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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