Palestra ministrada no TDC POA em 2016 sobre como usar esses dois princípios podem ajudar a melhorar seu código evitando alto acoplamento e problemas com referências nulas.

1. MELHORANDO SEU
CÓDIGO
Law of Demeter
Tell,
don’t ask

2. Sobre mim
@nelson_senna
https://nelsonsar.github.io

3. DRY?
YAGNI?
SOLID?
TELL DON’T ASK
LAW OF DEMETER
DDD!

4. –Alan Kay
“The key in making great and growable
systems is much more to design how its
modules communicate rather than what
their internal properties and behaviors
should be.”

6. class Paperboy
attr_reader :wallet
def initialize(wallet)
@wallet = wallet
end
def receive_from(customer, amount)
if customer.wallet.try(:amount).to_f > amount
@wallet.amount = amount
customer.wallet = customer.wallet.amount - amount
else
nil
end
end
end

7. class Paperboy
attr_reader :wallet
def initialize(wallet)
@wallet = wallet
end
def receive_from(customer, amount)
if customer.wallet.try(:amount).to_f > amount
@wallet.amount = amount
customer.wallet = customer.wallet.amount - amount
else
nil
end
end
end

8. class Paperboy
attr_reader :wallet
def initialize(wallet)
@wallet = wallet
end
def receive_from(customer, amount)
if customer.wallet.try(:amount).to_f > amount
@wallet.amount = amount
customer.wallet = customer.wallet.amount - amount
else
nil
end
end
end

9. –Ruby Science
“Referencing another object’s state
directly couples two objects together
based on what they are, rather than on
what they do.”

10. DE FORMA PRÁTICA

11. describe Paperboy do
describe '#receive_from' do
let(:paperboy) { described_class.new(Wallet.new(0)) }
context 'when customer has enough money to pay' do
it 'add due amount to wallet' do
customer = double
allow(customer).to receive(:wallet).and_return(Wallet.new(10))
paperboy.receive_from(user, 5)
expect(paperboy.wallet.amount).to eq(5)
end
end
end
end

12. describe Paperboy do
describe '#receive_from' do
let(:paperboy) { described_class.new(Wallet.new(0)) }
context 'when customer has enough money to pay' do
it 'add due amount to wallet' do
customer = double
allow(customer).to receive(:wallet).and_return(Wallet.new(10))
paperboy.receive_from(user, 5)
expect(paperboy.wallet.amount).to eq(5)
end
end
end
end

13. describe Paperboy do
describe '#receive_from' do
let(:paperboy) { described_class.new(Wallet.new(0)) }
context 'when customer has enough money to pay' do
it 'add due amount to wallet' do
customer = double
allow(customer).to receive(:wallet).and_return(Wallet.new(10))
paperboy.receive_from(user, 5)
expect(paperboy.wallet.amount).to eq(5)
end
end
end
end

14. E ISSO TEM UM
CHEIRINHO…

15. –Reek doumentation
“Feature Envy occurs when a code fragment
references another object more often than it
references itself, or when several clients do the
same series of manipulations on a particular type of
object.”

16. NÃO SE CONVENCEU?

18. def associate_user
@order ||= current_order
if try_spree_current_user && @order
if @order.user.blank? || @order.email.blank?
@order.associate_user!(try_spree_current_user)
end
end
end

20. TELL, DON’T ASK

21. –Martin Fowler
“It reminds us that rather than asking an
object for data and acting on that data,
we should instead tell an object what
to do.”

22. class Order
def apply_discount(promotion)
if promotion.eligible?(shipment)
promotion.activate(shipment)
end
end
end

23. class Order
def apply_discount(promotion)
if promotion.eligible?(shipment)
promotion.activate(shipment)
end
end
end

24. LAW OF DEMETER

25. –Ruby Science
“The law restricts how deeply a method
can reach into another object’s dependency
graph, preventing any one method from
becoming tightly coupled to another
object’s structure.”

26. class Paperboy
attr_reader :wallet
def initialize(wallet)
@wallet = wallet
end
def receive_from(customer, amount)
if customer.wallet.try(:amount).to_f > amount
@wallet.amount = amount
customer.wallet = customer.wallet.amount - amount
else
nil
end
end
end

27. class Paperboy
attr_reader :wallet
def initialize(wallet)
@wallet = wallet
end
def receive_from(customer, amount)
self.wallet.add(customer.pay(amount))
end
end

28. NULL OBJECT

29. –Martin Fowler
“Instead of returning null, or some
odd value, return a Special Case that
has the same interface as what the
caller expects.”

30. def associate_user
@order ||= current_order
if try_spree_current_user && @order
if @order.user.blank? || @order.email.blank?
@order.associate_user!(try_spree_current_user)
end
end
end

31. def try_spree_current_user
if respond_to?(:spree_current_user)
spree_current_user
elsif respond_to?(:current_spree_user)
current_spree_user
else
Guest.new
end
end

32. DESVANTAGENS E
CUIDADOS

33. https://github.com/troessner/reek

34. PRINCÍPIOS DEVERIAM
AJUDAR E NÃO
CONFUNDIR

35. DÚVIDAS

36. REFERÊNCIAS
• http://www.dan-manges.com/blog/37
• http://www.virtuouscode.com/2011/06/28/do-or-do-not-there-is-no-try/
• http://gmoeck.github.io/2011/09/28/why-you-should-care-about-information-hiding.html
• http://blog.davidchelimsky.net/blog/2006/11/27/fighting-the-urge-to-ask/
• http://www.ccs.neu.edu/research/demeter/demeter-method/LawOfDemeter/paper-boy/demeter.pdf
• martinfowler.com/bliki/TellDontAsk.html
• https://adamcod.es/2013/11/22/tell-dont-ask.html
• https://pragprog.com/articles/tell-dont-ask
• https://edelpero.svbtle.com/most-common-mistakes-on-legacy-rails-apps
• http://www.mockobjects.com/2006/10/tell-dont-ask-and-mock-objects.html
• https://robots.thoughtbot.com/tell-dont-ask
• https://skillsmatter.com/skillscasts/8611-tell-dont-ask
• https://www.javacodegeeks.com/2015/11/tell-dont-ask.html
• http://natpryce.com/articles/000777.html

37. REFERÊNCIAS
• http://martinfowler.com/bliki/GetterEradicator.html
• http://verraes.net/2014/09/objects-as-contracts-for-behaviour/
• https://medium.com/skyfishtech/retracing-original-object-oriented-
programming-f8b689c4ce50#.yk9k1s7ku
• http://brightonruby.com/2016/the-point-of-objects-john-cinnamond/
• https://github.com/spree/spree/blob/master/core/app/models/spree/
promotion_handler/page.rb
• https://github.com/troessner/reek/blob/master/docs/Feature-
Envy.md