RenderingSystem.scala

package gdx.scala.demo

import java.util.Comparator

import com.badlogic.ashley.core.{Component, ComponentMapper, Entity, Family}
import com.badlogic.ashley.systems.SortedIteratingSystem
import gdx.scala.demo.components.PositionComponent


class RenderingSystem() extends 
// Cannot resolve constructor on SortedInteratingSystem (https://github.com/libgdx/ashley/blob/master/ashley/src/com/badlogic/ashley/systems/SortedIteratingSystem.java)
SortedIteratingSystem(Family.all(classOf[RenderableComponent],
  classOf[PositionComponent]), new ZOrdering(ComponentMapper.getFor(classOf[PositionComponent]))) {

  var rm: ComponentMapper[RenderableComponent] = ComponentMapper.getFor(classOf[RenderableComponent])
  var pm: ComponentMapper[PositionComponent] = ComponentMapper.getFor(classOf[PositionComponent])
}

class ZOrdering(pm: ComponentMapper[PositionComponent]) extends Ordering[Entity] {
  override def compare(o1: Entity, o2: Entity): Int = Math.signum(pm.get(o1).z - pm.get(o2).z).toInt
}