Designing and Maintaining Software (DAMS) Louis Rose
Single Responsibility Principle
A class should have only one reason to change. - Martin and Martin Chapter 8, Agile Principles, Patterns and Practices in C#, Prentice Hall, 2009
Finding responsibilities class Pizza attr_reader :toppings attr_reader :likes def initialize(toppings) @toppings = toppings @likes = 0 end def title toppings.title end def cost toppings.cost + 4 end
def rating if likes > 1000 then "A" elsif likes > 500 then "B" elsif likes > 250 then "C" elsif likes > 100 then "D" else "E" end end def worse_rating if rating == "E" nil else rating.succ end end end
def like! @likes += 1 end
Based on: https://www.youtube.com/watch?v=5yX6ADjyqyE
Finding responsibilities class Pizza attr_reader :toppings attr_reader :likes def initialize(toppings) @toppings = toppings @likes = 0 end def title toppings.title end def cost toppings.cost + 4 end def like! @likes += 1 end
def rating if likes > 1000 then "A" elsif likes > 500 then "B" elsif likes > 250 then "C" elsif likes > 100 then "D" else "E" end end def worse_rating if rating == "E" nil else rating.succ end end end
Finding responsibilities class Pizza attr_reader :toppings attr_reader :likes def initialize(toppings) @toppings = toppings @likes = 0 end def title toppings.title end def cost toppings.cost + 4 end def like! @likes += 1 end
def rating if likes > 1000 then "A" elsif likes > 500 then "B" elsif likes > 250 then "C" elsif likes > 100 then "D" else "E" end end def worse_rating if rating == "E" nil else rating.succ end end end
Finding responsibilities class Pizza attr_reader :toppings attr_reader :likes def initialize(toppings) @toppings = toppings @likes = 0 end def title toppings.title end def cost toppings.cost + 4 end def like! @likes += 1 end
def rating if likes > 1000 then "A" elsif likes > 500 then "B" elsif likes > 250 then "C" elsif likes > 100 then "D" else "E" end end def worse_rating if rating == "E" nil else rating.succ end end end
Finding responsibilities class Pizza attr_reader :toppings attr_reader :likes def initialize(toppings) @toppings = toppings @likes = 0 end def title toppings.title end def cost toppings.cost + 4 end def like! @likes += 1 end
def rating if likes > 1000 then "A" elsif likes > 500 then "B" elsif likes > 250 then "C" elsif likes > 100 then "D" else "E" end end def worse_rating if rating == "E" nil else rating.succ end end end
Finding responsibilities class Pizza attr_reader :toppings attr_reader :likes def initialize(toppings) @toppings = toppings @likes = 0 end def title toppings.title end def cost toppings.cost + 4 end def like! @likes += 1 end
def rating if likes > 1000 then "A" elsif likes > 500 then "B" elsif likes > 250 then "C" elsif likes > 100 then "D" else "E" end end def worse_rating if rating == "E" nil else rating.succ end end end
Finding responsibilities class Pizza attr_reader :toppings attr_reader :likes def initialize(toppings) @toppings = toppings @likes = 0 end def title toppings.title end def cost toppings.cost + 4 end def like! @likes += 1 end
def rating if likes > 1000 then "A" elsif likes > 500 then "B" elsif likes > 250 then "C" elsif likes > 100 then "D" else "E" end end def worse_rating if rating == "E" nil else rating.succ end end end
Tactics Extract objects from primitives and data clumps Move methods to avoid feature envy Extract wrappers for ancillary responsibilities
Primitive Obsession class Pizza attr_reader :toppings attr_reader :likes def initialize(toppings) @toppings = toppings @likes = 0 end def title toppings.title end def cost toppings.cost + 4 end def like! @likes += 1 end
def rating if likes > 1000 then "A" elsif likes > 500 then "B" elsif likes > 250 then "C" elsif likes > 100 then "D" else "E" end end def worse_rating if rating == "E" nil else rating.succ end end end
Primitive Obsession class Pizza def like! @likes += 1 end def rating if likes > 1000 then "A" elsif likes > 500 then "B" elsif likes > 250 then "C" elsif likes > 100 then "D" else "E" end end def worse_rating if rating == "E" nil else rating.succ end end
end
Extract Primitive class Pizza def like! @likes += 1 end def rating Rating.from_likes(likes) end def worse_rating if rating.letter == "E" nil else rating.letter.succ end end end
class Rating def self.from_likes(likes) if likes > 1000 then new("A") elsif likes > 500 then new("B") elsif likes > 250 then new("C") elsif likes > 100 then new("D") else new("E") end end attr_reader :letter def initialize(letter) @letter = letter end end
Extract Primitive class Pizza def like! @likes += 1 end def rating Rating.from_likes(likes) end def worse_rating if rating.letter == "E" nil else rating.letter.succ end end end
class Rating def self.from_likes(likes) if likes > 1000 then new("A") elsif likes > 500 then new("B") elsif likes > 250 then new("C") elsif likes > 100 then new("D") else new("E") end end attr_reader :letter def initialize(letter) @letter = letter end end
Feature Envy class Pizza def like! @likes += 1 end def rating Rating.from_likes(likes) end def worse_rating if rating.letter == "E" nil else rating.letter.succ end end end
