抽象工廠（Abstract factory）

目的：

以同一個介面來建立一整族相關或相依的物件，不需要點明個物件真正所屬的具象類別。

結構：

程式碼範例

class AbstractFactory
  def create_product_a
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end

  def create_product_b
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end
end

class ConcreteFactory1 < AbstractFactory
  def create_product_a
    ConcreteProductA1.new
  end

  def create_product_b
    ConcreteProductB1.new
  end
end

class ConcreteFactory2 < AbstractFactory
  def create_product_a
    ConcreteProductA2.new
  end

  def create_product_b
    ConcreteProductB2.new
  end
end

class AbstractProductA

  def useful_function_a
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end
end

class ConcreteProductA1 < AbstractProductA
  def useful_function_a
    'The result of the product A1.'
  end
end

class ConcreteProductA2 < AbstractProductA
  def useful_function_a
    'The result of the product A2.'
  end
end

class AbstractProductB
  def useful_function_b
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end

  def another_useful_function_b(_collaborator)
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end
end

class ConcreteProductB1 < AbstractProductB
  def useful_function_b
    'The result of the product B1.'
  end

  def another_useful_function_b(collaborator)
    result = collaborator.useful_function_a
    "The result of the B1 collaborating with the (#{result})"
  end
end

class ConcreteProductB2 < AbstractProductB
  def useful_function_b
    'The result of the product B2.'
  end

  def another_useful_function_b(collaborator)
    result = collaborator.useful_function_a
    "The result of the B2 collaborating with the (#{result})"
  end
end

def client_code(factory)
  product_a = factory.create_product_a
  product_b = factory.create_product_b

  puts product_b.useful_function_b.to_s
  puts product_b.another_useful_function_b(product_a).to_s
end

puts 'Client: Testing client code with the first factory type:'
client_code(ConcreteFactory1.new)

puts "\n"

puts 'Client: Testing the same client code with the second factory type:'
client_code(ConcreteFactory2.new)

結果

Client: Testing client code with the first factory type:
The result of the product B1.
The result of the B1 collaborating with the (The result of the product A1.)

Client: Testing the same client code with the second factory type:
The result of the product B2.
The result of the B2 collaborating with the (The result of the product A2.)

解釋

在討論整個抽象工廠運作流程以前，最重要的是先了解客戶端如何啟動抽象工廠！
上述的程式碼中的client_code內所投入的factory參數，是工廠運作的開始，也是確立哪一個工廠應該開始作用
確認工廠後再來就是確認哪種產品是需要的，所以再來使用create_product_a或者create_product_b來製作特定產品
而製造產品的方法內部細節，會額外抽離到ConcreteProduct的類別之中
最後獲得產品物件後，就可以實踐各個方法

此外Abstract的部分是宣告抽象介面，利用與Concrete繼承關係，來讓客戶端知道哪些工廠或者產品可以實作
而Concrete則是負責實作具象成品（所以客戶端在產生物件時，會直接new出特定工廠來實作特定產品）

優點

1. 因為client端所輸入的是特定工廠，所以確保一系列產品的產出
2. 避免client與產品耦合
3. SRP，工廠跟產品職責分離
4. Open/Closed Principle，可以隨時增加工廠與其產品，不用擔心修改其他工廠或產品的內容

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