lazy_ivar_init.rb

module LazyInit
  class Delay # < BlankSlate or similar so other method calls aren't answered by Delay
    def initialize(obj, var_name, &block)
      @parent_obj, @var_name, @block = obj, var_name, block
    end

    def method_missing(method, *args, &block)
      realized_object = @block.call
      @parent_obj.instance_variable_set(@var_name, realized_object)
      realized_object.send(method, *args, &block)
    end
  end
  def lazy_init(var, &block)
    var_name = "@#{var}"
    self.instance_variable_set(var_name, Delay.new(self, var_name, &block))
  end
end


class Foo
  def initialize
    puts "Hi, I'm a Foo object that was just created."
  end
  def something
    puts "Yo, I'm doing something"
    42
  end
end

class SomethingDoer
  include LazyInit

  def initialize
    lazy_init(:foo) { Foo.new }
  end

  def do_something
    @foo.something # doesn't blow up because it auto-init'd
  end
end


doer = SomethingDoer.new
puts "A doer whas just created..."
puts "Now, I will call #do_something to force the realization of the @foo instance var..."
puts doer.do_something
puts doer.instance_variable_get("@foo").inspect # make sure the delay object was replaced with Foo object so the operation is transparent to clients

## Output:
# A doer whas just created...
# Now, I will call #do_something to force the realization of the @foo instance var...
# Hi, I'm a Foo object that was just created.
# Yo, I'm doing something
# 42
#<Foo:0x1001b9eb8>
#
#