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A Case for Use Cases

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Use Cases VS The State (of Rails applications)

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Shevaun Coker @shevauncoker Prosecutor

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Shevaun Coker Court Reporter

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Use Case Defendant

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Charges

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Reducing coupling between the Rails framework and business domain Charges

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Increasing visibility of features within the codebase Charges

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Encapsulating complex business logic Charges

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The following presentation contains explicit code scenes. Viewer discretion is advised. Warning!

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Opening Statement

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Sole Requirement: A buyer can purchase a photo

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Photo Buyer Purchase

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$ rails g model Buyer

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$ rails g model Buyer class Buyer < ActiveRecord::Base end

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$ rails g model Photo

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class Photo < ActiveRecord::Base end $ rails g model Photo

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$ rails g model Purchase

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$ rails g model Purchase class Purchase < ActiveRecord::Base belongs_to :photo belongs_to :buyer end

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$ rails g controller Purchases

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$ rails g controller Purchases class PurchasesController < ApplicationController def create Purchase.create!(buyer: buyer, photo: photo) redirect_to purchase_completed_path end end

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$ rails g controller Purchases class PurchasesController < ApplicationController def create Purchase.create!(buyer: buyer, photo: photo) redirect_to purchase_completed_path end end

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class PurchasesController < ApplicationController def create Purchase.create!(buyer, photo) redirect_to purchase_completed_path end end $ rails g controller Purchases

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class PurchasesController < ApplicationController def create Purchase.create!(buyer, photo) redirect_to purchase_completed_path end end $ rails g controller Purchases

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SOME TIME LATER…

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Sole Requirement: A buyer can purchase a photo s A buyer receives a purchase invoice Record the sales count of each photo A buyer can review a purchased photo Updated

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class PurchasesController def create purchase = Purchase.create!(buyer, photo) BuyerMail.invoice.deliver(buyer, purchase) photo.increment_sales_count buyer.enable_review_of(photo) redirect_to purchase_completed_path end end

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class PurchasesController def create purchase = Purchase.create!(buyer, photo) BuyerMailer.send_invoice(buyer, purchase) photo.increment_sales_count buyer.enable_review_of(photo) redirect_to purchase_completed_path end end

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class PurchasesController def create purchase = Purchase.create!(buyer, photo) BuyerMailer.send_invoice(buyer, purchase) photo.increment_sales_count buyer.enable_review_of(photo) redirect_to purchase_completed_path end end

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class PurchasesController def create purchase = Purchase.create!(buyer, photo) BuyerMailer.send_invoice(buyer, purchase) photo.increment_sales_count buyer.enable_review_of(photo) redirect_to purchase_completed_path end end

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class Purchase < ActiveRecord::Base def self.create_for_buyer(buyer, photo) purchase = self.create!(buyer, photo) BuyerMailer.send_invoice(buyer, purchase) photo.increment_sales_count buyer.enable_review_of(photo) end end

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class Purchase < ActiveRecord::Base def self.create_for_buyer(buyer, photo) purchase = self.create!(buyer, photo) BuyerMailer.send_invoice(buyer, purchase) photo.increment_sales_count buyer.enable_review_of(photo) end end

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class PurchasesController def create purchase = Purchase.create!(buyer, photo) BuyerMailer.send_invoice(buyer, purchase) photo.increment_sales_count buyer.enable_review_of(photo) redirect_to purchase_completed_path end end

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class PurchasesController def create purchase = Purchase.create!(buyer, photo) BuyerMailer.send_invoice(buyer, purchase) photo.increment_sales_count buyer.enable_review_of(photo) redirect_to purchase_completed_path end end

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class Purchase < ActiveRecord::Base def self.create_for_buyer(buyer, photo) purchase = self.create!(buyer, photo) BuyerMailer.send_invoice(buyer, purchase) photo.increment_sales_count buyer.enable_review_of(photo) end end

