World.as update sequence control.

    /**
     * Brings the Entity to be first updated in update sequence.
     * @param e Entity which will be moved in update sequence.
     */
    public function bringUpdateFirst(e:Entity):Boolean
    {
      // _updateFirst exists and not equals to me.
      if (_updateFirst !== e) return false;

      // Remove me from update sequence
      if (e._updatePrev) e._updatePrev._updateNext = e._updateNext; // If I'm last in update sequence, updateNext will be null anyway, so I don't care about checking
      if (e._updateNext) e._updateNext._updatePrev = e._updatePrev;

      // Bring me to front.
      _updateFirst._updatePrev = e;
      e._updateNext = _updateFirst;
      e._updatePrev = null;
      _updateFirst = e;
      return true;
    }

    /**
     * Send the Entity to be updated last in update sequence
     * @param e Entity which will be moved in update sequence.
     */
    public function sendUpdateLast(e:Entity):Boolean
    {
      // _updateFirst exists and I'm is not last updated entity
      if (_updateFirst && e._updateNext)
      {
        var lastEnt:Entity = e._updateNext;

        // Remove me from update sequence
        if (e._updatePrev) e._updatePrev._updateNext = lastEnt;

        if (e === _updateFirst) _updateFirst = e._updateNext; // If first, set next as first
        lastEnt._updatePrev = e._updatePrev;

        // Find last updated entity
        while (lastEnt._updateNext) lastEnt = lastEnt._updateNext;

        // Set as last updated entity
        lastEnt._updateNext = e;
        e._updatePrev = lastEnt;
        e._updateNext = null;
        return true;
      }
      return false;
    }

    /**
     * Brings the Entity to be updated right before second given entity.
     * @param e Entity which will be moved in update sequence.
     * @param before Entity, in front of which will be inserted first Entity.
     */
    public function bringUpdateBefore(e:Entity, before:Entity):Boolean
    {
      if (_updateFirst && before._updatePrev !== e)
      {
        // Remove from current sequence
        if (e._updatePrev) e._updatePrev._updateNext = e._updateNext;
        if (e._updateNext) e._updateNext._updatePrev = e._updatePrev;

        // Add to new sequence
        e._updatePrev = before._updatePrev;
        if (e._updatePrev) e._updatePrev._updateNext = e;

        e._updateNext = before;
        before._updatePrev = e;

        if (_updateFirst === before) _updateFirst = e;
        return true;
      }
      return false;
    }

    /**
     * Sends the entity to be updated right after the second given entity.
     * @param e Entity which will be moved in update sequence.
     * @param after Entity, after which will be inserted first Entity.
     */
    public function sendUpdateAfter(e:Entity, after:Entity):Boolean
    {
      if (_updateFirst && after._updateNext !== e)
      {
        // Remove from current sequence
        if (e === _updateFirst) _updateFirst = e._updateNext;
        if (e._updatePrev) e._updatePrev._updateNext = e._updateNext;
        if (e._updateNext) e._updateNext._updatePrev = e._updatePrev;

        // Add to new sequence
        e._updateNext = after._updateNext;
        if (e._updateNext) e._updateNext._updatePrev = e;

        e._updatePrev = after;
        after._updateNext = e;
        return true;
      }
      return false;
    }

    /**
     * Beings the entity forward in update sequence. (Will be updated earlier)
     * @param e Entity which will be moved in update sequence.
     */
    public function bringUpdateForward(e:Entity):Boolean
    {
      if (_updateFirst && e._updatePrev)
      {
        var prev:Entity = e._updatePrev;
        prev._updateNext = e._updateNext;
        if (e._updateNext) prev._updateNext._updatePrev = prev;

        e._updatePrev = prev._updatePrev;
        if (prev._updatePrev) e._updatePrev._updateNext = e;

        e._updateNext = prev;
        prev._updatePrev = e;
        if (prev === _updateFirst) _updateFirst = e;
        return true;
      }
      return false;
    }

    /**
     * Sends the entity backward in update sequence. (Will be updated later)
     * @param e Entity which will be moved in update sequence.
     */
    public function sendUpdateBackward(e:Entity):Boolean
    {
      if (_updateFirst && e._updateNext)
      {
        var next:Entity = e._updateNext;
        next._updatePrev = e._updatePrev;
        if (e._updatePrev) next._updatePrev._updateNext = next;

        e._updateNext = next._updateNext;
        if (next._updateNext) e._updateNext._updatePrev = e;

        next._updateNext = e;
        e._updatePrev = next;
        if (e === _updateFirst) _updateFirst = next;
        return true;
      }
      return false;
    }

    /**
     * Swaps two entities in update sequence.
     * @param first First entity to swap.
     * @param second Second entity to swap.
     */
    public function swapUpdateEntities(first:Entity, second:Entity):Boolean
    {
      if (_updateFirst)
      {
        var temp:Entity = first._updatePrev;
        first._updatePrev = second._updatePrev;
        second._updatePrev = temp;

        temp = first._updateNext;
        first._updateNext = second._updateNext;
        second._updateNext = temp;

        return true;
      }
      return false;
    }