Created
July 6, 2012 17:29
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Exceptions and the observer pattern
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class CardCreator | |
include Publisher | |
def create(iteration, attributes) | |
card = iteration.cards.build(attributes) | |
card.save! | |
rescue ActiveRecord::RecordInvalid | |
notify_subscribers(:create_failed, card) | |
else | |
notify_subscribers(:created, card) | |
end | |
end |
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class CardPersistenceResponder < Struct.new(:controller) | |
def created(card) | |
controller.instance_eval do | |
redirect_to planning_path(card.project) | |
end | |
end | |
def create_failed(card) | |
controller.instance_eval do | |
@title = 'New card' | |
@card = card | |
flash.now[:alert] = 'Your card needs a title' | |
render :action => 'new' | |
end | |
end | |
end |
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class CardsController < ApplicationController | |
def create | |
creator = CardCreator.new | |
creator.add_subscriber(CardPersistenceResponder.new(self)) | |
creator.create(project.backlog, card_params) | |
end | |
end |
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module Publisher | |
def add_subscriber(object) | |
@subscribers ||= [] | |
@subscribers << object | |
end | |
def notify_subscribers(message, *args) | |
return if @subscribers.blank? | |
@subscribers.each do |subscriber| | |
subscriber.send(message, *args) if subscriber.respond_to?(message) | |
end | |
end | |
end |
Since I made this gist I've written the process up: http://theagileplanner.com/blog/building-agile-planner/refactoring-with-hexagonal-rails
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Having just applied this refactoring to my app, I've had to deal with where to put the code for
CardUpdater
. Do you mergeCardCreator
andCardUpdater
, and what about the responder? Make separate ones for create/update?Keeping
CardCreator
andCardUpdater
separate makes a lot of sense, as my update logic is actually quite complicated. The responder stuff is pretty simple, so I've added anupdated
callback to the existing responder. It feels nice.I couldn't help feeling that the observer pattern is overkill given that I've only got one object that wants to observe it. But then I got to updating my unit tests, and enjoyed the fact that if I didn't add an observer to the creator/updater (in the test code) that it didn't mind at all. I didn't need to stub it; the callbacks just didn't run. I do like the observer pattern…