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class Purchase < ActiveRecord::Base def self.create_for_buyer(buyer, photo) purchase = self.create!(buyer, photo) BuyerMailer.send_invoice(buyer, purchase) photo.increment_sales_count buyer.enable_review_of(photo) end end

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class PurchasesController def create Purchase.create_for_buyer!(buyer, photo) redirect_to purchase_completed_path end end

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class PurchasesController def create Purchase.create_for_buyer!(buyer, photo) redirect_to purchase_completed_path end end

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http://wall.alphacoders.com/big.php?i=117571

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class Purchase < ActiveRecord::Base def self.create_for_buyer(buyer, photo) purchase = self.create!(buyer, photo) BuyerMailer.send_invoice(buyer, purchase) photo.increment_sales_count buyer.enable_review_of(photo) end end

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class Purchase < ActiveRecord::Base def self.create_for_buyer(buyer, photo) purchase = self.create!(buyer, photo) BuyerMailer.send_invoice(buyer, purchase) photo.increment_sales_count buyer.enable_review_of(photo) end end

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class Purchase < ActiveRecord::Base; end def self.create_for_buyer(buyer, photo) purchase = self.create!(buyer, photo) BuyerMailer.send_invoice(buyer, purchase) photo.increment_sales_count buyer.enable_review_of(photo) end end

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class Purchase < ActiveRecord::Base; end def self.create_for_buyer(buyer, photo) purchase = self.create!(buyer, photo) BuyerMailer.send_invoice(buyer, purchase) photo.increment_sales_count buyer.enable_review_of(photo) end end > Purchase.instance_methods.size - Object.instance_methods.size => 214

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class Purchase < ActiveRecord::Base def self.create_for_buyer(buyer, photo) purchase = self.create!(buyer, photo) BuyerMailer.send_invoice(buyer, purchase) photo.increment_sales_count buyer.enable_review_of(photo) end end

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class Purchase < ActiveRecord::Base def self.create_for_buyer(buyer, photo) purchase = self.create!(buyer, photo) BuyerMailer.send_invoice(buyer, purchase) photo.increment_sales_count buyer.enable_review_of(photo) end end

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class Purchase < ActiveRecord::Base def self.create_for_buyer(buyer, photo) purchase = self.create!(buyer, photo) BuyerMailer.send_invoice(buyer, purchase) photo.increment_sales_count buyer.enable_review_of(photo) end end

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class Purchase < ActiveRecord::Base def self.create_for_buyer(buyer, photo) purchase = self.create!(buyer, photo) BuyerMailer.send_invoice(buyer, purchase) photo.increment_sales_count buyer.enable_review_of(photo) end end

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Too many responsibilities

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Harder to change

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More error & conflict prone

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Huge class files

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http://techmaster.vn/2014/07/blogging-rails-02-mvc-trong-rails/

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Taking the Stand

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What is a Use Case?

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A use case is a written description of how users will perform tasks on your website. usability.gov

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It outlines, from a user's point of view, a system's behaviour as it responds to a request. usability.gov

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Each use case is represented as a sequence of simple steps, beginning with a user's goal and ending when that goal is fulfilled. usability.gov

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http://commons.wikimedia.org/wiki/File:CCTS_CI_Use_Case.png

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Reflect the architecture in the code

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Reflect the use cases in the code

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Examining the Witness

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How did we implement use cases at Envato?

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“an action a user of a system takes to achieve a goal”

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* PurchasePhoto * ConfirmEmail * ResetPassword * VerifySignIn

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$ls app/use_cases purchase_photo.rb confirm_email.rb reset_password.rb verify_sign_in.rb

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module UseCase extend ActiveSupport::Concern module ClassMethods def perform(*args) new(*args).tap { |use_case| use_case.perform } end end def perform raise NotImplementedError end end

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module UseCase extend ActiveSupport::Concern module ClassMethods def perform(*args) new(*args).tap { |use_case| use_case.perform } end end def perform raise NotImplementedError end end