class Rating def self.from_likes(likes) if likes > 1000 then new("A") elsif likes > 500 then new("B") elsif likes > 250 then new("C") elsif likes > 100 then new("D") else new("E") end end attr_reader :letter def initialize(letter) @letter = letter end end
Move Method class Pizza def like! @likes += 1 end def rating Rating.from_likes(likes) end def worse_rating if rating.letter == "E" nil else rating.letter.succ end end end
class Rating def self.from_likes(likes) if likes > 1000 then new("A") elsif likes > 500 then new("B") elsif likes > 250 then new("C") elsif likes > 100 then new("D") else new("E") end end attr_reader :letter def initialize(letter) @letter = letter end def worse_rating if letter == "E" nil else
Move Method class Pizza def like! @likes += 1 end def rating Rating.from_likes(likes) end def worse_rating rating.worse_rating end end
class Rating def self.from_likes(likes) if likes > 1000 then new("A") elsif likes > 500 then new("B") elsif likes > 250 then new("C") elsif likes > 100 then new("D") else new("E") end end attr_reader :letter def initialize(letter) @letter = letter end def worse_rating if letter == "E" nil else
Move Method class Pizza def like! @likes += 1 end def rating Rating.from_likes(likes) end def worse_rating rating.worse_rating end end
class Rating def self.from_likes(likes) if likes > 1000 then new("A") elsif likes > 500 then new("B") elsif likes > 250 then new("C") elsif likes > 100 then new("D") else new("E") end end def initialize(letter) @letter = letter end def worse_rating if @letter == "E" nil else @letter.succ end
Move Method class Pizza def like! @likes += 1 end def rating Rating.from_likes(likes) end def worse_rating rating.worse end end
class Rating def self.from_likes(likes) if likes > 1000 then new("A") elsif likes > 500 then new("B") elsif likes > 250 then new("C") elsif likes > 100 then new("D") else new("E") end end def initialize(letter) @letter = letter end def worse if @letter == "E" nil else @letter.succ end
Move Method class Pizza def like! @likes += 1 end def rating Rating.from_likes(likes) end end
class Rating def self.from_likes(likes) if likes > 1000 then new("A") elsif likes > 500 then new("B") elsif likes > 250 then new("C") elsif likes > 100 then new("D") else new("E") end end def initialize(letter) @letter = letter end def worse if @letter == "E" nil else @letter.succ end
Back to responsibilities class Pizza attr_reader :toppings attr_reader :likes def initialize(toppings) @toppings = toppings @likes = 0 end def title toppings.title end def cost toppings.cost + 4 end def like! @likes += 1 end
def rating if likes > 1000 then "A" elsif likes > 500 then "B" elsif likes > 250 then "C" elsif likes > 100 then "D" else "E" end end def worse_rating if rating == "E" nil else rating.succ end end end
Back to responsibilities class Pizza attr_reader :toppings attr_reader :likes def initialize(toppings) @toppings = toppings @likes = 0 end def title toppings.title end def cost toppings.cost + 4 end def like! @likes += 1 end
def rating Rating.from_likes(likes) end end
Extract ancillaries… class Pizza attr_reader :toppings attr_reader :likes def initialize(toppings) @toppings = toppings @likes = 0 end def title toppings.title end def cost toppings.cost + 4 end def like! @likes += 1 end
def rating Rating.from_likes(likes) end end
… using delegates class Pizza attr_reader :toppings def initialize(toppings) @toppings = toppings end def title toppings.title end def cost toppings.cost + 4 end end
class Likeable < SimpleDelegator attr_reader :likes def initialize(subject) @likes = 0 super end def like! @likes += 1 end def rating Rating.from_likes(likes) end end
… using delegates class Pizza attr_reader :toppings def initialize(toppings) @toppings = toppings end def title toppings.title end def cost toppings.cost + 4 end end
class Likeable < SimpleDelegator attr_reader :likes def initialize(subject) @likes = 0 super end def like! @likes += 1 end def rating Rating.from_likes(likes) end end
Cohesion & Coupling Before the refactoring: Client 1
Client 2
Pizza
Topping
Cohesion & Coupling After the refactoring: Client 1
Pizza
Topping
Client 2
Likeable
Rating
Cohesion & Coupling All clients were coupled to all of Pizza Client 1
Pizza Likeable
Client 2
Rating
Topping
Cohesion & Coupling Clients that don’t need Likeable aren’t affected by it Client 1
Pizza
Topping
Client 2
Likeable
Rating
Cohesion & Coupling Likeable can be re-used in new contexts Client 1
Pizza
Topping
Client 2
Likeable
Rating
Client 3
Summary Classes should have a single responsibility: a single reason to change Classes with low cohesion normally have more than one responsibility Extract classes to redistribute responsibilities
Designing and Maintaining Software (DAMS) - GitHub
Getting Cohesion. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Single Responsibility. Principle. A class should have only one reason to change. - Martin and Martin. Chapter 8, Agile Principles, Patterns and Practices in C#, Prentice Hall, 2009. Page 3. Finding responsibilities class Pizza attr_reader : ...
