Scala futures and method call backs using spray routing - scala

Question

I have this problem where I want to call a future in spray routing. I have tried below and I have it working and return the future value in onComplete
def sendFuture(input):Future[String]= Future {
//some string manipulation
result
}
post {
entity(as[String]) {
input =>
{
onComplete(sendFuture(input)) {
case Success(result) => {
complete("received: " + result)
}
case Failure(ex) => complete("An error occurred: " + ex.getMessage())
}
}
}
}
Now, I now want to do something more complex. I want to do a call a callback that accepts an argument inside sendFuture and return that string to sendFuture before sendFuture returns it to onComplete. Something like below:
def sendFuture(input):Future[String]= Future {
//some string manipulation
callBack.call(
(arg:String)=>
{
//some string manipulation
return callBackResult
}
)
//do something with result based on callBackResult
result
}
post {
entity(as[String]) {
input =>
{
onComplete(sendFuture(input)) {
case Success(result) => {
complete("received: " + result)
}
case Failure(ex) => complete("An error occurred: " + ex.getMessage())
}
}
}
}
If only callBack.call() would return a string but I it is a void/unit function.

Answer 1

You need to split sendFuture into two function and use map on the result (which is a future) of the first function, but i don't know where you get this callback.call stuff, that's not Scala Future. Anyway the logic is simple:
The first part of the function should contain a promise, which would expect the result of your callback (everything belongs to Scala implementation of Futures), e.g:
def computeFirstpart(input: String): Future[String] = {
val promise: Promise[String] = Promise()
// do something with your string
callback.onComplete { case v => promise.complete(v) }
promise.future
}
This part returns the result of your callback. Now you can attach more computations:
def sendFuture(input: String): Future[String] = {
computeFirstPart(input) flatMap { resultOfFirstPart =>
// do what you need
result // NOTE 1
}
}
NOTE 1 The result should be in Future context, you are returning simple type (like String) use map, not flatMap. Then use it in your route structure:
post {
entity(as[String]) { input =>
onComplete(sendFuture(input)) {
case Success(result) =>
complete("received: " + result)
case Failure(ex) =>
complete("An error occurred: " + ex.getMessage())
}
}
}
}