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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
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