ASTs are tree data structures that can be analysed for meaning (following JLJ in SYAC 2014/15) ... More Cohesive. Avoids Duplication. Clearer. More Extensible.
Open-source. Influenced by Perl, Smalltalk, Eiffel, Ada and Lisp. Dynamic. Purely object-oriented. Some elements of functional programming. Duck-typed class Numeric def plus(x) self.+(x) end end y = 5.plus(6) https://www.ruby-lang.org/en/about · http
Page 1. Getting Lean. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Lean software⦠Has no extra parts. Solves the problem at hand and no more. Is often easier to change (i.e., is more habitable). Page 3. The Advice I Want to Give.
Why not duplicate? Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Habitable Software. Leaner. Less Complex. Loosely Coupled. More Cohesive. Avoids Duplication. Clearer. More Extensible ??? Page 3. Bad Practice. Page 4. Don't Repeat Yo
âWe have tried to demonstrate that it is almost always incorrect to begin the decomposition of a system into modules on the basis of a flowchart. We propose instead that one begins with a list of difficult design decisions or design decisions which
Tools: Vagrant. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Bugs that appear in production and that can't be reproduced by a developer on their machine are really hard to fix. Problem: âIt works on my machineâ. Page 3. Why does
Clear Documentation. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Bad documentation. Misleading or contradictory find_customer(id). CustomerGateway. Used to look up a customer by their customer number. Page 3. Bad documentation. Red
%w.rack tilt date INT TERM..map{|l|trap(l){$r.stop}rescue require l};. $u=Date;$z=($u.new.year + 145).abs;puts "== Almost Sinatra/No Version has taken the stage on #$z for development with backup from Webrick". $n=Module.new{extend. Rack;a,D,S,q=Rack
R&D: sketch habitable solutions on paper, using UML. 4. Evaluate solutions and implement the best, using TDD. Probably start again at 3. 5. Give to the product owner to validate. Probably start again at 1. 6. Put into production for customers to eval
Habitable Software. Leaner. Less Complex. Loosely Coupled. More Cohesive. Avoids Duplication. Clearer. More Extensible ??? Page 3. Lean. âPerfection is finally achieved not when there is no longer anything to add, but when there is no longer anythi
Fixes issue #42. Users were being redirected to the home page after login, which is less useful than redirecting to the page they had originally requested before being redirected to the login form. * Store requested path in a session variable. * Redi
What is it? Several pieces of data are often used together. Why is it problematic? Behaviour that operates on the clump has no home. (and consequently is often duplicated). When does it arise? High cohesion of the clump has not been detected. D
Observers. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Page 3. Delivery people need to know when pizzas are ready class Pizza def initialize(delivery_person). @delivery_person = delivery_person end def bake cook # blocking call. @d
âWe want the reading of code to be easy, even it makes the writing harder. (Of course, there's no way to write code without also reading it, soâ¦)â - Bob Martin. Clean Code. Prentice Hall, 2009. Page 5. Page 6. User Experience. âA person of av
Getting loose coupling. Designing and Maintaining Software (DAMS). Louis Rose ... should not depend on low-level modules. Both should depend on abstractions.â âAbstractions should not depend on details. Details should depend on abstractions.â -
Automatically detect similar fragments of code. class StuffedCrust def title. "Stuffed Crust " +. @toppings.title +. " Pizza" end def cost. @toppings.cost + 6 end end class DeepPan def title. "Deep Pan " +. @ingredients.title +. " Pizza" end def cost
Ruby Testing Frameworks. 3 popular options are: RSpec, Minitest and Test::Unit. We'll use RSpec, as it has the most comprehensive docs. Introductory videos are at: http://rspec.info ...
Clear Names. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Naming is hard. âThere are only two hard things in Computer. Science: cache invalidation and naming things.â - Phil Karlton http://martinfowler.com/bliki/TwoHardThings.ht
Coupling Between Objects. Counts the number of other classes to which a class is coupled (other than via inheritance). CBO(c) = |d â C - (1cl U Ancestors(C))| uses(c, d) V uses(d, c). - Chidamber and Kemerer. A metrics suite for object-oriented des
Reducing duplication. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Tactics. Accentuate similarities to find differences. Favour composition over inheritance. Know when to reach for advanced tools. (metaprogramming, code generation).
Plug-ins. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Problem. Page 3. Current Architecture. Shareable. Likeable. Food. Pizza. Liking and sharing foods are primary business concerns, so shouldn't be implemented as delegators. Page
When we are testing the way that a unit behaves when a condition is met, use a stub to setup the condition. Solution: use stubs for queries class Subscription ... def bill(amount) unless payments.exists(subscription_id: id) payments.charge(subscripti
Size != Complexity. âImagine a small (50 line) program comprising. 25 consecutive "IF THEN" constructs. Such a program could have as many as 33.5 million distinct control paths.â - Thomas J. McCabe. IEEE Transactions on Software Engineering, 2:4,