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Refactoring

You probably have already done some refactoring as part of code katas.

Refactoring, like TDD, is a disciplined skill with rules.

  • Refactoring leaves the external behaviour of code the same.
  • Good tests[1] are essential to refactoring. Disciplined TDD is a great way to have a reliable test suite.
  • We refactor by applying small changes to our code (a single refactoring) from a catalog of refactorings, or by generalising code e.g. turning duplicated if statements into a loop
  • Your tests should be passing before and after each refactoring or generalisation.
  • By performing multiple refactorings in a row it is possible to significantly change the structure of your code.

A Single Refactoring: what is a small change?

Small reversible operations. They should have a name, with a clearly defined before and after state.

When mobbing or pairing it’s useful to be able to immediately describe a refactoring.

Generalising code: I can’t find it in the catalog!

While in the blue phase (refactoring step) we should look for appropriate generalisations to make.

A good rule of thumb is the RuleOfThree - if you see the same thing duplicated three times, create a more general solution.

Generalising code is part of refactoring. However, they do not result in like-for-like behaviour, so you will not find them in the catalog.

The process

Here is the process for performing the Extract Method refactoring, which is a single refactoring that you can find in the catalog of refactorings.

  1. Run your tests. Do they pass? If they don’t you need to solve that first.
  2. Apply the refactoring

Before

class PayslipPrinter
  def print(payslip)
    puts "======================"
    puts "Owed #{payslip.total_owed}."
    puts "Total Deducted #{payslip.total_deducted}"
  end
end
     .
       .
   . ;.
    .;
     ;;.
   ;.;;
   ;;;;.
   ;;;;;
   ;;;;;
   ;;;;;
   ;;;;;
   ;;;;;
 ..;;;;;..
  ':::::'
    ':`

After

class PayslipPrinter
  def print(payslip)
    puts "======================"
    print_totals(payslip.total_owed, payslip.total_deducted)
  end

  def print_totals(total_owed, total_deducted)
    puts "Owed #{total_owed}."
    puts "Total Deducted #{total_deducted}"
  end
end
  1. Run your tests. Do they pass? If they don’t, you’ve incorrectly performed your refactoring.

Stop and understand

  • Why do we run the tests before and afterwards?
  • Why not make lots of changes in one go?

To Do

Using the Yatzy Refactoring Kata by Emily Bache, practice applying refactorings from the catalog of refactorings.

Notes

  • [1] - https://www.madetech.com/blog/semantically-stable-test-suites

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