SRP


Single Responsibility Principle

Replay "POODR" book 
by Sandi Metz

Thomas Petrachi


Twitter: @thmszpy
Youtube: http://www.youtube.com/channel/thomasPetrachi

Vodeclic: www.vodeclic.com
Blog: blog.space-a.fr

O.O. Design


Easy-to-change application

  • No unexpected side effects
  • Easy to reuse
  • Small product changes == small code changes

Single responsibility


Deciding what belongs in a class

Example


Bicycles + gears

Example


The "Gear" class

Example


Adding more logic

Example


The "Gear" class

Single Responsibility




Is this the best way to organize code?

Single Responsibility


Why does SRP matters?


Without SRP:
  • Difficult to reuse
  • Leads to code duplication
  • Or to useless dependencies

Single Responsibility


Determining SRP:
  1. Mr Gear, what is your ratio?
  2. Seems fine


  1. Mr Gear, what is your tire?
  2. Seems ridiculous

Single Responsibility



When to change

"What is the future cost of doing nothing today?"

Coding Techniques


A few guidelines:
  1. Hide instance variables
  2. Hide data structures
  3. Extract responsibilities (from methods)
  4. Isolate responsibilities

Hide Instance Variables


Always use accessors
Then you can override if nexessary

Hide Data Structures

Each method has knowledge of the @data structure


Each method will need to change if @data changes

Hide Data Structures


Encapsulate knowledge of @data structure
Only #weelify method will change if @data changes

Extract Responsibilities


Two responsibilities in one method
becomes
Avoids the need for comments and improves reusability

Isolate Responsibilities


#diameter method doesn't belong in Gear class

Conclusion



SRP allows you to:
  • Anticipate future changes (like a real Wheel class)
  • Change without consequence
  • Reuse without duplication

Thanks !

Any questions?

SRP

By Thomas Petrachi

SRP

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