在Scala中通过Observable实现和折叠

Question

Why this code never fires the subscription block?

Observable(1, 2, 3).materialize.foldLeft(0) { (acc, tn) =>
  tn match {
    case OnNext(n) => { println(acc, n); acc + n }
  }
}.subscribe({s => println("THIS IS NOT FIRED", s) })

This is the output: (0,1) (1,2) (3,3)

Answer 1

I am a Scala beginner as well, so bear with me if anything is not explained properly.
It seems that you are missing the onCompleted case:

Try it with:

 Observable(1, 2, 3).materialize.foldLeft(0) { (acc, tn) =>
   tn match {
   case Notification.OnNext(n)  => { println(acc, n); acc + n }
   case Notification.OnCompleted() => acc
  }
}.subscribe({s => println("THIS IS NOW FIRED", s) })

But since the value returned from foldLeft is essentially one (the sum), it'd become more readable if we rewrite it as follows:

   val sum = Observable(1, 2, 3).materialize.foldLeft(0) { (acc, tn) =>
       tn match {
       case Notification.OnNext(n)  => { println(acc, n); acc + n }
       case Notification.OnCompleted() => acc
      }
    }.toBlockingObservable.single

   println(sum) // prints 6

Needless to say that the previous solutions hold True only for cases where there is no element in the stream that raises an Error.

I found this link very helpful to understand Observables.