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重構Ruby程式碼(學習筆記)
2015.Feb.25

這篇文章是閱讀『Refactoring: Ruby Edition』這本書的學習筆記,會記錄在書中看到的觀念,以及書中提供的重構範例。

重構初體驗

起點

影片出租店的客戶請你做一隻程式:

『在影片出租店中,顧客可以選擇不同的影片、不同的租期,而商家會依照影片的租期來計算費用。影片類型分為三種:一般片、兒童片、新片。程式要能夠計算費用、並輸出收據,收據上會有收費明細與顧客的點數,點數計算方法會依據影片是否為新片而有所不同。』

起始例子:

Movie Rental Customer
price_code days_rented  
    statement()

Movie

class Movie
  REGULAR     = 0
  NEW_RELEASE = 1
  CHILDREN    = 2

  attr_reader   :title
  attr_accessor :price_code

  def initialize(title, price_code)
    @title, @price_code = title, price_code
  end
end

Rental

class Rental
  attr_reader :movie, :days_rented

  def initialize(movie, days_rented)
    @movie, @days_rented = movie, days_rented
  end
end

Customer

class Customer
  attr_reader :name

  def initialize(name)
    @name = name
    @rentals = []
  end

  def add_rental(arg)
    @rentals << arg
  end

  def statement
    total_amount, frequent_renter_points = 0, 0
    result = "Rental Record for #{@name}\n"
    @rentals.each do |element|
      this_amount = 0

      # determine amounts for each line
      case element.movie.price_code
      when Movie::REGULAR
        this_amount += 2
        this_amount += (element.days_rented - 2) * 1.5 if element.days_rented > 2
      when Movie::NEW_RELEASE
        this_amount += element.days_rented * 3
      when Movie::CHILDREN
        this_amount += 1.5
        this_amount += (element.days_rented - 3) * 1.5 if element.days_rented > 3
      end

      # add frequent renter points
      frequent_renter_points += 1
      # add bonus for a two day new release rental
      if element.movie.price_code == Movie::NEW_RELEASE && element.days_rented > 1
        frequent_renter_points += 1
      end

      #show figures for this rental
      result += element.movie.title + "\t" + this_amount.to_s + "\n"
      total_amount += this_amount
    end
    # add footer lines
    result += "Amount owed is #{total_amount}\n"
    result += "You earned #{frequent_renter_points} frequent renter points"
    result
  end
end

對以上程式碼的評語

  • Customer類別裡的statement方法太長!
  • statement裡放很多應是其他類別做的事情。
  • 對這段醜陋的程式碼來說,『電腦』不關心它美不美,但『人』會關心!
    • 設計糟糕的系統會讓『人』很難維護。
  • 當客戶想將收據輸出成html格式時,目前的statement方法完全不能直接搬過去,除了寫一個全新的html_statement方法來複製大多數行為。
    • 如果收費標準改了,得同時去修改statement和html_statement的方法才行!
    • 如果還要做其他修改,就得一直複製貼上、複製貼上…(潛在威脅誕生)
    • 當程式碼越來越複雜,就很難保證修改的地方都是一致的。
  • 最可怕的是,當客戶說:『我們想要修改影片分類的方式,但我們還沒想好要怎麼改>_^』
    • 身為一個專業的工程師,雖然什麼都不確定,但能肯定的是六個月內一定會再修改程式碼。
    • 雖然目前程式可以動,程式界的老話:『只要它還沒壞,就別去碰它』。但已經可以預見之後的日子很不好過…此時不重構程式碼,更待何時?

小提示:當你需要向一段程式裡添加功能,而程式的結構又不便於動手時,首要之務,就是重構程式碼來降低添加功能的難度,然後再加入需要的功能。

第一步

require 'minitest/autorun'
require_relative 'ch1'

describe Customer, "#statement" do
  it "generate statement" do
    movie = Movie.new("Star War", Movie::NEW_RELEASE)
    rental = Rental.new(movie, 3)
    customer = Customer.new("John Doe")

    customer.add_rental(rental)
    customer.statement.must_equal <<-EOS.gsub(/^\s+/, '').gsub(/\n$/,'')
      Rental Record for John Doe
      Star War\t9
      Amount owed is 9
      You earned 2 frequent renter points
    EOS
  end
end

小提示:進行重構之前,要先準備好一段可靠的測試。一步一步地進行修改與測試,犯錯時才好發現bug的位置。

statement方法的分解和再組合

目的:將太長的方法分解成小片段,並移到更適合它們的類別中

  • 觀察程式碼片段中所有作用域中的變數(含區域變數與參數):rental, this_amount
  • 先將一部分程式碼抽取出去(如下),並先給個方法名字為:amount_for
def statement
  total_amount, frequent_renter_points = 0, 0
  result = "Rental Record for #{@name}\n"
  @rentals.each do |element|
    this_amount = amount_for(element) #這行改變了!

