Customising attr_reader to do lazy instantiation of attributes

Question

(Big edit, I got part of the way there…) I've been hacking away and I've come up with this as a way to specify things that need to be done before attributes are read:

class Class
  def attr_reader(*params)
    if block_given?
      params.each do |sym|
        define_method(sym) do
          yield
          self.instance_variable_get("@#{sym}")
        end
      end
    else
      params.each do |sym|
        attr sym
      end
    end
  end
end

class Test
  attr_reader :normal
  attr_reader(:jp,:nope) { changethings if @nope.nil? }

  def initialize
    @normal = "Normal"
    @jp = "JP"
    @done = false
  end

  def changethings
    p "doing"
    @jp = "Haha!"
    @nope = "poop"
  end

end

j = Test.new

p j.normal
p j.jp

But changethings isn't being recognised as a method — anyone got any ideas?

Answer 1

You need to evaluate the block in the context of the instance. yield by default will evaluate it in its native context.

class Class
  def attr_reader(*params, &blk)
    if block_given?
      params.each do |sym|
        define_method(sym) do
          self.instance_eval(&blk)
          self.instance_variable_get("@#{sym}")
        end
      end
    else
      params.each do |sym|
        attr sym
      end
    end
  end
end

Answer 2

Here's another alternative approach you can look at. It's not as elegant as what you're trying to do using define_method but it's maybe worth looking at.

Add a new method lazy_attr_reader to Class

class Class
  def lazy_attr_reader(*vars)
    options = vars.last.is_a?(::Hash) ? vars.pop : {}
    # get the name of the method that will populate the attribute from options
    # default to 'get_things'
    init_method = options[:via] || 'get_things'
    vars.each do |var|
      class_eval("def #{var}; #{init_method} if !defined? @#{var}; @#{var}; end")
    end
  end
end

Then use it like this:

class Test
  lazy_attr_reader :name, :via => "name_loader"

  def name_loader
    @name = "Bob"
  end
end

In action:

irb(main):145:0> t = Test.new
=> #<Test:0x2d6291c>
irb(main):146:0> t.name
=> "Bob"

Answer 3

IMHO changing the context of the block is pretty counter-intuitive, from a perspective of someone who would use such attr_reader on steroids.

Perhaps you should consider plain ol' "specify method name using optional arguments" approach:

def lazy_attr_reader(*args, params)
  args.each do |e|
    define_method(e) do
      send(params[:init]) if params[:init] && !instance_variable_get("@#{e}")
      instance_variable_get("@#{e}")
    end
  end
end

class Foo
  lazy_attr_reader :foo, :bar, :init => :load

  def load
    @foo = 'foo'
    @bar = 'bar'
  end
end

f = Foo.new
puts f.bar
#=> bar