An Example of the Null Object Pattern

0_old_pim_event_subscription.rb

# This class throws a NoMethod exception in #active_for_user? when it's initialized
# with a ride which has no user. We'd like to just turn the class's behaviour off in that case
# since the whole functionality of it really doesn't make sense for rides which are userless.

# Usage: PimEventSubscription.new(ride).subscribe/unsubscribe
class PimEventSubscription
  attr_reader :ride
  def initialize(ride)
    @ride=ride
  end
  def subscribe
     log_active
     return unless active_for_user?
     "subscribe"
  end
  def unsubscribe
     log_active
     return unless active_for_user?
     "unsubscribe"
  end
  
  private
  def log_active
    return unless ride.user.present?
    Rails.logger.error("#{prefix} Activated for User:#{Features[ride.user].id}:#{active_for_user?.inspect}")
  end
  
  def active_for_user?
    Features[ride.user].pim_event_feed_enabled? # explodes when ride.user is null
  end
end

1_fixed_pim_event_subscription.rb

# We can certainly fix it by adding if statements through out the entire class in all the areas affected...
# But that leads to painful duplication of branching.

# Usage: PimEventSubscription.new(ride).subscribe/unsubscribe
class PimEventSubscription
  attr_reader :ride
  def initialize(ride)
    @ride=ride
  end
  def subscribe
     log_active
     return unless active_for_user?
     "subscribe"
  end
  def unsubscribe
     log_active
     return unless active_for_user?
     "unsubscribe"
  end
  
  private
  def log_active
    return unless ride.user.present? #EW. We had to fix it in two places.
    Rails.logger.error("#{prefix} Activated for User:#{Features[ride.user].id}:#{active_for_user?.inspect}")
  end
  
  def active_for_user?
    return if ride.user.present? #EW. Duplication usually means you're missing something shared.
    Features[ride.user].pim_event_feed_enabled? 
  end
end

2_new_pim_event_subscription.rb

# Lets solve that duplication by using the Null Object Pattern

# If we make an object which responds to all the same methods that 
# the primary object responds to _from its external callers_...
class InvalidPimEventSubscription
 def subscribe
 end
 def unsubscribe
 end
end
class PimEventSubscription
  def self.for_ride(ride)
        return PimEventSubscription.new(ride) if ride.user.present?
        return InvalidPimEventSubscription.new #Then callers wont be the wiser.
  end
  # ... rest of original (step 0) PimEventSubscription code here. 
end

2_ride.rb

# And we can use the new for_ride method here, and see how the null objects have the same protocol as the primary objects.


  def enable_rips_gps_updates
    #- PimEventSubscription.new(self).subscribe
    PimEventSubscription.for_ride(self).subscribe
    update_attribute(:rips_hook_enabled, RipsHook.register_gps(self))
  end

  def disable_rips_gps_updates
    #- PimEventSubscription.new(self).unsubscribe
    PimEventSubscription.for_ride(self).unsubscribe
    RipsHook.deregister_gps(self)
    # even if the deregister fails proceed as though 
    # it succeeded...
    update_attribute(:rips_hook_enabled, false)
  end