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 Active Record 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 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 include Singleton class << self # Attaches the observer to the supplied model classes. def observe(*models) models.flatten! models.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[/(.*)Observer/, 1] observed_class_name.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.sum([]) { |klass| klass.send(:subclasses) } end def add_observer!(klass) klass.add_observer(self) klass.class_eval 'def after_find() end' unless klass.method_defined?(:after_find) end end end