scala - 当两个 promise 完成时触发 Action

Question

我希望在两个 promise 都成功时完成一些操作。所以我创建了这样一段代码:

val promiseForPersistence = Promise[Unit]()
val promiseForReplication = Promise[Unit]()

promiseForPersistence.future.onComplete {
  case scala.util.Success(_) => {
    if (promiseForReplication.isCompleted) {
      println(s"Acking from persistence promise (replication one already completed)" )
    }
  }
  case _ =>
}

promiseForReplication.future.onComplete {
  case scala.util.Success(_) => {
    if (promiseForPersistence.isCompleted) {
      println(s"Acking from replication promise (persistence one already completed)" )
    }
  }
  case _ =>
}

promiseForPersistence.success()
promiseForReplication.success()

问题是，两条日志消息都被打印出来了。我希望 Action 只被解雇一次。为什么它会被触发两次，我怎样才能让它只触发一次？

Answer 1

您不应为此目的在 Future 上使用 isCompleted 方法。您应该像这样使用 flatMap 将 2 个 futures 组合成一个 futures:

for {
  _ <- promiseForPersistence.future
  _ <- promiseForReplication.future
} println("Both are completed" )

onComplete 中的回调可以在两个 Promise 完成后进行评估。所以你会在回调和 2 个 println 消息中得到 isCompleted == true。

有效的方法是在回调中添加回调:

promiseForPersistence.future.onSuccess { _ =>
  promiseForReplication.future.onSuccess { _ =>
    println(s"Acking from replication promise (persistence one already completed)" )
  }
}

使用 flatMap(for-comprehension)，您将获得完全相同的行为，但语法不那么冗长。