Add Cancellable utility method

src/main/scala/markatta/futiles/Cancellable.scala

@@ -0,0 +1,47 @@
+package markatta.futiles
+
+import scala.concurrent.ExecutionContext
+import scala.concurrent.Future
+import scala.concurrent.Promise
+import scala.util.Try
+import java.util.concurrent.{Callable, FutureTask}
+
+class Cancellable[T](executionContext: ExecutionContext, block: => T) {
+  private val promise = Promise[T]()
+
+  /** @return
+    *   The underlying [[scala.concurrent.Future]] which you can use
+    */
+  def future: Future[T] = promise.future
+
+  private val jf: FutureTask[T] = new FutureTask[T](
+    new Callable[T] {
+      override def call(): T = block
+    }
+  ) {
+    override def done() = promise.complete(Try(get()))
+  }
+
+  /** Attempts to cancel the underlying [[scala.concurrent.Future]]. Note that this is a best effort attempt
+    */
+  def cancel(): Unit = jf.cancel(true)
+
+  executionContext.execute(jf)
+}
+
+object Cancellable {
+
+  /** Allows you to put a computation inside of a [[scala.concurrent.Future]] which can later be cancelled
+    * @param block
+    *   The computation to run inside of the [[scala.concurrent.Future]]
+    * @param executionContext
+    *   The [[scala.concurrent.ExecutionContext]] to run the [[scala.concurrent.Future]] on
+    * @return
+    *   A [[markatta.futiles.Cancellable]] providing both the [[scala.concurrent.Future]] and a `cancel` method allowing
+    *   you to terminate the [[scala.concurrent.Future]] at any time
+    * @see
+    *   Adapted from https://stackoverflow.com/a/39986418/1519631
+    */
+  def apply[T](block: => T)(implicit executionContext: ExecutionContext): Cancellable[T] =
+    new Cancellable[T](executionContext, block)
+}

src/test/scala/markatta/futiles/CancellableSpec.scala

@@ -0,0 +1,59 @@
+package markatta.futiles
+
+import java.util.concurrent.atomic.AtomicBoolean
+import scala.concurrent.{CancellationException, ExecutionException}
+
+class CancellableSpec extends Spec {
+  describe("The cancellable utility") {
+
+    describe("without cancellation") {
+
+      it("works as a normal Future") {
+        val cancellable = Cancellable {
+          ()
+        }
+
+        cancellable.future.futureValue shouldEqual ()
+      }
+
+      it("throws an Exception correctly") {
+        val cancellable = Cancellable {
+          throw new IllegalArgumentException
+        }
+
+        val exception = cancellable.future.failed.futureValue
+        exception shouldBe an[ExecutionException]
+        exception.getCause shouldBe an[IllegalArgumentException]
+      }
+
+    }
+
+    describe("with cancellation") {
+
+      it("prevents Future from completing") {
+        val atomicBoolean = new AtomicBoolean(true)
+
+        val cancellable = Cancellable {
+          Thread.sleep(100)
+          atomicBoolean.set(false)
+        }
+
+        Thread.sleep(50)
+        cancellable.cancel()
+        Thread.sleep(100)
+        atomicBoolean.get() shouldEqual true
+      }
+
+      it("throws a CancellationException exception") {
+        val cancellable = Cancellable {
+          Thread.sleep(100)
+        }
+        cancellable.cancel()
+        cancellable.future.failed.futureValue shouldBe an[CancellationException]
+      }
+
+    }
+
+  }
+
+}