From: Rick DeNatale <rick.denatale@...>
Date: 2009-03-16T01:36:29+09:00
Subject: Re: Basics of Require

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There's a very nice, underused method of module which can help with loading
code on demand.  Despite being quite fluent in Ruby, I only found out about
it a few weeks ago, thanks to Chad Fowler.

It's Module#autoload.  It takes two arguments. The first is the NAME of a
constant as either a String or a Symbol, the second is a string with the
same meaning of the argument to require.  What it does is to associate the
first argument with the second, and if and when an undefined constant
matching the first argument is encountered in the scope of the module, to
effectively require the file named by the path argument.  It can be used at
the top level to autoload definitions of contstants in the global scope.
 For example

# at top level
autoload :Foo, 'lib/foo'  # this will load lib/foo.rb the first time anyone
references ::Foo

require 'something.rb'

# in file lib/foo.rb
class Foo
   autoload :Bar, 'lib/foo/bar'  # loads lib/foo/bar.rb when ::Foo::Bar is
referenced when undefined
end

# in file 'lib/foo/bar.rb'
module Foo::Bar
  # code
end

# in file 'something.rb'

   class MyClass < Foo
#...
   end

When the 'class Myclass < Foo' is executed, Foo will be undefined, this will
trigger the load of 'lib/foo.rb'  which is expected to define the constant
Foo which it does. Similarly lib/foo/bar.rb would be loaded if an when some
code defined ::Foo::Bar as an undefined constant.

This is somewhat similar to the automatic class loading built into Rails
(actually the ActiveSupport component of rails) using constant_missing, but
it's actually built into Ruby, and has been for quite a while.  In fact
ActiveSupport and the rest of rails is evolving to implement automatic class
loading using autoload, which is how I learned about it. Chad was doing a
walkthrough of ActiveRecord at RubyRx and showed the new code.

-- 
Rick DeNatale

Blog: http://talklikeaduck.denhaven2.com/
Twitter: http://twitter.com/RickDeNatale
WWR: http://www.workingwithrails.com/person/9021-rick-denatale
LinkedIn: http://www.linkedin.com/in/rickdenatale

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