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