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module UseCase extend ActiveSupport::Concern module ClassMethods def perform(*args) new(*args).tap { |use_case| use_case.perform } end end def perform raise NotImplementedError end end

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module UseCase extend ActiveSupport::Concern module ClassMethods def perform(*args) new(*args).tap { |use_case| use_case.perform } end end def perform raise NotImplementedError end end

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module UseCase extend ActiveSupport::Concern module ClassMethods def perform(*args) new(*args).tap { |use_case| use_case.perform } end end def perform raise NotImplementedError end end

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class PurchasePhoto include UseCase def initialize(buyer, photo) @buyer = buyer @photo = photo end def perform create_purchase send_invoice increment_sales_count allow_buyer_to_review_photo end end

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class PurchasePhoto include UseCase def initialize(buyer, photo) @buyer = buyer @photo = photo end def perform create_purchase send_invoice increment_sales_count allow_buyer_to_review_photo end end

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class PurchasePhoto include UseCase def initialize(buyer, photo) @buyer = buyer @photo = photo end def perform create_purchase send_invoice increment_sales_count allow_buyer_to_review_photo end end

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class PurchasePhoto include UseCase def initialize(buyer, photo) @buyer = buyer @photo = photo end def perform create_purchase send_invoice increment_sales_count allow_buyer_to_review_photo end end

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class PurchasePhoto include UseCase def initialize(buyer, photo) @buyer = buyer @photo = photo end def perform create_purchase send_invoice increment_sales_count allow_buyer_to_review_photo end end

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class PurchasePhoto include UseCase def initialize(buyer, photo) @buyer = buyer @photo = photo end def perform create_purchase send_invoice increment_sales_count allow_buyer_to_review_photo end end

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private def create_purchase @purchase = Purchase.create!(buyer, photo) end def send_invoice Buyer::InvoiceMailer.enqueue(purchase) end def increment_sales_count Photo::SalesCount.increment(photo) end def allow_buyer_to_review_photo Photo::ReviewPermissions.add_buyer(buyer, photo) end

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class PurchasePhoto include UseCase include ActiveModel::Validations def success? errors.none? end end

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class PurchasesController def create use_case = PurchasePhoto.perform if use_case.success? redirect_to purchase_completed_path else redirect_to purchase_failed_path end end end

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class PurchasesController def create use_case = PurchasePhoto.perform if use_case.success? redirect_to purchase_completed_path else redirect_to purchase_failed_path end end end

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class PurchasesController def create use_case = PurchasePhoto.perform if use_case.success? redirect_to purchase_completed_path else redirect_to purchase_failed_path end end end

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class PurchasesController def create use_case = PurchasePhoto.perform if use_case.success? redirect_to purchase_completed_path else redirect_to purchase_failed_path end end end

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Hearing from the Defence

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Lessons Learned

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Command Query Separation Bertrand Meyer, Object Oriented Software Construction Lesson 1

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Command: has side effects, no return value

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Query: idempotent, returns a value

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“We conflate commands and queries at our peril” Sandi Metz, Magic Tricks of Testing 2013

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A great fit for Integration Tests Lesson 2

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acceptance integration unit Test Coverage

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RSpec.describe PurchasePhoto do describe “#perform” do subject(:perform) { PurchasePhoto.perform(buyer, photo) } it “creates a purchase record” do expect { perform }.to change { Purchase.count }.by(1) end it “sends an invoice” do perform expect(Mailer.deliveries.last). to have_attributes(subject: “Purchase Invoice”) end end end

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RSpec.describe PurchasePhoto do describe “#perform” do subject(:perform) { PurchasePhoto.perform(buyer, photo) } it “creates a purchase record” do expect { perform }.to change { Purchase.count }.by(1) end it “sends an invoice” do perform expect(Mailer.deliveries.last). to have_attributes(subject: “Purchase Invoice”) end end end

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RSpec.describe PurchasePhoto do describe “#perform” do subject(:perform) { PurchasePhoto.perform(buyer, photo) } it “creates a purchase record” do expect { perform }.to change { Purchase.count }.by(1) end it “sends an invoice” do perform expect(Mailer.deliveries.last). to have_attributes(subject: “Purchase Invoice”) end end end