    # add frequent renter points
    frequent_renter_points += 1
    # add bonus for a two day new release rental
    if element.movie.price_code == Movie::NEW_RELEASE && element.days_rented > 1
      frequent_renter_points += 1
    end

    #show figures for this rental
    result += element.movie.title + "\t" + this_amount.to_s + "\n"
    total_amount += this_amount
  end
  # add footer lines
  result += "Amount owed is #{total_amount}\n"
  result += "You earned #{frequent_renter_points} frequent renter points"
  result
end
def amount_for(element)
  this_amount = 0
  case element.movie.price_code
  when Movie::REGULAR
    this_amount += 2
    this_amount += (element.days_rented - 2) * 1.5 if element.days_rented > 2
  when Movie::NEW_RELEASE
    this_amount += element.days_rented * 3
  when Movie::CHILDREN
    this_amount += 1.5
    this_amount += (element.days_rented - 3) * 1.5 if element.days_rented > 3
  end
  this_amount
end
  • 切開來後,就可以一一針對片段做修改。
  • 先改命名!(閱讀程式碼,在理解程式的同時,把這些理解嵌入程式碼中,以防止將來忘記從中領悟到的東西。)
def amount_for(rental)
  result = 0
  case element.movie.price_code
  when Movie::REGULAR
    result += 2
    result += (element.days_rented - 2) * 1.5 if element.days_rented > 2
  when Movie::NEW_RELEASE
    result += element.days_rented * 3
  when Movie::CHILDREN
    result += 1.5
    result += (element.days_rented - 3) * 1.5 if element.days_rented > 3
  end
  result
end

小提示:傻瓜寫的程式碼只有電腦才能理解,優秀的工程師寫的程式碼可以讓其他人都能看懂。

  • 觀察amount_for,其實它使用的是來自rental的內容,而非customer。
    • amount_for放錯class了,通常,一個方法應該放在它資料來源的對象中,因此要移回rental中。
    • 為了適應新位置,需調整,此處要將參數刪掉,並給了個新名字。
class Rental
  def charge
    result = 0
    case movie.price_code
    when Movie::REGULAR
      result += 2
      result += (days_rented - 2) * 1.5 if days_rented > 2
    when Movie::NEW_RELEASE
      result += days_rented * 3
    when Movie::CHILDREN
      result += 1.5
      result += (days_rented - 3) * 1.5 if days_rented > 3
    end
    result
  end
end
  • 再把程式中所有引用舊方法的地方改成引用新方法。
class Customer
  def statement
    ...
    @rentals.each do |element|
      this_amount = element.charge
    ...
  end
end

目前程式狀態:

Movie Rental Customer
price_code days_rented  
  charge() statement()
  • Rental.charge暫時先放下,先回頭看看Customer.statement
def statement
  total_amount, frequent_renter_points = 0, 0
  result = "Rental Record for #{@name}\n"
  @rentals.each do |element|
    this_amount = element.charge

    # add frequent renter points
    frequent_renter_points += 1
    # add bonus for a two day new release rental
    if element.movie.price_code == Movie::NEW_RELEASE && element.days_rented > 1
      frequent_renter_points += 1
    end

    #show figures for this rental
    result += element.movie.title + "\t" + this_amount.to_s + "\n"
    total_amount += this_amount
  end
  # add footer lines
  result += "Amount owed is #{total_amount}\n"
  result += "You earned #{frequent_renter_points} frequent renter points"
  result
end
  • 目前,statement方法中的this_amount變數變成多餘了,此時可以用Replace Temp with Query方法來去掉this_amount。
def statement
  total_amount, frequent_renter_points = 0, 0
  result = "Rental Record for #{@name}\n"
  @rentals.each do |element|

    # add frequent renter points
    frequent_renter_points += 1
    # add bonus for a two day new release rental
    if element.movie.price_code == Movie::NEW_RELEASE && element.days_rented > 1
      frequent_renter_points += 1
    end

    #show figures for this rental #下面這兩行改變了~~
    result += element.movie.title + "\t" + element.charge.to_s + "\n"
    total_amount += element.charge
  end
  # add footer lines
  result += "Amount owed is #{total_amount}\n"
  result += "You earned #{frequent_renter_points} frequent renter points"
  result
end
  • 接下來,處理計算點數規則的部分,想把此段切出去,下方為原本的位置:
def statement
  ...
  @rentals.each do |element|

