A “promised” can be used by the producer of a value to notify an interested parties when the value becomes available. Zero or more promises can be created for the promised value via the promise() method. Promised extends Downstream, which represents the write side of async values. To supply the promised value, simply call exactly one of the methods inherited from Downstream.

A “promised” can have zero or more listeners, but can only be fulfilled once. All of the methods inherited from Downstream will throw an Promised.AlreadySuppliedException if a value, error or completion have already been signalled for this topic.

 import ratpack.test.exec.ExecHarness;
 import ratpack.exec.Execution;
 import ratpack.exec.util.Promised;

 import java.util.concurrent.atomic.AtomicReference;
 import java.util.concurrent.CountDownLatch;

 import static org.junit.jupiter.api.Assertions.assertEquals;

 public class Example {
   public static void main(String... args) throws Exception {
     AtomicReference<Long> ref = new AtomicReference<>();
     CountDownLatch latch = new CountDownLatch(1);

     ExecHarness.runSingle(e -> {
       Promised<Long> topic = new Promised<>();

       // create a listener
       Execution.fork().start(e1 ->
         topic.promise().then(v -> {
           ref.set(v);
           latch.countDown();
         })
       );

       // fulfill the topic, notifying listeners
       topic.success(1l);
     });

     latch.await();
     assertEquals(1l, ref.get().longValue());
   }
 }