问个 akka stream 的问题

打算从 issues 入手读 akka 源码， 给 stream operator 写写文档示例，

lazy source 的描述:

  /**
   * Defers invoking the `create` function to create a future source until there is downstream demand.
   *
   * The returned source will emit downstream and behave just like it was the outer source. Downstream completes
   * when the created source completes and fails when the created source fails.
   *
   * Note that asynchronous boundaries (and other operators) in the stream may do pre-fetching which counter acts
   * the laziness and will trigger the factory immediately.
   *
   * The materialized future value is completed with the materialized value of the created source when
   * it has been materialized. If the function throws or the source materialization fails the future materialized value
   * is failed with the thrown exception.
   *
   * If downstream cancels or fails before the function is invoked the materialized value
   * is failed with a [[akka.stream.NeverMaterializedException]]
   */
  def lazySource[T, M](create: () => Source[T, M]): Source[T, Future[M]] =

请教下 v 友什么场景会使用到lazy source啊, 实在想不到