NAME

attributes.rb

INSTALL

gem install attributes

URIS

http://rubyforge.org/projects/codeforpeople/
http://codeforpeople.com/lib/ruby

SYNOPSIS

attributes.rb provides a set of attr_* like method with several user
friendly additions.  attributes.rb is similar to the traits.rb package but
sacrifices a few features for simplicity of implementation: attributes.rb is
only 42 lines of code.

the implimentation of attributes.rb borrows many of the best ideas from the
metakoans.rb ruby quiz

  http://www.rubyquiz.com/quiz67.html

in particular the solutions of Christian Neukirchen and Florian Gross.

HISTORY

3.7.0
  small patch to use 'instance_variable_defined?' instead of defined?
  keyword

3.5.0
  migrated to a pervasives based impl to attributes should work on any
  object - even blankslate objects

3.3.0

  moved to an instance variable-less model using an module level closure for
  the attributes list

SAMPLES

<========< samples/a.rb >========>

~ > cat samples/a.rb

  #
  # basic usage is like attr, but note that attribute defines three methods,
  # getter, setter, and query
  #
    require 'attributes'

    class C
      attribute 'a'
    end

    c = C.new

    c.a = 42              # setter
    p c.a                 # getter
    p 'forty-two' if c.a? # query

  #
  # also not that attribute(s) works as well for objects as classes
  #
    o = Object.new
    o.attribute 'answer' => 42
    p o.answer

~ > ruby samples/a.rb

  42
  "forty-two"
  42

<========< samples/b.rb >========>

~ > cat samples/b.rb

  #
  # default values may be given either directly or as a block which will be
  # evaluated in the context of self.  in both cases (value or block) the
  # default is set only once and only if needed - it's a lazy evaluation.
  #
    require 'attributes'

    class C
      attribute :a => 42
      attribute(:b){ Float a }
    end

    c = C.new

    p c.a
    p c.b

~ > ruby samples/b.rb

  42
  42.0

<========< samples/c.rb >========>

~ > cat samples/c.rb

  #
  # multiple values may by given, plain names and key/val pairs may be mixed.
  #
    require 'attributes'

    class C
      attributes 'x', 'y' => 0b101000, 'z' => 0b10
    end

    c = C.new
    c.x = c.y + c.z
    p c.x

~ > ruby samples/c.rb

  42

<========< samples/d.rb >========>

~ > cat samples/d.rb

  #
  # a nice feature is that all attributes are enumerated in the class.  this,
  # combined with the fact that the getter method is defined so as to delegate
  # to the setter when an argument is given, means bulk initialization and/or
  # attribute traversal is very easy.
  #
    require 'attributes'

    class C
      attributes %w( x y z )

      def attributes
        self.class.attributes
      end

      def initialize
        attributes.each_with_index{|a,i| send a, i}
      end

      def to_hash
        attributes.inject({}){|h,a| h.update a => send(a)}
      end

      def inspect
        to_hash.inspect
      end
    end

    c = C.new
    p c.attributes 
    p c 

    c.x 'forty-two' 
    p c.x

~ > ruby samples/d.rb

  ["x", "y", "z"]
  {"x"=>0, "y"=>1, "z"=>2}
  "forty-two"

<========< samples/e.rb >========>

~ > cat samples/e.rb

  #
  # my favourite element of attributes is that getters can also be setters.
  # this allows incredibly clean looking code like
  #
    require 'attributes'

    class Config
      attributes %w( host port)
      def initialize(&block) instance_eval &block end
    end

    conf = Config.new{
      host 'codeforpeople.org'

      port 80
    }

    p conf

~ > ruby samples/e.rb

  #<Config:0x1e834 @port=80, @host="codeforpeople.org">