Iterating in reverse dependency order, as asked in http://stackoverflow.com/questions/31688017/how-to-observe-on-reverse-dependency-order/32006887#32006887

new_gist_file.scala

import scala.collection.JavaConverters._
import com.amazonaws.services.s3.AmazonS3Client
import rx.lang.scala._
import rx.lang.scala.schedulers._
import com.amazonaws.services.s3.model._


/**
 * @author jfaerman
 */
object RevDepIterator extends App {
  val S3 = new AmazonS3Client

  val buckets = Observable.from {
    S3.listBuckets.asScala
  }

  val things = buckets.flatMap { b => Observable.from(
    S3.listObjects(new ListObjectsRequest().withBucketName(b.getName))
      .getObjectSummaries
      .asScala)  :+ b
  }
  
  for (t <- things) t match {
    case b:Bucket => println(s"delete bucket ${b.getName}")
    case o:S3ObjectSummary => println(s"delete object ${o.getBucketName}/${o.getKey}")
  }
  
}