext): Directive1[Try[T]]

The signature shown is simplified, the real signature uses magnets. [1]

[1]See The Magnet Pattern for an explanation of magnet-based overloading.


The evaluation of the inner route passed to a onComplete directive is deferred until the given future has completed and provided with a extraction of type Try[T].

It is necessary to bring a ExecutionContext into implicit scope for this directive to work.

To handle the Failure case automatically and only work with the result value, use onSuccess. To complete with a successful result automatically and just handle the failure result, use onFailure, instead.


def divide(a: Int, b: Int): Future[Int] = Future {
  a / b

val route =
  path("divide" / IntNumber / IntNumber) { (a, b) =>
    onComplete(divide(a, b)) {
      case Success(value) => complete(s"The result was $value")
      case Failure(ex)    => complete(InternalServerError, s"An error occurred: ${ex.getMessage}")

Get("/divide/10/2") ~> route ~> check {
  responseAs[String] === "The result was 5"

Get("/divide/10/0") ~> sealRoute(route) ~> check {
  status === InternalServerError
  responseAs[String] === "An error occurred: / by zero"