ActivityAggregator.rb

class ActivityAggregator
  def initialize(streams)
    @streams = streams
  end
  def next_activities(limit)
    activities = []
    while (activities.size < limit) && more_activites? do
      activities << next_activity
    end
    activities
  end
private
 
  def next_activity
    @streams.select{ |s| has_next?(s) }.sort_by{ |s| s.peek.created_at }.last.next
  end
  def more_activities?
    @streams.any?{ |s| has_next?(s) }
  end
  def has_next?(stream)
    stream.peek
    true
  rescue
    false
  end
end