说明：iterator模式包括内部遍历（internal iterator ）和外部遍历(external iterator)两种，内部遍历是指对象自己已经存在的遍历方法，比如在array中的each方法，外部遍历是用户自定义的方法，额外定义在对象上。下面介绍两种遍历方法的示范代码

#external iterator
fruits = %w(orange apple mango)
fruits.each do |fruit|
  puts fruit
end

#internal iterator
class ExternalIterator 
  def initialize(array)
    @array = array
    @index = 0
  end

  def first
    @array[0]
  end

  def next
    value = @array[@index]
    @index += 1
    value
  end

  def is_done?
    @index == @array.length - 1
  end

  def current_item
    @array[@index]
  end

  def has_next?
    @index < @array.length
  end

  def previous
    value = @array[@index]
    @index -= 1
    value
  end

  def has_previous?
    @index > 0
  end

  def rewind
    @index = 0
  end
end

fruits = %w(orange apple mango)
array_iterator = ExternalIterator.new(fruits)

while array_iterator.has_next?
  puts array_iterator.next_item
end

https://medium.com/@dljerome/design-patterns-in-ruby-iterator-beead2c6492