Refactoring: Refactorings/Extract Method

Turn a section of code inside a long method into its own method (shortens the long method). Resulting code looks more like easy-to-understand pseudocode.

Mechanics

1. Create a new method, named after 'what it does (not how it does it)

   Note: If code in question is simple, only extract if new method better reveals intention. If you can't come up with a more meaningful name, don't extract the code.

2. Copy extracted code from source method into new method
3. Scan extracted code for references to local variables. Make these variables parameters
4. Scan extracted code for temporary variables declared in source method. If so, declare them in target method instead.
5. If any local variables inside source method are modified by extracted code, treat extracted code as a query and assign result to said variable

   {{note|If more than 1 modified variable, Split Temporary Variable or Replace Temp with Query

6. Create function call to target method above extracted code; don't forget necessary local-scope variables for parameters
7. Delete extracted code in source method, replacing it with function call from previous step.
8. Check for unused temporary variables that have been declared inside the newly extracted method and remove them.
9. Test

Example

Before

def print_owing(previous_amount)
  outstanding = previous_amount * 1.2

  # print banner
  puts "*************************"
  puts "***** Customer Owes *****"
  puts "*************************"

  # calculate outstanding
  @orders.each do |order|
    outstanding += order.amount
  end

  # print details
  puts "name: #{@name}"
  puts "amount: #{outstanding}"
end

After

def print_owing(previous_amount)
  print_banner
  outstanding = calculate_outstanding(previous_amount * 1.2)
  print_details outstanding
end

def print_banner
  puts "*************************"
  puts "***** Customer Owes *****"
  puts "*************************"
end

def calculate_outstanding(initial_value)
  @orders.inject(initial_value) { |result, order| result + order.amount }
end

def print_details(outstanding)
  puts "name: #{@name}"
  puts "amount: #{outstanding}"
end