    # add frequent renter points
    frequent_renter_points += 1
    # add bonus for a two day new release rental
    if element.movie.price_code == Movie::NEW_RELEASE && element.days_rented > 1
      frequent_renter_points += 1
    end
  ...
  end
  ...
end
  • 一樣,先找區域變數:
    • element:可以被當作參數傳進來。
    • frequent_renter_points:已經在前面附值過,而且沒有讀取的動作,就不需要當參數傳,只需要寫加法動作即可。
class Customer
  def statement
    total_amount, frequent_renter_points = 0, 0
    result = "Rental Record for #{@name}\n"
    @rentals.each do |element|

      frequent_renter_points += element.frequent_renter_points #改在這裡!!

      #show figures for this rental
      result += element.movie.title + "\t" + element.charge.to_s + "\n"
      total_amount += element.charge
    end
    # add footer lines
    result += "Amount owed is #{total_amount}\n"
    result += "You earned #{frequent_renter_points} frequent renter points"
    result
  end
end
class Rental
  def frequent_renter_points
    (movie.price_code == Movie.NEW_RELEASE && days_rented > 1) ? 2 : 1
  end
end

目前程式狀態:

Movie Rental Customer
price_code days_rented  
  charge() statement()
  frequent_renter_points()  
  • 現在要來移除區域變數
    • 原因:區域變數只有在自己的方法裡才有用,因此很容易寫出很長很複雜的方法。
    • 作法:把Customer類別中的total_amount, frequent_renter_points替換成相對應的查詢方式(Replace Temp with Query)。
class Customer
  def statement
    frequent_renter_points = 0 #先拿掉total_amount
    result = "Rental Record for #{@name}\n"
    @rentals.each do |element|

      frequent_renter_points += element.frequent_renter_points

      #show figures for this rental
      result += element.movie.title + "\t" + element.charge.to_s + "\n"
    end
    # add footer lines
    result += "Amount owed is #{total_charge}\n" #這裡也改了!
    result += "You earned #{frequent_renter_points} frequent renter points"
    result
  end

  private

  def total_charge
    result = 0
    @rentals.each do |element|
      result += element.charge
    end
    result
  end
end
  • 上面private中的total_charge方法還可再簡化(運用Collection Closure Method)
def total_charge
  @rentals.inject(0) { |sum, rental| sum + rental.charge }
end
  • 接著對另一個區域變數frequent_renter_points進行相同的操作
class Customer
  def statement
    result = "Rental Record for #{@name}\n"
    @rentals.each do |element|
      #show figures for this rental
      result += element.movie.title + "\t" + element.charge.to_s + "\n"
    end
    # add footer lines
    result += "Amount owed is #{total_charge}\n"
    result += "You earned #{total_frequent_renter_points} frequent renter points"
     #上面那行改了!
    result
  end

  private

  def total_frequent_renter_points
    @rentals.inject(0){ |sum, rental| sum + rental.frequent_renter_points }
  end
end

目前程式狀態:

Movie Rental Customer
price_code days_rented  
  charge() statement()
  frequent_renter_points() total_charge()
    total_frequent_renter_points()
  • 產生效能疑慮
    • 此時很多人可會對效能產生疑慮(把區域變數切開來所做的修改),但先把程式碼整理清楚比較重要,之後可以再通過Profiler處理效能問題!
    • 而且方法切開後,如果其他部分需要一樣的內容,很容易就能使用。

接下來,暫時放下重構,加入html_statement方法

class Customer
  def html_statement
    result = "<h1>Rentals for <em>#{@name}</em></h1><p>\n"
    @rentals.each do |element|
      # show figures for this rental
      result += element.movie.title + ": " + element.charge.to_s + "<br>\n"
    end
    #add footer lines
    result += "<p>You owe <em>#{total_charge}</em></p>\n"
    result += "On this rental you earned " +
           "<em>#{total_frequent_renter_points}</em> " +
           "frequent renter points</p>"
    result
  end
end
  • 製作這個新方法速度沒花太多時間,是因為共用了statement的方法

