gistfile1.rb

class Hoge
  def initialize(state, foo)
    @state = state
    @foo = foo
  end
  
  def state=(new_state)
    @state = new_state
  end
  
  def print
    puts "@state is #{@state} and @foo is #{@foo}"
  end
end

h = Hoge.new("aho", 1)
h.print #=> @state is aho and @foo is 1

h.state = "baka" # 8 行目の state= を呼ぶ
h.print #=> @state is baka and @foo is 1