Question

I am looking to do some method chaining. I have the following code:

class MyClass

  attr_accessor :handler

  def do_a
    puts 'i just did a'
    self.handler = 'a'
    self
  end

  def do_b_if_a
    puts 'i just did b' if handler == 'a'
  end

 end

So the following works:

irb > test = MyClass.new
=> #<MyClass:0x007fa44ced9a70 @handler=nil>
irb > test.do_a
'i just did a'
irb > test.do_a.do_b_if_a
'i just did a'
'i just did b'

What I DONT want to work is when I call do_a the first time it sets the handler, which means now do_b_if_a can be called at any time. But I only want it to be called when it is chained with do_a, how do I do that?

Answer 1

I think you’re tracking state in the wrong place. You’d be better off with something similar to ActiveRecord’s query interface, for example:

class MyClass
  attr_accessor :handler

  def do_a
    puts 'i just did a'
    with_handler 'a'
  end

  def do_b_if_a
    puts 'i just did b' if handler == 'a'
  end

private

  def with_handler(h)
    o = dup
    o.handler = h
    o
  end

end

That way you always have an instance of MyClass but you have a throw-away copy that keeps track of its history. This is similar to cHao’s approach but it doesn’t need an extra decorator class as MyClass can decorate itself.