Add support for handling async event results

dew-common/src/dev/shiza/dew/result/ResultHandlerService.java

@@ -3,6 +3,7 @@
 import dev.shiza.dew.event.Event;
 import java.util.Map;
 import java.util.Map.Entry;
+import java.util.concurrent.CompletableFuture;
 
 final class ResultHandlerService implements ResultHandlerFacade {
 
@@ -24,6 +25,12 @@ public <E extends Event, T> void process(final E event, final T value) {
       return;
     }
 
+    final Class<?> resultType = value.getClass();
+    if (resultType == CompletableFuture.class) {
+      processPromise((CompletableFuture<?>) value, event);
+      return;
+    }
+
     final ResultHandler<?, ?> resultHandler = getResultHandler(value.getClass());
     if (resultHandler == null) {
       throw new ResultHandlingException(
@@ -34,6 +41,19 @@ public <E extends Event, T> void process(final E event, final T value) {
     ((ResultHandler<E, T>) resultHandler).handle(event, value);
   }
 
+  private <E extends Event, T> void processPromise(
+      final CompletableFuture<T> resultFuture, final E event) {
+    resultFuture
+        .whenComplete((result, cause) -> process(event, result))
+        .exceptionally(
+            cause -> {
+              throw new ResultHandlingException(
+                  "Could not handle result of type %s, because of an exception."
+                      .formatted(cause.getClass().getName()),
+                  cause);
+            });
+  }
+
   private ResultHandler<?, ?> getResultHandler(final Class<?> clazz) {
     final ResultHandler<?, ?> resultHandler = handlers.get(clazz);
     if (resultHandler != null) {

dew-common/src/dev/shiza/dew/result/ResultHandlingException.java

@@ -5,4 +5,8 @@ public final class ResultHandlingException extends IllegalStateException {
   public ResultHandlingException(final String message) {
     super(message);
   }
+
+  public ResultHandlingException(final String message, final Throwable cause) {
+    super(message, cause);
+  }
 }