Question

I am using a function from third party code, which looks like this:

scala> def willCallback(fun: Function1[Int, Unit]) {
     | doWork()
     | fun(1)
     | }
willCallback: (fun: Int => Unit)Unit

In my code, I define a function and would like it to return the callback function parameter, to achieve this:

scala> def callbackResult():Int = {
     |   willCallback( (i:Int) => {
     |     // What do I put here 
     |     // to make the return value of callbackResult to be i?
     |   })
     | }

What can I do to make it work?

Thanks.

Answer 1

From your comment it seems that you want willCallback to block until the callback has executed. Here’s how you can do it with the Future/Promise API in Scala 2.10,

import scala.concurrent._

// Library code (which you can't modify)
def willCallback(fun: Int => Unit) { fun(1) }

// Your code. You can fulfill a `promise` in your callback
def callbackResult(): Int = {
  val p = promise[Int]
  val f = p.future

  willCallback { i: Int =>
    p.success(i)
  }

  // Block until the promise has been fulfilled (the callback has executed).
  // In this case, the return value is 1
  Await.result(f, duration.Duration.Inf)
}

Of course, there is the danger that if the callback never executes, your callbackResult() method will hang indefinitely. To avoid this danger, it might be better for callbackResult() to return f: Future[Int] rather than await its Int value, as Régis Jean-Gilles suggests.