RubyMotionでテーブルビューのページング

UITableViewの一番下のセルまでスクロールしたら、次のX件読み込む、っていうやつ。

items_controller.rb

class ItemsController < UITableViewController
  READ_COUNT = 20
  ITEM_CELL_ID = 'Item'

  def viewDidLoad
    super

    self.title = 'hoge'
    @items = []
    @page = 0
    @last_items_size = 0

    @indicator = UIActivityIndicatorView.alloc.initWithActivityIndicatorStyle(UIActivityIndicatorViewStyleGray)
    @indicator.stopAnimating

    load_items
  end

  def load_items
    Item.fetch(READ_COUNT, @page * READ_COUNT) do |items, error_message|
      @items += items
      @last_items_size += items.size
      self.tableView.reloadData
    end
  end

  def load_more_items
    @page += 1
    load_items
    end_indicator
  end

  def tableView(tableView, numberOfRowsInSection: section)
    @items.size
  end

  def tableView(tableView, cellForRowAtIndexPath: indexPath)
    cell = tableView.dequeueReusableCellWithIdentifier(ITEM_CELL_ID) ||
      UITableViewCell.alloc.initWithStyle(UITableViewCellStyleSubtitle, reuseIdentifier: ITEM_CELL_ID)

    item = @items[indexPath.row]
    cell.textLabel.text = item.title
    cell.detailTextLabel.text = item.url
    cell
  end

  def scrollViewDidScroll(scrollView)
    if self.tableView.contentOffset.y >= self.tableView.contentSize.height - self.tableView.bounds.size.height
      return if @indicator.isAnimating

      if @last_items_size >= (@page + 1) * READ_COUNT
        start_indicator
        self.performSelector('load_more_items', withObject: nil, afterDelay: 1.0)
      end
    end
  end

  def start_indicator
    @indicator.startAnimating
    @indicator.frame = [[0, 0], [self.view.frame.size.width / 2, self.view.frame.size.height / 8]]  # このへんは適当に
    self.tableView.setTableFooterView @indicator
  end

  def end_indicator
    @indicator.stopAnimating
  end
end

item.rb

class Item
  attr_accessor :title, :url

  BASE_URL = 'https://example.com/'

  def initialize(data)
    @title = data['title']
    @url = data['url']
  end


  def self.fetch(fetch_count, offset, &block)
    url = "#{BASE_URL}?count=#{fetch_count}&offset=#{offset}"

    AFMotion::JSON.get(url) do |result|
      items = []
      error_message = nil

      if result.success?
        items = result.object.map{|data| self.new(data) }
      end
      block.call(items, error_message)
    end
  end
end