用多態替換價格代碼中的條件邏輯

  • Rental.charge的方法中用到了case
    • case所操作的卻是movie的價錢,應該要把charge移到Movie類別中。
    • 下方是原本的程式碼:
class Rental
  def charge
    result = 0
    case movie.price_code
    when Movie::REGULAR
      result += 2
      result += (days_rented - 2) * 1.5 if days_rented > 2
    when Movie::NEW_RELEASE
      result += days_rented * 3
    when Movie::CHILDREN
      result += 1.5
      result += (days_rented - 3) * 1.5 if days_rented > 3
    end
    result
  end
end
  • 移到Movie後:
class Rental
  def charge
    movie.charge(days_rented)
  end
end

class Movie
  def charge(days_rented)
    result = 0
    case price_code
    when REGULAR
      result += 2
      result += (days_rented - 2) * 1.5 if days_rented > 2
    when NEW_RELEASE
      result += days_rented * 3
    when CHILDREN
      result += 1.5
      result += (days_rented - 3) * 1.5 if days_rented > 3
    end
    result
  end
end
  • 為什麼要移到Movie然後傳入租期參數,而不是在Rental裡傳入影片類型參數呢?
    • 因為客戶希望修改影片分類的方式,要把之後修改的連鎖反應減至最小。
  • 另外再把顧客點數計算一起移到Movie,下方為原本的:
class Rental
  def frequent_renter_points
    (movie.price_code == Movie.NEW_RELEASE && days_rented > 1) ? 2 : 1
  end
end
  • 移到Movie後:
class Rental
  def frequent_renter_points
    movie.frequent_renter_points(days_rented)
  end
end

class Movie
  def frequent_renter_points(days_rented)
    (price_code == NEW_RELEASE && days_rented > 1) ? 2 : 1
  end
end

目前程式狀態:

Movie Rental Customer
price_code days_rented  
charge(days_rented) charge() statement()
frequent_renter_points(days_rented) frequent_renter_points() total_charge()
    total_frequent_renter_points()
    html_statement()

最後…要來處理繼承!

  • 為了替換掉case語句,利用Replace Type Code with State/Strategy方式。
    • 自定義一個price_code的setter方法(因為再往後要做好玩的事情)
    • 下方是原本的:
class Movie
  REGULAR     = 0
  NEW_RELEASE = 1
  CHILDREN    = 2

  attr_reader   :title
  attr_accessor :price_code

  def initialize(title, price_code)
    @title, @price_code = title, price_code
  end
end
  • 修改後
class Movie
  ...
  attr_reader :price_code #改成getter而已

  def price_code= (value) #自己定義了setter
    @price_code = value
  end

  def initialize(title, the_price_code) #這裡也有改
    @title, self.price_code = title, the_price_code
  end
end
  • 接下來添加三個類別:
class RegularPrice
end

class NewReleasePrice
end

class ChildrensPrice
end
  • 繼續在剛剛自定義的price_code setter中做些好玩的事:
def price_code= (value)
  @price_code = value
  @price = case price_code
    when REGULAR     : RegularPrice.new
    when NEW_RELEASE : NewReleasePrice.new
    when CHILDRENS   : ChildrensPrice.new
  end
end
  • 作者提到,上方所出現的case將會是重構完成後,唯一一處case語句!
    • 接下來將其他有case的地方改掉,下方是原本的:
class Movie
  def charge(days_rented)
    result = 0
    case price_code
    when REGULAR
      result += 2
      result += (days_rented - 2) * 1.5 if days_rented > 2
    when NEW_RELEASE
      result += days_rented * 3
    when CHILDREN
      result += 1.5
      result += (days_rented - 3) * 1.5 if days_rented > 3
    end
    result
  end
end
  • 改成:
class RegularPrice
  def charge(days_rented)
    result = 2
    result += (days_rented - 2) * 1.5 if days_rented > 2
    result
  end
end

class NewReleasePrice
  def charge(days_rented)
    days_rented * 3
  end
end

class ChildrensPrice
  def charge(days_rented)
    result = 1.5
    result += (days_rented - 3) * 1.5 if days_rented > 3
    result
  end
end

class Movie
  def charge(days_rented)
    @price.charge(days_rented)
  end
end
  • 接下來輪到frequent_renter_points,下方為原本的:
class Movie
  def frequent_renter_points(days_rented)
    (price_code == NEW_RELEASE && days_rented > 1) ? 2 : 1
  end
end
  • 透過Extract Module:
module DefaultPrice
  def frequent_renter_points(days_rented)
    1
  end
end

class RegularPrice
  include DefaultPrice
  ...
end

class ChildrensPrice
  include DefaultPrice
  ...
end

class NewReleasePrice
  def frequent_renter_points(days_rented)
    days_rented > 1 ? 2 : 1
  end
end

