Current (working but not polished) implementation of onUI and onBG

AndroidUtils.scala

import scala.collection.mutable.{HashMap, Map}
import scala.concurrent.Channel
import scala.concurrent.ops.spawn

import android.content.Context
import android.os.{Handler, Message}

object AndroidBG {

  private val androidBGs: Map[Context, AndroidBG] = new HashMap()

  implicit def toAndroidBG(context: Context) = synchronized {
    androidBGs.getOrElseUpdate(context, new AndroidBG(context))
  }

  def remove(context: Context) = synchronized {
    androidBGs.remove(context)
  }

}

class UIResult[A] extends Function0[A] {

  private val channel: Channel[A] = new Channel
  lazy val content: A = channel.read
  override def apply = content

  def write(data: A) = channel.write(data)

}

object UIResult {

  implicit def toA[A](result: UIResult[A]) = result()

}

class AndroidBG(context: Context) {

  AndroidBG.androidBGs.put(context, this)

  val currentHandler = new Handler {
    override def handleMessage(message: Message) = message.obj.asInstanceOf[() => Unit]()
  }

  def onBG(block: => Any): Unit = {
    spawn {
      try {
        block
      } finally {
        AndroidBG.remove(context)
      }
    }
  }

  def onUI[A](block: => A): UIResult[A] = {
    val answer: UIResult[A] = new UIResult
    currentHandler.obtainMessage(0, () => answer.write(block)).sendToTarget
    answer
  }

}