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require 'singleton'
require 'set'
module ActiveRecord
module Observing # :nodoc:
def self.included(base)
base.extend ClassMethods
end
module ClassMethods
# Activates the observers assigned. Examples:
#
# # Calls PersonObserver.instance
# ActiveRecord::Base.observers = :person_observer
#
# # Calls Cacher.instance and GarbageCollector.instance
# ActiveRecord::Base.observers = :cacher, :garbage_collector
#
# # Same as above, just using explicit class references
# ActiveRecord::Base.observers = Cacher, GarbageCollector
#
# Note: Setting this does not instantiate the observers yet. #instantiate_observers is
# called during startup, and before each development request.
def observers=(*observers)
@observers = observers.flatten
end
# Gets the current observers.
def observers
@observers ||= []
end
# Instantiate the global ActiveRecord observers
def instantiate_observers
return if @observers.blank?
@observers.each do |observer|
if observer.respond_to?(:to_sym) # Symbol or String
observer.to_s.camelize.constantize.instance
elsif observer.respond_to?(:instance)
observer.instance
else
raise ArgumentError, "#{observer} must be a lowercase, underscored class name (or an instance of the class itself) responding to the instance method. Example: Person.observers = :big_brother # calls BigBrother.instance"
end
end
end
protected
# Notify observers when the observed class is subclassed.
def inherited(subclass)
super
changed
notify_observers :observed_class_inherited, subclass
end
end
end
# 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.
#
class Observer
include Singleton
class << self
# Attaches the observer to the supplied model classes.
def observe(*models)
models.flatten.collect! { |model| model.is_a?(Symbol) ? model.to_s.camelize.constantize : model }
define_method(:observed_classes) { Set.new(models) }
end
# The class observed by default is inferred from the observer's class name:
# assert_equal [Person], PersonObserver.observed_class
def observed_class
if observed_class_name = name.scan(/(.*)Observer/)[0]
observed_class_name[0].constantize
else
nil
end
end
end
# Start observing the declared classes and their subclasses.
def initialize
Set.new(observed_classes + observed_subclasses).each { |klass| add_observer! klass }
end
# Send observed_method(object) if the method exists.
def update(observed_method, object) #:nodoc:
send(observed_method, object) if respond_to?(observed_method)
end
# Special method sent by the observed class when it is inherited.
# Passes the new subclass.
def observed_class_inherited(subclass) #:nodoc:
self.class.observe(observed_classes + [subclass])
add_observer!(subclass)
end
protected
def observed_classes
Set.new([self.class.observed_class].compact.flatten)
end
def observed_subclasses
observed_classes.collect(&:subclasses).flatten
end
def add_observer!(klass)
klass.add_observer(self)
klass.class_eval 'def after_find() end' unless klass.respond_to?(:after_find)
end
end
end
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