class Movie
  def frequent_renter_points(days_rented)
    @price.frequent_renter_points(days_rented)
  end
end
  • 最後一步,從price_code的setter方法移除case語句,原本要這樣:
# calling code
movie = Movie.new("模仿遊戲", Movie::NEW_RELEASE)
# and later...
movie.price_code = Movie::REGULAR

class Movie
  ...
  def price_code= (value)
    @price_code = value
    @price = case price_code
      when REGULAR     : RegularPrice.new
      when NEW_RELEASE : NewReleasePrice.new
      when CHILDRENS   : ChildrensPrice.new
    end
  end
end
  • 重構後:
# calling code
movie = Movie.new("模仿遊戲", NewReleasePrice.new)
# and later...
movie.price = RegularPrice.new

class Movie
  attr_writer :price
end

大功告成

最後程式狀態:

Movie Rental Customer
title days_rented name
charge(days_rented) charge(days_rented) statement()
frequent_renter_points(days_rented) frequent_renter_points total_charge()
    total_frequent_renter_points()
    html_statement()
New Release Price Regular Price Childrens Price Default Price
charge(days_rented) charge(days_rented) charge(days_rented) frequent_renter_points(days_rented)
frequent_renter_points(days_rented)      
protocol
charge(days_rented)

最終程式碼:

Movie

class Movie
  attr_reader :title
  attr_writer :price

  def initialize(title, price)
    @title, @price = title, price
  end

  def charge(days_rented)
    @price.charge(days_rented)
  end

  def frequent_renter_points(days_rented)
    @price.frequent_renter_points(days_rented)
  end
end

Rental

class Rental
  attr_reader :movie, :days_rented

  def initialize(movie, days_rented)
    @movie, @days_rented = movie, days_rented
  end

  def charge
    movie.charge(days_rented)
  end
  
  def frequent_renter_points
    movie.frequent_renter_points(days_rented)
  end
end

Customer

class Customer
  attr_reader :name

  def initialize(name)
    @name = name
    @rentals = []
  end

  def add_rental(arg)
    @rentals << arg
  end
  
  def statement
    result = "Rental Record for #{@name}\n"
    @rentals.each do |element|
      #show figures for this rental
      result += element.movie.title + "\t" + element.charge.to_s + "\n"
    end
    # add footer lines
    result += "Amount owed is #{total_charge}\n"
    result += "You earned #{total_frequent_renter_points} frequent renter points"
    result
  end

  def html_statement
    result = "<h1>Rentals for <em>#{@name}</em></h1><p>\n"
    @rentals.each do |element|
      # show figures for this rental
      result += element.movie.title + ": " + element.charge.to_s + "<br>\n"
    end
    #add footer lines
    result += "<p>You owe <em>#{total_charge}</em></p>\n"
    result += "On this rental you earned " +
           "<em>#{total_frequent_renter_points}</em> " +
           "frequent renter points</p>"
    result
  end

  private

  def total_charge
    @rentals.inject(0) { |sum, rental| sum + rental.charge }
  end

  def total_frequent_renter_points
    @rentals.inject(0) { |sum, rental| sum + rental.frequent_renter_points }
  end
end

Default Price

module DefaultPrice
  def frequent_renter_points(days_rented)
    1
  end
end

Regular Price

class RegularPrice
  include DefaultPrice

  def charge(days_rented)
    result = 2
    result += (days_rented - 2) * 1.5 if days_rented > 2
    result
  end
end

Childrens Price

class ChildrensPrice
  include DefaultPrice

  def charge(days_rented)
    result = 1.5
    result += (days_rented - 3) * 1.5 if days_rented > 3
    result
  end
end

New Release Price

class NewReleasePrice
  def charge(days_rented)
    days_rented * 3
  end

  def frequent_renter_points(days_rented)
    days_rented > 1 ? 2 : 1
  end
end

最終程式碼的Test

  • 因為Movie.new要傳的參數改變了,所以測試也要修改一下:
require 'minitest/autorun'
require_relative 'ch1'

describe Customer, "#statement" do
  it "generate statement" do
    # movie = Movie.new("Star War", Movie::NEW_RELEASE) # 這行是原本的
    movie = Movie.new("Star War", NewReleasePrice.new) # 改成這行
    rental = Rental.new(movie, 3)
    customer = Customer.new("John Doe")

    customer.add_rental(rental)
    customer.statement.must_equal <<-EOS.gsub(/^\s+/, '').gsub(/\n$/,'')
      Rental Record for John Doe
      Star War\t9
      Amount owed is 9
      You earned 2 frequent renter points
    EOS
  end
end
我的Ruby學習之路
2016.Oct.04
A ruby week in Japan
2014.Sep.29
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