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require 'observer'
require 'singleton'
require 'active_support/core_ext/array/wrap'
require 'active_support/core_ext/module/aliasing'
require 'active_support/core_ext/string/inflections'
module ActiveModel
module Observing
extend ActiveSupport::Concern
included do
extend Observable
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=(*values)
@observers = values.flatten
end
# Gets the current observers.
def observers
@observers ||= []
end
# Instantiate the global Active Record observers.
def instantiate_observers
observers.each { |o| instantiate_observer(o) }
end
# Wraps methods with before and after notifications.
#
# wrap_with_notifications :create, :save, :update, :destroy
def wrap_with_notifications(*methods)
methods.each do |method|
class_eval(<<-EOS, __FILE__, __LINE__ + 1)
def #{method}_with_notifications(*args, &block)
notify_observers(:before_#{method})
result = #{method}_without_notifications(*args, &block)
notify_observers(:after_#{method})
result
end
EOS
alias_method_chain(method, :notifications)
end
end
protected
def instantiate_observer(observer)
# string/symbol
if observer.respond_to?(:to_sym)
observer = 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
# Notify observers when the observed class is subclassed.
def inherited(subclass)
super
changed
notify_observers :observed_class_inherited, subclass
end
end
private
def notify_observers(method)
self.class.changed
self.class.notify_observers(method, self)
end
end
class Observer
include Singleton
class << self
# Attaches the observer to the supplied model classes.
def observe(*models)
models.flatten!
models.collect! { |model| model.respond_to?(:to_sym) ? model.to_s.camelize.constantize : model }
define_method(:observed_classes) { models }
end
def observed_classes
Array.wrap(observed_class)
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
observed_classes.each { |klass| add_observer!(klass) }
end
def observed_classes
self.class.observed_classes
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 add_observer!(klass)
klass.add_observer(self)
end
end
end
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