require 'active_support/core_ext/class/attribute' 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 config.active_record.observers configuration setting in your # config/environment.rb 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 ModelObserver.instance before. Observers are # singletons and that call instantiates and registers them. # class Observer < ActiveModel::Observer class_attribute :observed_methods self.observed_methods = [] def initialize super observed_subclasses.each { |klass| add_observer!(klass) } end def self.method_added(method) self.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.class.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