class TextField
  attr_accessor :value

  def initialize
    @value = ""
  end
end

class TextFieldWithObserver < TextField
  attr_reader :observers

  def initialize
    @observers = []
    super
  end

  def value=(value)
    super
    observers.each { |observer| observer.text_field_changed }
  end
end

class List
  attr_accessor :words

  def initialize(words)
    @words = words
  end
end

class ListSearchMediator

  def initialize(text_field, list)
    @text_field = text_field
    @text_field.observers << self
    @list = list
  end

  def text_field_changed
    @list.words = @list.words.inject([]) do |narrowed_list, word|
      if word.start_with?(@text_field.value)
        narrowed_list.push(word)
      end
      narrowed_list
    end
  end
end