require 'singleton'
require 'cassandra_mapper/support/concern'
require 'cassandra_mapper/core_ext/array/wrap'
require 'cassandra_mapper/core_ext/module/aliasing'
require 'cassandra_mapper/core_ext/string/inflections'

module CassandraMapper
  module Support
    module Observing
      extend CassandraMapper::Support::Concern

      module ClassMethods
        # == Active Model Observers Activation
        #
        # 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

        def add_observer(observer)
          unless observer.respond_to? :update
            raise ArgumentError, "observer needs to respond to `update'"
          end
          @observer_instances ||= []
          @observer_instances << observer
        end

        def notify_observers(*arg)
          if defined? @observer_instances
            for observer in @observer_instances
              observer.update(*arg)
            end
          end
        end

        def count_observers
          @observer_instances.size
        end

        protected

        def instantiate_observer(observer) #:nodoc:
          # 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
          notify_observers :observed_class_inherited, subclass
        end
      end

      private
        # Fires notifications to model's observers
        #
        # def save
        #   notify_observers(:before_save)
        #   ...
        #   notify_observers(:after_save)
        # end
        def notify_observers(method)
          self.class.notify_observers(method, self)
        end
    end

    # == Active Model Observers
    #
    # Observer classes respond to lifecycle 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(account)
    #       AuditTrail.new(account, "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 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

        # 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
        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 #:nodoc:
        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) #:nodoc:
        klass.add_observer(self)
      end
    end
  end
end