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module ActiveRecord
# Observer classes respond to lifecycle callbacks to implement trigger-like
# behavior outside the original class. This is a great way to reduce the
# clutter that normally comes when the model class is burdened with
# functionality that doesn't pertain to the core responsibility of the
# class. Example:
#
# class CommentObserver < ActiveRecord::Observer
# def after_save(comment)
# Notifications.deliver_comment("admin@do.com", "New comment was posted", comment)
# end
# end
#
# This Observer sends an email when a Comment#save is finished.
#
# class ContactObserver < ActiveRecord::Observer
# def after_create(contact)
# contact.logger.info('New contact added!')
# end
#
# def after_destroy(contact)
# contact.logger.warn("Contact with an id of #{contact.id} was destroyed!")
# end
# end
#
# This Observer uses logger to log when specific callbacks are triggered.
#
# == Observing a class that can't be inferred
#
# Observers will by default be mapped to the class with which they share a name. So CommentObserver will
# be tied to observing Comment, ProductManagerObserver to ProductManager, and so on. If you want to name your observer
# differently than the class you're interested in observing, you can use the Observer.observe class method which takes
# either the concrete class (Product) or a symbol for that class (:product):
#
# class AuditObserver < ActiveRecord::Observer
# observe :account
#
# def after_update(account)
# AuditTrail.new(account, "UPDATED")
# end
# end
#
# If the audit observer needs to watch more than one kind of object, this can be specified with multiple arguments:
#
# class AuditObserver < ActiveRecord::Observer
# observe :account, :balance
#
# def after_update(record)
# AuditTrail.new(record, "UPDATED")
# end
# end
#
# The AuditObserver will now act on both updates to Account and Balance by treating them both as records.
#
# == Available callback methods
#
# The observer can implement callback methods for each of the methods described in the Callbacks module.
#
# == Storing Observers in Rails
#
# If you're using Active Record within Rails, observer classes are usually stored in app/models with the
# naming convention of app/models/audit_observer.rb.
#
# == Configuration
#
# In order to activate an observer, list it in the <tt>config.active_record.observers</tt> configuration setting in your
# <tt>config/environment.rb</tt> file.
#
# config.active_record.observers = :comment_observer, :signup_observer
#
# Observers will not be invoked unless you define these in your application configuration.
#
# == Loading
#
# Observers register themselves in the model class they observe, since it is the class that
# notifies them of events when they occur. As a side-effect, when an observer is loaded its
# corresponding model class is loaded.
#
# Up to (and including) Rails 2.0.2 observers were instantiated between plugins and
# application initializers. Now observers are loaded after application initializers,
# so observed models can make use of extensions.
#
# If by any chance you are using observed models in the initialization you can still
# load their observers by calling <tt>ModelObserver.instance</tt> before. Observers are
# singletons and that call instantiates and registers them.
#
class Observer < ActiveModel::Observer
extlib_inheritable_accessor(:observed_methods){ [] }
def initialize
super
observed_subclasses.each { |klass| add_observer!(klass) }
end
def self.method_added(method)
observed_methods << method if ActiveRecord::Callbacks::CALLBACKS.include?(method.to_sym)
end
protected
def observed_subclasses
observed_classes.sum([]) { |klass| klass.send(:subclasses) }
end
def add_observer!(klass)
super
# Check if a notifier callback was already added to the given class. If
# it was not, add it.
self.observed_methods.each do |method|
callback = :"_notify_observers_for_#{method}"
if (klass.instance_methods & [callback, callback.to_s]).empty?
klass.class_eval "def #{callback}; notify_observers(:#{method}); end"
klass.send(method, callback)
end
end
end
end
end
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