Design Patterns in Ruby [Digest 6] Iterator

最新推荐文章于 2020-09-15 18:56:16 发布

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本文介绍了迭代器模式的概念，并通过Java和Ruby两种语言展示了外部迭代器和内部迭代器的实现方式及其区别。外部迭代器更容易控制迭代过程，且可以被多个方法或对象共享。

The Iterator pattern is a technique that allows an aggregate object to provide the outside world with a way to access its collection of subobjects.

In Java we use java.util.Iterator interface to be implemented.

eg. StringTokenizer ResultSet

In Ruby We can write Iterator:

class ArrayIterator
  def initialize(array)
    @array = array
    @index = 0
  end
  def has_next?
    @index < @array.length
  end
  def item
    @array[@index]
  end
  def next_item
    value = @array[@index]
    @index += 1
    value
  end
end

array = ['red', 'green', 'blue']
i = ArrayIterator.new(array)
while i.has_next?
   puts("item: #{i.next_item}")
end

The Internal Iterator is:

def for_each_element(array)
  i = 0
  while i < array.length
    yield(array[i])
    i += 1
  end
end

There are some differences between the two implementations:

external version will be easier to control the process:

"With an external iterator, you won’t call next until you are good and ready for the next element. With an internal iterator, by contrast, the aggregate relentlessly pushes the code block to accept item

after item."

"A second advantage of external iterators is that, because they are external, you can share them—you can pass them around to other methods and objects."