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RSpec.describe PurchasePhoto do describe “#perform” do subject(:perform) { PurchasePhoto.perform(buyer, photo) } it “creates a purchase record” do expect { perform }.to change { Purchase.count }.by(1) end it “sends an invoice” do perform expect(Mailer.deliveries.last). to have_attributes(subject: “Purchase Invoice”) end end end

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Don’t put low level logic in your UseCase Lesson 3

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private def create_purchase @purchase = Purchase.create!(buyer, photo) end def send_invoice Buyer::InvoiceMailer.enqueue(purchase) end def increment_sales_count Photo::SalesCount.increment(photo) end def allow_buyer_to_review_photo Photo::ReviewPermissions.add_buyer(buyer, photo) end

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Don’t make your UseCases generic Lesson 4

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http://veryfunnypics.eu/2013/07/19/canadian-protest-signs/

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Presenting the Evidence

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A Consistent Interface Provides

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PurchasePhoto.perform(buyer, photo) class PurchasePhoto include UseCase #... end

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PurchasePhoto.perform(buyer, photo) class PurchasePhoto include UseCase #... end

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Self Documenting & Readable Code Provides

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class PurchasePhoto include UseCase def perform create_purchase send_invoice increment_sales_count allow_buyer_to_review_photo end end

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private def create_purchase @purchase = Purchase.create!(buyer, photo) end def send_invoice Buyer::InvoiceMailer.enqueue(purchase) end def increment_sales_count Photo::SalesCount.increment(photo) end def allow_buyer_to_review_photo Photo::ReviewPermissions.add_buyer(buyer, photo) end

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Context & Encapsulation Provides

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class PurchasePhoto def initialize(buyer, photo) @buyer = buyer @photo = photo end def perform # steps end end

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Extensibility Provides

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private def create_purchase @purchase = Purchase.create!(buyer, photo) end def send_invoice Buyer::InvoiceMailer.enqueue(purchase) end # ... def send_purchase_confirmation Purchase::ConfirmationMailer.enqueue(purchase) end def allow_buyer_to_review_photo

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private def create_purchase @purchase = Purchase.create!(buyer, photo) end def send_invoice Buyer::InvoiceMailer.enqueue(purchase) end # ... def send_confirmation_email Purchase::ConfirmationMailer.enqueue(purchase) end def allow_buyer_to_review_photo

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class PurchasePhoto include UseCase def perform create_purchase send_invoice increment_sales_count allow_buyer_to_review_photo end end

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class PurchasePhoto include UseCase def perform create_purchase send_invoice increment_sales_count allow_buyer_to_review_photo send_confirmation_email end end

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Decoupling from your framework Provides

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A codebase that reveals its features Provides

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$ls app/use_cases purchase_photo.rb confirm_email.rb reset_password.rb verify_sign_in.rb

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Objection!

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http://i.imgur.com/aXTps4g.jpg

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Use Case is a perfectly cromulent name

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Illustration by Leslie Herman

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cjohansen/use_case cypriss/mutations stevehodgkiss/interaction collectiveidea/interactor https://github.com/

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cjohansen/use_case cypriss/mutations stevehodgkiss/interaction collectiveidea/interactor https://github.com/

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cjohansen/use_case cypriss/mutations stevehodgkiss/interaction collectiveidea/interactor https://github.com/

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cjohansen/use_case cypriss/mutations stevehodgkiss/interaction collectiveidea/interactor https://github.com/

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cjohansen/use_case cypriss/mutations stevehodgkiss/interaction collectiveidea/interactor https://github.com/

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Closing Statement

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Reduces Coupling Increases feature visibility Encapsulates complex logic

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Reduces Coupling Increases feature visibility Encapsulates complex logic GUILTY

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CASE CLOSED

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webuild.envato.com/blog/a-case-for-use-cases