Refactoring: Refactorings/Replace Method with Method Object

Turns a method into its own object so local variables become instance variables on that object. Code then becomes much easier to refactor in other ways.

Useful in a long method that uses local variables so you can't use Extract Method.

Mechanics

1. Create a new class. Name it after the method
2. Give the new class an attribute for the object that once hosted the original method (source object), and an attribute for temporary variables and parameters in the method
3. Give the new class a constructor that takes the source object and each parameter
4. Give the new class a method named "compute"
5. Copy the body of the original method into compute, using the source object instance variable for any reference to methods on the source object
6. Test
7. Replace old method with one that creates new object and calls compute.

Example

Before

class Account
  def gamma(input_val, quantity, year_to_date)
    important_value1 = (input_val * quantity) + delta
    important_value2 = (input_val * year_to_date) + 100
    if (year_to_date - important_value1) > 100
      important_value2 -= 20
    end
    important_value3 = important_value2 * 7
    # ...
    important_value3 - 2 * important_value1
  end
end

After

class Gamma
  attr_reader :account, :input_val, :quantity, :year_to_date, :important_value1, :important_value2, :important_value3

  def initialize(account, input_val_arg, quantity_arg, year_to_date_arg)
    @account = account
    @input_val = input_val_arg
    @quantity = quantity_arg
    @year_to_date = year_to_date_arg
  end

  def compute
    @important_value1 = (input_val * quantity) + @account.delta
    @important_value2 = (input_val * year_to_date) + 100
    if (year_to_date - important_value1) > 100     # now we could easily Extract Method on this
      @important_value2 -= 20
    end
    @important_value3 = important_value2 * 7
    # ...
    @important_value3 - 2 * important_value1
  end
end

class Account
  def gamma(input_val, quantity, year_to_date)
    Gamma.new(self, input_val, quantity, year_to_date).compute
  end
end