til/ruby/turning-any-class-into-an-enumerator.md

Turning Any Class Into An Enumerator

At the core of any enumerator is the ability to respond to an #each call. With that in mind, we prepare any class for being turned into an enumerator.

Consider this class SquaresCollection that allows you to turn an array of integers into an array of its squares.

class SquaresCollection
  def initialize(items)
    @items = items
  end

  def run
    @items.map { |item| item * item }
  end
end

SquaresCollection.new([1,2,3]).run #=> [1,4,9]

We can work with this, but it opts out of Ruby's enumerator offerings.

We can, instead, provide an #each method which allows instances of our SquaresCollection class to be turned into enumerators.

class SquaresCollection
  def initialize(items)
    @items = items
  end

  def each
    return enum_for(:each) unless block_given?

    @items.each do |item|
      yield item * item
    end
  end
end

Here is how we can use it:

sc = SquaresCollection.new([1,2,3])

puts sc.to_enum
#=> <Enumerator ...>

puts sc.to_enum.map { |item| "* #{item} *" }
#=> ["* 1 *", "* 4 *", "* 9 *"]

The #to_enum method looks for an #each method on the instance's class and uses that to create an enumerator.

source