Here is a quick scalafiddle that runs the code from 2 years ago :)
https://scalafiddle.io/sf/xaGO6Yr/0
On Mon, Nov 5, 2018 at 8:18 AM Martynas Mickevičius <2...@lightbend.com>
wrote:
> Did you send any elements to the stream? internalCounter is incremented
> in the map operator for every incoming stream element.
>
> Trait is only used to hide the implementation details of the counter.
>
> On Thu, Nov 1, 2018 at 12:04 AM <jurgis.p...@dataqube.de> wrote:
>
>> Hi Rafal,
>>
>> I just stumbled upon your reply. Could you explain why you have to wrap
>> the AtomicLong into the Counter trait? I tried returning the naked
>> AtomicLong in the 'mapMaterializedValue' function, but the resulting value
>> was always 0. Why does wrapping in a trait produce a different result?
>>
>> Best,
>> Jurgis
>>
>>
>> On Sunday, March 6, 2016 at 1:46:12 AM UTC+1, Rafał Krzewski wrote:
>>>
>>> Hi,
>>> there are a few ways of doing that. Probably the simplest one is using
>>> Flow.mapMaterializedValue. Suppose you'd like to create a Flow that counts
>>> the elements that pass through it and makes the current count available
>>> through a "side channel":
>>>
>>> trait Counter {
>>> def get: Long
>>> }
>>>
>>> def counter[T]: Flow[T, T, Counter] = {
>>> val internalCounter = new AtomicLong(0)
>>> Flow[T].map{ elem ⇒
>>> internalCounter.incrementAndGet()
>>> elem
>>> }.mapMaterializedValue(_ ⇒ new Counter{
>>> override def get = internalCounter.get
>>> })
>>> }
>>>
>>> Another way is using a GraphStageWithMaterializedValue while building a
>>> custom Flow / Sink / Source. Instead of returning a GraphStageLogic,
>>> like an ordinary GraphStage, you return a pair of GraphStageLogic and
>>> the materialized value.
>>>
>>> Cheers,
>>> Rafał
>>>
>>> W dniu niedziela, 6 marca 2016 01:02:56 UTC+1 użytkownik Arun Sethia
>>> napisał:
>>>>
>>>> Hi,
>>>>
>>>> can some explain what does it mean of materialized value ? I have see
>>>> documentation at
>>>> http://doc.akka.io/docs/akka-stream-and-http-experimental/2.0.3/scala/stream-quickstart.html#transforming-and-consuming-simple-streams
>>>>
>>>> I am not sure how Flow can define materialize type, for example the
>>>> following code has Input - Tweet, output - Int but Mat is Unit. I would
>>>> like to see how someone can define Mat as Int or any example where Flow or
>>>> source is defining Mat other than Unit.
>>>>
>>>>
>>>> - val count: Flow[Tweet, Int, Unit] = Flow[Tweet].map(_ => 1)
>>>>
>>>>
>>>>
>>>> It is quite confusing for me to understand difference between "out"
>>>> and "Mat".
>>>>
>>>>
>>>> Thanks
>>>>
>>>> As
>>>>
>>>> --
>>
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>>>>>>>>>>
>>>>>>>>>> Read the docs: http://akka.io/docs/
>>>>>>>>>> Check the FAQ:
>>>>>>>>>> http://doc.akka.io/docs/akka/current/additional/faq.html
>>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user
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