CallbacksTest.scala

import java.util.concurrent.Semaphore
import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global
import org.scalatest.FunSuite

class CallbacksTest extends FunSuite {
  test("Callback are complicated") {
    var result: Int = 0; val semaphore = new Semaphore(1, true)

    generateNumber(generatedNumber => addThree(generatedNumber,
      addingResult => {
        result = addingResult
        semaphore.release()
      }))

    semaphore.acquire()
    assert(result == 10)
  }

  private def generateNumber[A](onSuccess: Int => Unit) = Future {
    Thread.sleep(100); onSuccess(7)
  }

  private def addThree[A](value: Int, onSuccess: Int => Unit): Unit = onSuccess(value + 3)
}