Class: ActiveModel::Observer

Inherits:
Object
  • Object
show all
Includes:
Singleton
Defined in:
activemodel/lib/active_model/observing.rb

Overview

Active Model Observers

Observer classes respond to life cycle 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 < ActiveModel::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 < ActiveModel::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 < ActiveModel::Observer
  observe :account

  def after_update()
    AuditTrail.new(, "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 < ActiveModel::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.

Class Method Summary (collapse)

Instance Method Summary (collapse)

Constructor Details

- (Observer) initialize

Start observing the declared classes and their subclasses.



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# File 'activemodel/lib/active_model/observing.rb', line 190

def initialize
  observed_classes.each { |klass| add_observer!(klass) }
end

Class Method Details

+ (Object) observe(*models)

Attaches the observer to the supplied model classes.



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# File 'activemodel/lib/active_model/observing.rb', line 158

def observe(*models)
  models.flatten!
  models.collect! { |model| model.respond_to?(:to_sym) ? model.to_s.camelize.constantize : model }
  remove_possible_method(:observed_classes)
  define_method(:observed_classes) { models }
end

+ (Object) observed_class

The class observed by default is inferred from the observer's class name:

assert_equal Person, PersonObserver.observed_class


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# File 'activemodel/lib/active_model/observing.rb', line 180

def observed_class
  if observed_class_name = name[/(.*)Observer/, 1]
    observed_class_name.constantize
  else
    nil
  end
end

+ (Object) observed_classes

Returns an array of Classes to observe.

You can override this instead of using the observe helper.

class AuditObserver < ActiveModel::Observer
  def self.observed_classes
    [Account, Balance]
  end
end


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# File 'activemodel/lib/active_model/observing.rb', line 174

def observed_classes
  Array.wrap(observed_class)
end

Instance Method Details

- (Object) observed_class_inherited(subclass)

Special method sent by the observed class when it is inherited. Passes the new subclass.



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# File 'activemodel/lib/active_model/observing.rb', line 205

def observed_class_inherited(subclass) #:nodoc:
  self.class.observe(observed_classes + [subclass])
  add_observer!(subclass)
end

- (Object) observed_classes

:nodoc:



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# File 'activemodel/lib/active_model/observing.rb', line 194

def observed_classes #:nodoc:
  self.class.observed_classes
end

- (Object) update(observed_method, object)

Send observed_method(object) if the method exists.



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# File 'activemodel/lib/active_model/observing.rb', line 199

def update(observed_method, object) #:nodoc:
  send(observed_method, object) if respond_to?(observed_method)
end