(原創) 我的Design Pattern之旅[5]:Facade Pattern (OO) (Design Pattern) (C/C++)
Abstract
facade是我學到的第三個pattern,也是個很簡單的pattern,主要是將眾多class重新包裝成一個更好用的class。
Intent
替子系統裡的一堆介面定義一套統一的高階介面,讓子系統更易使用。
Introduction
Facade常用在兩個地方,第一種用法是將原來程序導向的系統,利用Facade包成OO,如MFC包裝Win32 API,第二種用法是將原來複雜的OO系統,再包成更簡單的OO,如.NET Enterprise Library將.NET Framework再做一次包裝,讓你更容易使用.NET。另外3-tier也是典型Facade的應用,DAL(Data Access Layer)就是一個Facade,讓BLL(Business Logic Layer)在存取資料時,不用對付ADO.NET複雜的機制,只需面對DAL即可。
Sample Code
如我們坐捷運買票,目前的賣票的機器只能吃銅板,所以得另外的提供換鈔機,讓旅客將紙幣換成銅板,也就是說,若一個旅客身上只有紙幣又要賣捷運票,他必須做兩個步驟,先將紙幣換成銅板,再用銅板去買車票。
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執行結果
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但對於旅客來說,似乎太嫌麻煩,買個票要排兩次隊,若我們能設計一部機器,能吃紙鈔,且能購票,那對旅客將非常的方便。事實上,我們並不需重新設計這樣的機器,只要將這兩台機器整合成一台即可。
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執行結果
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可以看到client的程式明顯乾淨多了
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這表示旅客可以用更簡單的方式買票了。
Consequence
1.避免外界觸及子系統的元件,降低外界所要面對的物件數量,讓子系統更容易使用。
2.促成外界與子系統之間的低偶合性。在子系統裡,元件之間的耦合性往往都很高,降低耦合性會使原件更容易改變而不波及外界。Facade有助於將系統分層、將物件之間的依存關係分層,順便剔除複雜的、循環的依存關係;這對於需要將client和子系統分開製作的專案而言,是非常重要的性質。
對大型軟體系統來說,降低編譯方面的依存關係是非常重要的;子系統一有變動,大家都希望重新編譯的時間能越短越好。只要用Facade降低編譯依存關係,不僅能將重新編譯的動作侷限在一小塊區域,更有利於軟體移植:只是重新編譯一塊子系統,不太需要其他地方也一起再編譯一遍。
3.如有必要,外界仍可直接接觸子系統內的類別,所以你可以在易用性與一般性之間做選擇。
Conclusion
或許你會覺得facade pattern沒什麼,的確,他只是一個概念而已,而且非常單純,但卻常常看的到。
Known use
1..NET Enterprise Library的DAAB
2..NET 2.0的My
3.網路架構的OSI模型
See Also
(原創) 深入探討Altera的Checksum Master範例 (SOC) (SOPC Builder) (Nios II) (DE2)
Reference
GoF,Design Patterns,Addison Weseley ,1995