Attributes-4.1.0

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.

the implementation 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 N. and Florian
Gross along
with concepts from the original traits lib

key features provided by attributes are

 - ability to specify default values for attrs and definition

time. values
can be literal objects or blocks, which are evaluated in the
context of
self to initialize the variable

 - classes remember which attributes they've defined and this

information is
available to client code

 - a whole suite of methods is defined by calls to #attributes

including
getter, setter, query (var?) and banger (var! - which forces
re-initialization from the default value)

 - ability to define multiple attributes at once using key =>

value pairs

 - fast lookup of whether or not a class has defined a certain

attribute

 - attributes can be defined on objects on a per singleton basis

as well

 - getters acts as setters if an argument is given to them

all this in < 100 lines of code

HISTORY
4.1.0
- 4.0.0 introduced a bug where a query (foo?) would not
initialize a var -
4.1.0 fixes that

4.0.0
- removed dependancy on, and bundle of, pervasives
- faster. as fast as normal method definition.
- faster lookup for MyClass.attributes.include?(‘foobar’)

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 a

suite of methods
#
require ‘attributes’

   class C
     attribute 'a'
   end

   c = C.new

   c.a = 42
   p c.a                 #=> 42
   p 'forty-two' if c.a? #=> 'forty-two'

 #
 # attributes works on object too
 #
   o = Object.new
   o.attribute 'answer' => 42
   p o.answer           #=> 42

~ > 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. the
# ‘banger’ method can be used to re-initialize a variable at any
point whether
# or not it’s already been initialized.
#
require ‘attributes’

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

   c = C.new
   p c.a #=> 42
   p c.b #=> 42.0

   c.a = 43
   p c.a #=> 43
   c.a!
   p c.a #=> 42

~ > ruby samples/b.rb

 42
 42.0
 43
 42

<========< 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 #=> 42

~ > 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:0x1ffb8 @port=80, @host="codeforpeople.org">

<========< samples/f.rb >========>

~ > cat samples/f.rb

 #
 # of course attributes works as well at class/module level as at

instance
# level
#
require ‘attributes’

   module Logging
     Level_names = {
       0 => 'INFO',
       # ...
       42 => 'DEBUG',
     }

     class << self
       attribute 'level' => 42
       attribute('level_name'){ Level_names[level] }
     end
   end

 p Logging.level
 p Logging.level_name

~ > ruby samples/f.rb

 42
 "DEBUG"

a @ http://codeforpeople.com/

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