require 'singleton' module ActiveRecord # Observers can be programmed to react to lifecycle callbacks in another class 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 excess responsibility that doesn't pertain to # the core and nature 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 is triggered when a Comment#save is finished and sends a notification about it to the administrator. # # == Observing a class that can't be infered # # 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 # something else than the class you're interested in observing, you can implement the observed_class class method. Like this: # # class AuditObserver < ActiveRecord::Observer # def self.observed_class() Account end # def after_update(account) # AuditTrail.new(account, "UPDATED") # end # end # # == Observing multiple classes at once # # If the audit observer needs to watch more than one kind of object, this can be specified in an array, like this: # # class AuditObserver < ActiveRecord::Observer # def self.observed_class() [ Account, Balance ] end # 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. # # The observer can implement callback methods for each of the methods described in the Callbacks module. class Observer include Singleton def self.observe(*models) define_method(:observed_class) { models } end def initialize [ observed_class ].flatten.each do |klass| klass.add_observer(self) klass.send(:define_method, :after_find) unless klass.respond_to?(:after_find) end end def update(callback_method, object) send(callback_method, object) if respond_to?(callback_method) end private def observed_class if self.class.respond_to? "observed_class" self.class.observed_class else Object.const_get(infer_observed_class_name) end end def infer_observed_class_name self.class.name.scan(/(.*)Observer/)[0][0] end end end