When
====
When adds :if and :unless conditions to ActiveRecord callbacks 
and validations and ActionController filters. It works exactly 
the way as the current implementation of #validates_acceptance_of.
It works on the 12 regular callbacks:
before_validation
before_validation_on_create
after_validation
after_validation_on_create
before_save
before_create
before_update
after_create
after_update
after_save
before_destroy
after_destroy
3 validations:
validate
validate_on_create
validate_on_update
and 1 filter:
before_filter
It works when :if or :unless is passed a Symbol, a Proc or a String.
They return or evaluate to a true or false value.
Example
=======
class Address < ActiveRecord::Base
  before_save :geolocate
  def geolocate
    if complete?
      ...
    end
  end
  def complete?
    street? && city? && state? && zip?
  end
end
In this case, we want to find the latitude and longitude of an address only if 
the address is complete.
Wrapping the entirety of a callback method with conditional logic is bad form.
The callback should execute WHEN the model's life cycle reaches its 
"before_save" point and WHEN its address is "complete."
With When, the WHEN responsibility is moved to where it belongs:
as part of the callback.
class Address < ActiveRecord::Base
  before_save :geolocate, 
    :if => :complete?
  def geolocate
    ...
  end
  def complete?
    street? && city? && state? && zip?
  end
end
before_create's single responsibility is to execute code WHEN certain conditions are met.
geolocate's single responsibility is to ... geolocate. It should not contain its own
preconditions. 
More Examples
=============
before_create :encrypt_password,
  :unless => lambda {|user| user.password_confirmation.blank?}
before_filter :log_in!,
  :only => [:new, :create],
  :unless => :logged_in?
What When does NOT support
==========================
# ActiveRecord Class callbacks
before_create PasswordEncryptor,
  :unless => lambda {|user| user.password_confirmation.blank?}
# ActionController Class filters
before_filter Authorizer,
  :unless => :logged_in?
When will not work if your code contains any of these.
In our experience we've never used class callbacks or filters and find them to be overkill.
Obsolete after Rails 2.0.3
==========================
Whenever the next release after Rails 2.0.2 comes out, this feature will be baked into Rails
via the ActiveSupport::Callbacks module. Happy coding!
Installation
============
piston import https://svn.thoughtbot.com/plugins/when/trunk vendor/plugins/when
Copyright (c) 2008 Jared Carroll, Dan Croak, and thoughtbot, inc. released under the MIT license