FileIO.toPath接收器完成时触发

而不是 alsoTo ,使用 alsoToMat 和 mapMaterializedValue Sink

val source: Source[ByteString, _] = ???

val cachedSource =
  source
    .alsoToMat(FileIO.toPath(path))(Keep.right)
    .mapMaterializedValue { fileResult => // fileResult is a Future[IOResult]
      // do something with fileResult
    }