pillowfactory.org : Archives for August 2008

Hacktastic! lazy_attr_accessor & lazy_attr_reader

Posted by Luke Pillow Thu, 28 Aug 2008 08:56:00 GMT

Whether or not you agree with the use of lazy initialized attributes, you’d probably agree that writing the accessor method can be a bit tedious at times; especially if you’re used to Ruby’s attr_accessor, attr_reader, and attr_writer methods.

This Hacktastic entry introduces a couple nice (at least fun to write) little Ruby class methods (based on Jay Fields’ blog entry, "Ruby: Lazy Initialized Attributes") to assist with lazy initialized attribute declarations.

Let’s take a quick look at how one might use lazy_attr_accessor & lazy_attr_reader

class Foo
  lazy_attr_accessor :bar => [], :woot => :bam
end

foo = Foo.new
puts "Before = #{foo.bar.inspect}" # Before = []
foo.bar << 2
puts foo.woot # bam
foo.woot = :wooooooooot 
puts foo.woot # wooooooooot 
puts "After = #{foo.bar.inspect}" # After = [2]

And the hack in all it’s glory…

class Object   
  def self.lazy_attr_reader(hsh)
    hsh.each do |method_name, default_value|
      self.send :define_method, method_name.to_sym do
        var_name = "@#{method_name}"
        instance_variable_set(var_name, default_value) unless instance_variable_defined? var_name
        instance_variable_get var_name
      end
    end
  end
   # creates both lazy_attr_reader and attr_writer  
  def self.lazy_attr_accessor(hsh)
    attr_writer *hsh.keys
    lazy_attr_reader hsh
  end
end